Climate Change and Climate Action – Page 4

It has been about 3 million years since the Earth’s atmosphere last had 400 ppm of carbon dioxide

Posted on August 17, 2018August 19, 2018 by r.d.pancost

In 2015, I was interviewed by Susan Kucera when she visited Bristol to show her beautiful film on climate change – Breath of Life. Working with Jeff Bridges, she has created a powerful new film – Living in the Future’s Past – that features those interviews with me and many other scientists, psychologists, politicians and philosophers. My own contributions on climate change reflect on the history of our planet and how that provides perspective for our current unprecedented rate of climate change. To elaborate on that, I am posting some recent press releases on the research that informed my reflections (and yet to be published at the time of interview).

Image result for living in the futures past

In particular, I discuss work in collaboration with colleagues at Southampton, showing that the Earth’s current pCO2 level of 400 ppm is higher than it has been for nearly 3 million years (this work also refers to our research on the Pliocene, which I have also written about here).

A multinational research team, led by scientists at the University of Southampton and the University of Bristol Cabot Institute, has developed new records of past CO₂ levels. These reveal that the CO₂ content of the Earth’s atmosphere between 2.8 to 3.3 million years ago, were higher than that of the pre-industrial Earth and likely higher than at any other point over the past two million years – but similar to values reached in the past decade.

The new records are based on geochemical analyses of marine sediments. These have been measured using techniques developed at Bristol and Southampton over the past decade. The Bristol team includes Professor Richard Pancost from the University of Bristol, Director of the Cabot Institute and the Primary Investigator of the wider grant under which this research was conducted, as well as Dr Marcus Badger, Professor Dan Lunt and Professor Daniela Schmidt. Professor Pancost explains: “We cannot directly measure the CO₂ levels on Earth prior to about 1 million years ago, and so we instead use proxies. In the case of our project, funded by the NERC, we used a combination of approaches based on the chemical signatures of organisms preserved in sediments at the bottom of the sea.”

By studying the relationship between CO₂ levels and climate change during a warmer period in Earth’s history, the team have been able to estimate how the climate will respond to increasing levels of carbon dioxide, a parameter known as ‘climate sensitivity’. The findings, which have been published in Nature, fall in line with estimates in the most recent IPCC report. “Today the Earth is still adjusting to the recent rapid rise of CO₂ caused by human activities, whereas the longer-term Pliocene records document the full response of CO₂-related warming,” says Southampton’s Dr Gavin Foster, co-lead author of the study. “Our estimates of climate sensitivity lie well within the range of 1.5 to 4.5°C warming per CO₂ doubling summarised in the latest IPCC report. ”

Professor Dan Lunt, also of the University of Bristol and the Cabot Institute adds: “We compared the temperature response to CO₂ change in the warm Pliocene to that during colder times, like the glacial cycles of the last 800 thousand years. The temperature response was around half that of the colder period, but that difference can be largely resolved by considering the growth and retreat of large continental ice sheets during more recent glacial cycles. These ice sheets reflect a lot of sunlight and their growth consequently amplifies the impact of CO₂ changes, but they were smaller and less variable during the warm Pliocene.”

“Our new records also reveal an important change at around 2.8 million years ago, when levels dropped to values of about 280 ppm, similar to those seen before the industrial revolution,” says lead author of the study Dr Miguel Martinez-Boti, also from Southampton. “This appears to have caused a dramatic global cooling that initiated the ice-age cycles that have dominated Earth’s climate ever since.”

Professor Pancost added: “When we account for the influence of the ice sheets, we can confirm that the Earth’s climate changed with a similar sensitivity to overall forcing during both warmer and colder climates. During the Pliocene the Earth was warmer by around 2°C than it is today and atmospheric CO₂ levels were around 350-400 parts per million (ppm), similar to the levels reached in recent years. This suggests that in the long term, we have already committed to 2 °C warming, and future CO₂ increases will only add to that.”

NOTE: Subsequent to this work, we pushed this methodology further back into Earth history, into the Eocene (30 to 50 million years ago). This was probably the last time the Earth had pCO2 levels similar to what we might reach by the end of the century (>800 ppm).

Plio-Pleistocene climate sensitivities evaluated using high-resolution CO₂ records by M.A. Martínez-Botí, G.L. Foster, T. B. Chalk, E.J. Rohling, P.F. Sexton, D.J. Lunt, R.D. Pancost, M.P.S. Badger & D.N. Schmidt DOI: 10.1038/nature14145

This work was funded by an NERC grant to Pancost (PI), G Foster, D Schmidt and D Lunt.

Back to the Future ‘Hothouse’

Posted on August 13, 2018August 14, 2018 by r.d.pancost

Our current global warming target and the trajectory it places us on, towards a future ‘Hothouse Earth’, has been the subject of much recent discussion, stimulated by a paper by Will Steffen and colleagues. In many respects, the key contribution of this paper and similar work is to extend the temporal framing of our climate discussions, beyond 2100 for several centuries or more. Analogously, it is useful to extend our perspective backwards to similar time periods, to reflect on the last time Earth experienced such a Hothouse state and what it means.

The Steffen et al paper allows for a variety of framings, all related to the range of natural physical, biological and chemical feedbacks that will amplify or mitigate the human intervention in climate. [Note: the authors frame their paper around the concept of a limited number of steady state scenarios/temperatures for the Earth. They then argue that aiming for 2C, potentially an unstable state, could trigger feedbacks tipping the world towards the 4C warmer Hothouse. I find that to be somewhat simplistic given the diversity of climate states that have existed, if even transiently, over the past 15 million years, but that is a discussion for another day.] From my perspective, the most useful framing – and one that remains true to the spirit of the paper is this: We have set a global warming limit of 2C by 2100, with an associated carbon budget. What feedback processes will that carbon budget and warming actually unleash over the coming century, how much additional warming will they add, and when?

That is a challenging set of questions that comes with a host of caveats, most related to the profound uncertainty in the interlinked biogeochemical processes that underpin climate feedbacks. For example, as global warming thaws the permafrost, will it release methane (with a high global warming potential than carbon dioxide)? Will the thawed organic matter oxidise to carbon dioxide or will it be washed and buried in the ocean? And will the increased growth of plants under warmer conditions lead instead to the sequestration of carbon dioxide? The authors refer to previous studies that suggest a permafrost feedback yielding an additional 0.1C warming by the end of the century; but there is great uncertainty in both the magnitude of that impact and its timing.

And timing is the great question at the heart of this perspective piece. I welcome it, because too often our perspective is fixed on the arbitrary date of 2100, knowing full well that the Earth will continue to warm and ice continue to melt long after that date. In this sense, Steffen et al is not a contradiction to what has been reported from the IPCC but an expansion on it.

Classically, we discuss these issues in terms of fast and slow feedbacks, but in fact there is a continuum between near instantaneous feedbacks and those that act over hundreds, thousands or even millions of years. A warmer atmosphere will almost immediately hold more water vapour, providing a rapid positive feedback on warming – and one that is included in all of those IPCC projections. More slowly, soil carbon, including permafrost, will begin to oxidise, with microbial activity stimulated and accelerated under warmer conditions – a feedback that is only just now being included in Earth system models. And longer term, all manner of processes will come into play – and eventually, they will include the negative feedbacks that have helped regulate Earth’s climate for the past 4 billion years.

There is enough uncertainty in these processes to express caution in some of the press’s more exuberant reporting of this topic. But lessons from the past certainly underscore the concerns articulated by Steffen et al. We think that the last time Earth had 410 ppm CO2, a level similar to what you are breathing right now, was the Pliocene about 3 million years ago. This was a world that was 1 to 2C warmer than today (i.e. 2 to 3C warmer than the pre-industrial Earth) and with sea levels about 10 m higher. This suggests that we are already locked into a world that far exceeds the ambitions and targets of the Paris Agreement. This is not certain as we live on a different planet and one where the great ice sheets of Greenland and Antarctica might not only be victims of climate change but climate stabilisers through ice-sheet hysteresis. And even if a Pliocene future is fixed, it might take centuries for that warming and sea level change to be realised.

But that analogue does suggest caution, as advocated by the Hothouse Earth authors.

It also prompts us to ask what the Earth was like the last time its atmosphere held about 500 ppm CO2, similar to the level needed to achieve the Paris Agreement to limit end-of-century warming below 2C. A useful analogue for those greenhouse gas levels is the Middle Miocene Climate Optimum, which occurred from 17 to 14.7 million years ago.

d11BAlkCenozoic_CO2_SameScale_AIv1_NoBilj-NoPagani99v2.jpg

Figure showing changes in ocean temperature (based on oxygen isotopic compositions of benthic foraminifera) and pCO2 over the past 60 million years (from Palaeo-CO2). Solid symbols are from the d11B isotope proxy and muted symbols are from the alkenone-based algal carbon isotope fractional proxy. Note the spike in pCO2 associated with the MMCO at about 15 million years ago.

As one would expect for a world with markedly higher carbon dioxide levels, the Miocene was hotter than the climate of today. And consistent with many of Steffen et al.’s arguments, it was about 4C hotter rather than a mere 2C, likely due to the range of carbon cycle and ice-albedo feedbacks they describe. But such warmth was not uniform – globally warmer temperatures of 4C manifest as far hotter temperatures in some parts of the world and only slightly warmer temperatures elsewhere. Pollen and microbial molecular fossils from the North Sea, for example, indicate that Northern Europe experienced sub-tropical climates.

But what were the impacts of this warmth? What is a 4C warmer world like? To understand that, we also need to understand the other ways in which the Miocene world differed from ours, not just due to carbon dioxide concentrations but also the ongoing movement of the continents and the continuing evolution of life. In both respects, the Miocene was broadly similar to today. The continents were in similar positions, and the geography of the Miocene is one we would recognise. But there were subtle differences, including the ongoing uplift of the Himalayas and the yet-to-be-closed gateway between North and South America, and these subtle differences could have had major impacts on Asian climate and the North Atlantic circulation, respectively.

Similarly, the major animal groups had evolved by this point, and mammals had firmly established their dominance in a world separated by 50 million years from the dinosaurs. Remnant groups from earlier times (hell pigs!) still terrorised the landscape, but many of the groups were the same or closely related to those we would recognise today. And although hominins would not appear until the end of the Miocene, the apes had become well established, represented by as many as a 100 species. In the oceans, the differences were perhaps more apparent, the seas thriving with the greatest diversity of cetaceans in the history of our planet and associated with them the gigantic macro-predators such as Charcharadon megalodon (The Meg^TM).

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Smithsonian mural showing Miocene Fauna and landscape.

But it is the plants that exhibit the most pronounced differences between modern and Miocene life. Grasses had only recent proliferated across the planet at the time of the MMCO, and the C4 plants had yet to expand to their current dominance. And in this regard, the long-term evolution of Earth’s climate likely played a crucial role. There are about 8100 species of C4 plants (although this comprises only 3% of the plant species known to us) and most of these are grasses with other notable species being maize and sugar cane. They are distinguished from the dominant C3 plants, which comprise almost all other species, by virtue of their carbon dioxide assimilation biochemistry (the Hatch-Slack mechanism) and their leaf cellular physiology (the Kranz leaf anatomy). It is a collective package that is exceptionally well adapted to low carbon dioxide conditions, and their global expansion about 7 million years ago was almost certainly related to the long-term decline in carbon dioxide from the high levels of the Middle Miocene. Although C4 plants only represent a small proportion of modern plant species, the Miocene world, bereft of them, would have looked far different than today – lacking nearly half of our modern grass species and by extension clear analogues to the vast African savannahs.

Aside from these, the most profound differences between the Miocene world and that of today would have been the direct impacts of higher global temperatures. There is strong evidence that the Greenland ice sheet was far reduced in size compared to that of today, and its extent and even whether or not it was a persistent ice sheet or an ephemeral one remains the subject of debate. Similarly, West Antarctica was likely devoid of permanent ice, and the East Antarctic Ice Sheet was probably smaller – perhaps far smaller – than it is today. And collectively, these smaller ice sheets were associated with a sea level that was about 40 m higher than that of today.

The hot Miocene world would have been different in other ways, including the hydrological cycle. Although less studied than for other ancient intervals, it is almost certain that elevated warmth – and markedly smaller equator-to-pole temperature differences – would have impacted the global distribution of water. More water was evidently exported to the high latitudes, resulting in a warmer and vegetated Antarctica where the ice had retreated. It was also likely associated with far more extreme rainfall events, with the hot air able to hold greater quantities of water. More work is needed, but it is tempting to imagine the impact of these hot temperatures and extreme rainfall events. They would have eroded the soil and flushed nutrients to the sea, perhaps bringing about the spread of anoxic dead zones, similar to the Oceanic Anoxic Events of the Mesozoic or the dead zones of modern oceans caused by agricultural run-off. Indeed, the Miocene is characterised by the deposition of some very organic-rich rocks, including the North Pacific Monterey Formation, speaking to the occurrence of reduced oxygen levels in parts of these ancient oceans.

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It is unclear if our ambitions to limit global warming to 2C by the end of this century really have put us on a trajectory for 4C. It is unclear if we are destined to return to the Miocene.

But if so, the Miocene world is one both similar to but markedly distinct from our own – a world of hotter temperatures, extremes of climate, fewer grasslands, Antarctic vegetation, Arctic forests and far higher sea levels. Crucially, it is not the world for which our current society, its roads, cities, power plants, dams, borders, farmlands and treaties, has been designed.

Moreover, the MMCO Earth is a world that slowly evolved from an even warmer one over millions of years*; and that then evolved over further millions of years to the one in which we now inhabit. It is not a world that formed in a hundred or even a thousand years. And that leaves us three final lessons from the past. First, we do not know how the life of this planet, from coral reefs to the great savannahs, will respond to such geologically rapid change. Second, we do not know how we will respond to such rapid change; if we must adapt, we must learn how to do so creatively, flexibly and equitably. And third, it is probably not too late to prevent such a future from materialising, but even if it is, we still must act to slow down that rate of change to which we must adapt.

And we still must act to ensure that our future world is only 4C hotter and analogous to the Miocene; if we fail to act, the world will be even hotter, and we will have to extend our geological search 10s of millions of years further into the past, back to the Eocene, to find an even hotter and extreme analogue for our future Hothouse World.

*The final jump into the MMCO appears to have been somewhat more sudden, but still spanned around two-hundred thousand years. A fast event geologically but not on the timescales of human history.

Why we must Bridge the Gap

Posted on August 11, 2018 by r.d.pancost

In 2015, Bristol was the European Green Capital. Bristol had and still has bold ambitions to be an environmentally thriving city, rich in wildlife and green spaces and committed to net zero carbon emissions by 2050. But this is a journey that requires the commitment of all, and as such, I was asked to help kick off the Green Capital year and frame its ambitions. This is what I wrote in January 2015; I’d write something similar today, but no less ambitious and no less determined.

Why we must Bridge the Gap

Environmental experts describe the gap between our green intentions and our green actions as “the green gap”. We asked Bristol-based climate scientist, Professor Richard Pancost, to explain the scale of the problem, and why we need to take action now to tackle climate change.

By Professor Richard D. Pancost, Director of the Cabot Institute

Photo: Martina Ebel

Much of the climate change of the past century has been caused by our burning of fossil fuels. And without a change in that fossil fuel use, continued climate change in the next century could have devastating impacts on our society.

It is likely to bring increased risk and hazards associated with extreme weather events. Refugee crises could be caused by rising sea levels or droughts that make some nations uninhabitable. And climate change will also make our world a more uncertain place to live, whether that be uncertainty in future rainfall patterns, the magnitude of sea level rise or the response of global fisheries to ocean acidification.

This uncertainty is particularly problematic because it makes it so much harder for industry or nations to plan and thrive.

Or to grapple with the other great challenge facing humanity – securing food, water and energy for 7 billion people (and growing).

Because of this, most nations have agreed that global warming should be held below 2°C. [Note: And of course, this was agreed at COP21 in the Paris Climate Change Agreement.]

Interconnected world

These climatic and environmental impacts will be felt at home in the South West of England.

We live in an interconnected world, such that drought in North America will raise the price of our food. The effects of ocean acidification on marine ecosystems and UK fisheries remain worryingly uncertain. The floods of last winter could have been a warning of life in a hotter and wetter world; moreover, it will only become harder to protect our lowlands from not only flooding but also salt water incursions as sea level rises.

JAN 7, 2014: An unknown jogger running along the sea wall between in Dawlish, South West England

Photo: Paul J Martin / Shutterstock.com

Climate change affects us all – globally, nationally and locally in Bristol, the 2015 European Green Capital. Preventing it requires reductions in emissions over the next decade. And it then requires putting an end to all fossil fuel emissions in the decades to come.

Recent discussions in Lima and likely those in Paris at the end of this year, focused on how we reduce emissions globally. But in order to end all fossil fuel emissions in future, we need to put in place an international treaty. And this is the most difficult but necessary action to achieve.

Carbon lifecycle

Carbon dioxide has a lifetime in the atmosphere of 1000s of years, such that slower emissions will only delay climate change.

That can be useful – if we must adapt to a changing world, having more time to do so will be beneficial. However, it is absolutely clear that emissions must stop if we are to meet our target of 2°C. In fact, according to most climate models as well as the geological history of climate, emissions must stop if we are to keep total warming below 5°C.

[Or we must spend a lot of money removing that CO2 from the atmosphere later.]

In short, we cannot use the majority of our coal, gas and petroleum assets for energy. They must stay buried.

Can we ‘geoengineer’ our way to alternative solution? Not according to recent research. Last November, a Royal Society Meeting showcased the results of three UK Research Council Funded investigations of geoengineering feasibility and consequences. They collectively illustrated that geoengineering a response to climate change was at best complicated and at worst a recipe for disaster and widespread global conflict.

The most prominent geoengineering solution is to offset the greenhouse gas induced rise in global temperatures via the injection of stratospheric particles that reflect some of the solar energy arriving at Earth. However, on the most basic level, a world with elevated CO2 levels and reflective particles in the atmosphere is not the same as a world with 280 ppm of CO2 and a pristine atmosphere.

To achieve the same average global temperature, some regions will be cooler and others warmer. Rainfall patterns will differ: regional patterns of flood and drought will differ. Even if it could be done, who are the arbitrators of a geoengineered world? The potential for conflict is profound.

In short, the geoengineering our climate is neither a feasible nor a just option.

And again, the conclusion is that we cannot use most of our fossil fuels.

Ratcliffe-on-Soar is one of the most efficient coal fired power stations in the UK, and removes 92% of sulphur dioxide from flue gas before it is released into the atmosphere. But it does not remove the CO2

Photo: Mark Burrows/ Shutterstock

Future generations

One might argue that we can adapt to climate change: why risk our economy now when we can adapt to the consequences of climate change later?

Many assessments suggest that this is not the best economic approach, but I understand the gamble: be cautious with a fragile economy now and deal with consequences later. This argument, however, ignores the vast inequity associated with climate change.

It is the future generations that will bear the cost of our inaction. Moreover, it appears that the most vulnerable to climate change are the poorest – and those who consume the least fossil fuels.

Those of us who burn are not those who will pay.

Arguably then, we in the UK have a particular obligation to the poor of the world and of our own country, as well as to our children and grandchildren, to soon cease the use of our fossil fuels.

Energy is at the foundation of modern society and it has been the basis for magnificent human achievement over the past 150 years, but it is clear that obtaining energy by burning fossil fuels is warming our planet and acidifying our oceans. The consequences for our climate, from extreme weather events to rising sea levels, is profound; even more worrying are the catastrophic risks that climate change poses for the food and water resources on which society depends. It is now time for us to mature beyond the 19th and 20th century fossil-fuel derived energy to a renewable energy system of the 21st century that is sustainable for us and our planet.

We must bridge the gap.

[This was written in Jan 2015, before the Paris Agreement was signed, before Brexit, before Trump, before plummeting costs of offshore wind, before reconsideration of nuclear energy as financially viable, before so much… But one thing is very clear – we have made a lot of progress but not enough and CO2 emissions have not only not fallen but they continue to rise.]

Climate Change Uncertainty is a reason for action not inaction

Posted on July 21, 2018July 23, 2018 by r.d.pancost

One of the primary motivations for the Uncertain World – from its conception to this website – is to explain the perils of uncertainty, how it undermines confidence, planning, strategy and adaptation and by extension every sector of modern society from farming to finance. Uncertainty is risk. And the greater that uncertainty, especially with respect to its intersection with complex ecological, biogeochemical and social systems, the greater the risk. Others, however, use uncertainty as a means to confuse and prevaricate: ‘The science is not settled.’ ‘We don’t know enough to act.’ This blog co-written with my friend and colleague Steve Lewandowsky tackles that issue (originally published in The Conversation).

Former environment minister Owen Paterson has called for the UK to scrap its climate change targets. In a speech to the Global Warming Policy Foundation, he cited “considerable uncertainty” over the impact of carbon emissions on global warming, a line that was displayed prominently in coverage by the Telegraph and the Daily Mail.

Paterson is far from alone: climate change debate has been suffused with appeals to “uncertainty” to delay policy action. Who hasn’t heard politicians or media personalities use uncertainty associated with some aspects of climate change to claim that the science is “not settled”?

Over in the US, this sort of thinking pops up quite often in the opinion pages of The Wall Street Journal. Its most recent article, by Professor Judith Curry, concludes that the ostensibly slowed rate of recent warming gives us “more time to find ways to decarbonise the economy affordably.”

At first glance, avoiding interference with the global economy may seem advisable when there is uncertainty about the future rate of warming or the severity of its consequences.

But delaying action because the facts are presumed to be unreliable reflects a misunderstanding of the science of uncertainty. Simply because a crucial parameter such as the climate system’s sensitivity to greenhouse gas emissions is expressed as a range – for example, that under some emissions scenarios we will experience 2.6°C to 4.8ºC of global warming or 0.3 to 1.7 m of sea level rise by 2100 – does not mean that the underlying science is poorly understood. We are very confident that temperatures and sea levels will rise by a considerable amount.

Perhaps more importantly, just because some aspects of climate change are difficult to predict (will your county experience more intense floods in a warmer world, or will the floods occur down the road?) does not negate our wider understanding of the climate. We can’t yet predict the floods of the future but we do know that precipitation will be more intense because more water will be stored in the atmosphere on a warmer planet.

This idea of uncertainty might be embedded deeply within science but is no one’s friend and it should be minimised to the greatest extent possible. It is an impetus to mitigative action rather than a reason for complacency.

Uncertainty means greater risk

There are three key aspects of scientific uncertainty surrounding climate change projections that exacerbate rather than ameliorate the risks to our future.

First, uncertainty has an asymmetrical effect on many climatic quantities. For example, a quantity known as Earth system sensitivity, which tells us how much the planet warms for each doubling of atmospheric carbon dioxide concentration, has been estimated to be between 1.5°C to 4.5ºC. However, it is highly unlikely, given the well-established understanding of how carbon dioxide absorbs long-wave radiation, that this value can be below 1ºC. There is a possibility, however, that sensitivity could be higher than 4.5ºC. For fundamental mathematical reasons, the uncertainty favours greater, rather than smaller, climate impacts than a simple range suggests.

Second, the uncertainty in our projections makes adaptation to climate change more expensive and challenging. Suppose we need to build flood defences for a coastal English town. If we could forecast a 1m sea level rise by 2100 without any uncertainty, the town could confidently build flood barriers 1m higher than they are today. However, although sea levels are most likely to rise by about 1m, we’re really looking at a range between 0.3m and 1.7m. Therefore, flood defences must be at least 1.7m higher than today – 70cm higher than they could be in the absence of uncertainty. And as uncertainty increases, so does the required height of flood defences for non-negotiable mathematical reasons.

And the problem doesn’t end there, as there is further uncertainty in forecasts of rainfall occurrence, intensity and storm surges. This could ultimately mandate a 2 to 3m-high flood defence to stay on the safe side, even if the most likely prediction is for only a 1m sea-level rise. Even then, as most uncertainty ranges are for 95% confidence, there is a 5% chance that those walls would still be too low. Maybe a town is willing to accept a 5% chance of a breach, but a nuclear power station cannot to take such risks.

Finally, some global warming consequences are associated with deep, so-called systemic uncertainty. For example, the combined impact on coral reefs of warmer oceans, more acidic waters and coastal run-off that becomes more silt-choked from more intense rainfalls is very difficult to predict. But we do know, from decades of study of complex systems, that those deep uncertainties may camouflage particularly grave risks. This is particularly concerning given that more than 2.6 billion people depend on the oceans as their primary source of protein.

Similarly, warming of Arctic permafrost could promote the growth of CO₂-sequestering plants, the release of warming-accelerating methane, or both. Warm worlds with very high levels of carbon dioxide did exist in the very distant past and these earlier worlds provide some insight into the response of the Earth system; however, we are accelerating into this new world at a rate that is unprecedented in Earth history, creating additional layers of complexity and uncertainty.

Uncertainty does not imply ignorance

Increasingly, arguments against climate mitigation are phrased as “I accept that humans are increasing CO₂ levels and that this will cause some warming but climate is so complicated we cannot understand what the impacts of that warming will be.”

This argument is incorrect – uncertainty does not imply ignorance. Indeed, whatever we don’t know mandates caution. No parent would argue “I accept that if my child kicks lions, this will irritate them, but a range of factors will dictate how the lions respond; therefore I will not stop my child from kicking lions.”

The deeper the uncertainty, the more greenhouse gas emissions should be perceived as a wild and poorly understood gamble. By extension, the only unequivocal tool for minimising climate change uncertainty is to decrease our greenhouse gas emissions.

Steve and I with many brilliant colleagues elaborated on these in a special publication of the Royal Society edited by Tim Ballard, Steve and myself – Uncertainty as Knowledge. The introduction is now available online for free and elaborates on these critical issues. And do watch the accompanying video!

The Pliocene – The last time Earth had 400 ppm of Carbon Dioxide

Posted on July 9, 2018 by r.d.pancost

One of the main approaches for better understanding our future Uncertain World, both with respect to minimising that uncertainty but also identifying the limits to our knowledge, is to look to the past. A few years ago, we passed 400 ppm of CO2 in the atmosphere, a level that we have not experienced for about 3 million years, during the Pliocene epoch. That coincided with Bristol hosting the 2nd International Conference on the Pliocene, providing an opportunity through this piece, originally published on The Conversation, and its embedded video to discuss how we try to unravel ancient climates to better understand the future. I have updated some of the text, reflecting some of the changing intellectual landscape. To provide some immediate context, below is the latest record of current atmospheric carbon dioxide concentrations from the Mauna Loa Observatory (i.e. The Keeling Curve).

How can air bubbles trapped in ice for millions of years, or fossilised fern fronds, or the chemical make-up of rocks that were underwater in the distant past provide us with an inkling of our future?

The answer lies in these clues provided by studying the Pliocene epoch, the span of geological time that stretched from 5.3 to 2.6 million years ago. This period of Earth’s history is interesting for many reasons, but one of the most profound is that the Earth’s atmosphere apparently contained high concentrations of carbon dioxide. Our best estimates suggest concentrations of about 300-400 parts per million (ppm) – much higher than concentrations of 100 years ago, but the same or lower than today after centuries of industrialisation and fossil fuel burning.

So studying the Pliocene could provide valuable insight into the type of planet we are creating via global warming. Our researchers at the Cabot Institute recently released a video on the topic, which has coincided with pronounced flooding across the UK [in Winter and Spring 2014] and renewed attention focused on our weather and climate. There is little doubt that increased carbon dioxide concentrations will cause global warming. The key questions are how much, and with what consequences.

One of the key lessons from Earth history is climate sensitivity. Climate sensitivity can be expressed in various ways, but in its simplest sense it is a measure of how much warmer the Earth becomes for a given doubling of atmospheric carbon dioxide concentrations.

This is well known for the Pleistocene, and especially the past 800,000 years of Earth history, a period for which we have detailed temperature reconstructions and carbon dioxide records derived from bubbles of gas trapped in ancient ice cores.

During that time, across several ice ages, the planet’s climate sensitivity showed warming of about 2.5-3°C for a doubling of carbon dioxide, which falls in the middle of the range of predictions given by models. Ice core records, however, extend back no more than a million years, and this time period is generally characterised by colder climates than those of today.

A section of an ice core of from 16,000 years ago. National Ice Core Laboratory

If we want to explore climate sensitivity on a warmer planet, we must look further back into Earth history, to times such as the Pliocene.

Reconstructing atmospheric carbon dioxide concentrations without relying on ice cores is admittedly more challenging. Instead of directly measuring the concentration of carbon dioxide in gas bubbles, we must rely on indirect records. For example, carbon dioxide concentration influences the number of stomata (pores) on plant leaves, and this can be measured on the fossils of ancient leaves. Alternatively, there are a number of geochemical tools based on how carbon dioxide affects the pH of seawater, or how algae take up carbon dioxide as they photosynthesise – these are recorded in the chemical composition of ancient fossils.

[For more info on how we reconstruct atmospheric carbon dioxide, especially in times pre-dating our ice core records, see this fantastic website maintained by Gavin Foster and friends.]

Atmospheric CO2 from AD 1000 to AD 2018 (right) from a mix of ice core records and measuresments of the astmosphere from Mauna Lao. On the left is a compilation of ice core CO2 (red) and boron isotope based estimates (blue). Note the age scales are different but y-axis is the same. See this document for references.

From p-CO2.org.

These means of drawing estimates come with larger margins of error, but they still provide key insights into climate sensitivity on a warmer Earth. Recent research indicates that these various carbon dioxide estimates of Pliocene carbon dioxide levels are converging, giving added confidence from which to derive estimates of climate sensitivity. In particular, it seems an increase of carbon dioxide from about 280ppm (equivalent to that before the industrial revolution) to about 400ppm in the Pliocene resulted in an Earth warmer by 2°C. The below figure shows the sea surface temperatures reconstructed for the Pliocene using a range of chemical and biological proxy data (a) and the difference (anomaly) between those temperatures and those of the modern pre-industrial world, i.e. before we started adding carbon dioxide to the atmosphere in significant quantities (b); notice how much hotter the oceans were, especially at high latitudes. From the Proceedings of the Royal Society.

Image result for Pliocene SSTs

This next figure is derived from an ensemble of climate models, which allows an extrapolation between data and therefore a comparison of temperatures on land and the modern pre-industrial world. Again, note how much higher temperatures were at high latitudes… but also continental interiors. We are seeing a manifestation of this now, with elevated temperatures occurring all over the globe but some areas experiencing much more dramatic warming. This work is from the amazing PlioMIP project and this figure is specifically from the PAGES website, adapted from Haywood et al., 2013.

Image result for images of pliocene warmth terrestrial

Taking into account other factors, this suggests a climate sensitivity of about 3°C, which confirms both the Pleistocene and model-based estimates. It also suggests that we have yet to experience the full consequences of the greenhouse gases already added to the atmosphere, let alone those we are still putting into it. [And finally, it suggests that there is a risk that we have already surpassed the agreed limits of the Paris Climate Agreement.]

So then, what was this much warmer world like? First of all, it was not an inhospitable planet – plants and animals thrived. This should not be a surprise – in fact, the Earth was much warmer even further back into the past. The changes in the climate we are inducing is a problem for us humans, and for our societies, not the planet we’re on. [And that is particularly evident, as I re-post this blog. We are experiencing a global heatwave that is causing forest fires (where associated with aridity), but it is also impacting infrastructure and the economy, warping rail lines, disrupting work patterns, driving up electricity usage. It is also causing deaths which raises a particularly acute and challenging question – are we and the ecosystems on which we depend prepared for the speed of this rapid global warming? Organisms and ecosystems had millions of years to evolve in a manner that allowed them to thrive in the Pliocene and previous greenhouse climates.]

Second, the Pliocene was a rather different world. For example, higher global temperatures were associated with a climate that was also wetter than at present. That provides important corroborating evidence for models that predict a warmer and wetter future.

Perhaps most striking, sea level in the Pliocene appears to have been between 10 to 40 metres higher than today, indicating that both the Greenland Ice Sheet and Antarctic Ice Sheet were markedly smaller. To put that into context, the Met Office has already commented on how flooding in the UK has been affected by sea level rise of 12cm over the last 100 years, and will be exacerbated further by another 5-7cm by 2030.

We must be careful in how we extract climate lessons from the geological record, and that is particularly true when we consider ice sheet behaviour. One widely discussed concept is ice sheet hysteresis. This is a fancy way of saying that due to feedback mechanisms, it could be easier to build an ice sheet on Greenland or Antarctica than it is to melt one. If hysteresis is a force stabilising our current ice sheets, then it may be that a planet with today’s carbon dioxide levels of 400 ppm will not necessarily have a sea level 20 metres higher than that of today – as it was during the Pliocene. On the other hand if hysteresis is rather weak, then the question is not whether we will see such a massive sea level change, but how long it will take to arrive (probably hundreds or even thousands of years).

Most importantly, the collective research into Earth history, including the Pliocene, reveals that Earth’s climate can and has changed. It also reveals that climate does not just change randomly: it changes when forced in ways that are relatively well understood – one of these is the concentration of carbon dioxide in our atmosphere. And consequently, there is little doubt from Earth’s history that transforming fossil carbon underground into carbon dioxide in the air – as we are doing today – will significantly affect the climate we experience for the forseeable future.

[Gerald Haug delivered the keynote for the Pliocene Conference and his outstanding public lecture is available here. With an introduction from the founder of the Organic Geochemistry Unit, Geoff Eglinton.]

[For a more fulsome discussion of how Warm Climates of the Past can hold Lessons for the Future, please check out our Special Royal Society Volume on the topic. Led by my Bristol colleague Dan Lunt but with lots of friends.]

The Origins of the Uncertain World

Posted on July 7, 2018July 7, 2018 by r.d.pancost

In late 2014, the Cabot Institute was in deep consultation with artists, colleagues, businesses and political leaders about our contribution to Bristol EU Green Capital 2015. Given the breadth of Cabot, we were keen to contribute in diverse ways, especially around sustainability solutions and the range of environmental challenges we face, from plastics in the sea to procuring safe, sustainable food. However, 2015 was also a fantastic chance to discuss climate change, its causes and impacts and how Bristol and the wider world would have to adapt – especially given that 2015 would culminate with the COP21 climate negotiations in Paris. At the same time, we wanted to examine climate change through a somewhat different lens than had been done in the past. Uncertainty was that lens. We wrote this at the end of 2014 announcing the Uncertain World as our framework for discussing these issues during 2015 and beyond. It went on to inform Bristol’s strong commitments to climate change and its Resilience Strategy.

Originally posted on the Cabot Institute blog, this was our statement of intent.

Over the next 18 months, in collaboration with Bristol Green Capital 2015 artists, civic leaders and innovative thinkers, the Cabot Institute will be participating in a series of activities in which we examine how human actions are making our planet a much more uncertain place to live.Fifty years ago, between 1962 and 1966, J. G. Ballard wrote a trio of seminal environmental disaster novels: The Drowned World, The Burning World and The Crystal World. These novels remain signposts to our future, the challenges we might face and the way people respond to rapid and unexpected change to their environment. In that spirit and coinciding with the Bristol Green Capital 2015, we introduce The Uncertain World, a world in which profound uncertainty becomes as much of a challenge to society as warming and rising sea levels.

J.G Ballard’s The Drowned World
taken from fantasticalandrewfox.com

For the past twenty years, the University of Bristol has been exploring how to better understand, mitigate and live with environmental uncertainty, with the Cabot Institute serving as the focus for that effort since its founding in 2010. Uncertainty is the oft-forgotten but arguably most challenging aspect of mankind’s centuries-long impact on the environment. We live our lives informed by the power of experience: our own as well as the collective experience of our families, communities and wider society. When my father started dairy farming he sought advice from my mother’s grandfather, our neighbours, and the grizzled veterans at the Middlefield auction house. Experience helps us make intelligent decisions, plan strategically and anticipate challenges.

Similarly, our weather projections, water management and hazard planning are also based on experience: tens to hundreds of years of observation inform our predictions of future floods, drought, hurricanes and heat waves. These records – this experience – can help us make sensible decisions about where to live, build and farm.

Now, however, we are changing our environment and our climate, such that the lessons of the past have less relevance to the planning of our future. In fact, many aspects of environmental change are unprecedented not only in human experience but in Earth history. As we change our climate, the great wealth of knowledge generated from human experience is losing capital every day.

The Uncertain World is not one of which we have no knowledge – we have high confidence that temperatures and sea level will rise, although there is uncertainty in the magnitude and speed of change. Nor should we view The Uncertain World with existential fear – we know that warm worlds have existed in the past. These were not inhospitable and most evidence from the past suggests that a climate ‘apocalypse’ resulting in an uninhabitable planet is unlikely.

Nonetheless, increasing uncertainty arising from human-induced changes to our global environment should cause deep concern. Crucial details of our climate remain difficult to predict, and it undermines our ability to plan for our future. We do not know whether many regions of the world will become wetter or dryer. This uncertainty propagates and multiplies through complex systems: how do we make sensible predictions of coastal flood risk when there is uncertainty in sea level rise estimates, rainfall patterns and the global warming that will impact both? We can make predictions even in such complex systems, but the predictions will inevitably come with a degree of uncertainty, a probabilistic prediction. How do we apply such predictions to decision making? Where can we build new homes, where do we build flood defences to protect existing ones, and where do we abandon land to the sea?

Methane escaping from Arctic
permafrost. Image: Treehugger.com

Perhaps most worrying, the consequences of these rapid changes on biological and chemical systems, and the people dependent upon them, are very poorly understood. For example, the synergistic impact of warmer temperatures, more acidic waters, and more silt-choked coastal waters on coral reefs and other marine ecosystems is very difficult to predict. This is particularly concerning given that more than 2.6 billion people depend on the oceans as their primary source of protein. Similarly, warming of Arctic permafrost could promote the growth of CO2-sequestering plants or the release of warming-accelerating methane – or both. Warm worlds with very high levels of carbon dioxide did exist in the past and these do provide some insight into the response of the Earth system, but we are accelerating into this new world at a rate that is unprecedented in Earth history, creating additional layers of uncertainty.
During late 2014 and 2015, the Cabot Institute will host a variety of events and collaborate with a variety of partners across Bristol and beyond to explore this Uncertain World and how we can live in it. How do we better explain uncertainty and what are the ‘logical’ decisions to make when faced with uncertainty? One of our first events will explore how uncertainty in climate change predictions should motivate us to action: the more uncertain our predictions the more we should employ mitigation rather than adaptation strategies. Future events will explore how past lessons from Earth history help us better understand potential future scenarios; how future scenario planning can inform the decisions we make today; and most importantly, how we build the necessary flexibility into social structures to thrive in this Uncertain World.

Ancient ‘Dead Seas’ offer a stark warning for our own future

Posted on June 30, 2018January 1, 2022 by r.d.pancost

The oceans are experiencing a devastating combination of stresses. Rising CO2 levels are raising temperatures while acidifying surface waters. More intense rainfall events, deforestation and intensive farming are causing soils and nutrients to be flushed to coastal seas. And increasingly, the oceans are being stripped of oxygen, with larger than expected dead zones being identified in an ever broadening range of settings. These dead zones appear to be primarily caused by the runoff of nutrients from our farmlands to the sea, but it is a process that could be exacerbated by climate change – as has happened in the past.

Recently, our group published a paper about the environmental conditions of the Zechstein Sea, which reached from Britain to Poland 270 million years ago. Our paper revealed that for tens of thousands of years, some parts – but only parts – of the Zechstein Sea were anoxic (devoid of oxygen). As such, it contributes to a vast body of research, spanning the past 40 years and representing the efforts of hundreds of scientists, which has collectively transformed our understanding of ancient oceans – and by extension future ones.

The types of processes that bring about anoxia are relatively well understood. Oxygen is consumed by animals and bacteria as they digest organic matter and convert it into energy. In areas where a great deal of organic matter has been produced and/or where the water circulation is stagnant such that the consumed oxygen cannot be rapidly replenished, concentrations can become very low. In severe cases, all oxygen can be consumed rendering the waters anoxic and inhospitable to animal life. This happens today in isolated fjords and basins, like the Black Sea. And it has happened throughout Earth history, allowing vast amounts of organic matter to escape degradation, yielding the fossil fuel deposits on which our economy is based, and changing the Earth’s climate by sequestering what had once been carbon dioxide in the atmosphere into organic carbon buried in sediments.

Red circles show the location and size of many dead zones. Black dots show Ocean dead zones of unknown size. Image source: Wikimedia Commons/NASA Earth Observatory

In some cases, this anoxia appears to have been widespread; for example, during several transient Cretaceous events, anoxia spanned much of what is now the Atlantic Ocean or maybe even almost all of the ancient oceans. These specific intervals were first identified and named oceanic anoxic events in landmark work by Seymour Schlanger and Hugh Jenkyns. In the 1970s, during the earliest days of the international Deep Sea Drilling Program (now the International Ocean Discovery Program, arguably the longest-running internationally coordinated scientific endeavor), they were the first to show that organic matter-rich deep sea deposits were the same age as similar deposits in the mountains of Italy. Given the importance of these deposits for our economy and our understanding of Earth and life history, scientists have studied them persistently over the past four decades, mapping them across the planet and interrogating them with all of our geochemical and palaeontological resources.

In my own work, I have used the by-products of certain bacterial pigments to interrogate the extent of that anoxia. The organisms are green sulfur bacteria (GSB), which require both sunlight and the chemical energy of hydrogen sulfide in order to conduct a rather exotic form of bacterial photosynthesis; crucially, hydrogen sulfide is only formed in the ocean from sulfate after the depletion of oxygen (because the latter yields much more energy when used to consume organic matter). Therefore, GSB can only live in a unique niche, where oxygen poor conditions have extended into the photic zone, the realm of light penetration at the very top of the oceans, typically only the upper 100 m. However, GSB still must compete for light with algae that live in even shallower and oxygen-rich waters, requiring the biosynthesis of light harvesting pigments distinct from those of plants, the carotenoids isorenieratene, chlorobactene and okenone. For the organism, this is an elegant modification of a molecular template to a specific ecological need. For the geochemist, this is an astonishingly fortuitous and useful synthesis of adaptation and environment – the pigments and their degradation products can be found in ancient rocks, serving as molecular fossil evidence for the presence of these exotic and diagnostic organisms.

And these compounds are common in the black shales that formed during oceanic anoxic events. And in particular, during the OAE that occurred 90 million years ago, OAE2, they are among the most abundant marker compounds in sediments found throughout the Atlantic Ocean and the Tethyan Ocean, what is now the Mediterranean Sea. It appears that during some of these events anoxia extended from the seafloor almost all the way to the ocean’s surface.

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Today, the deep sea is a dark and empty world. It is a world of animals and Bacteria and Archaea – and relatively few of those. Unlike almost every other ecosystem on our planet, it is bereft of light and therefore bereft of plants. The animals of the deep sea are still almost entirely dependent on photosynthetic energy, but it is energy generated kilometres above in the thin photic zone. Beneath this, both animals and bacteria largely live off the scraps of organic matter energy that somehow escape the vibrant recycling of the surface world and sink to the twilight realm below. In this energy-starved world, the animals live solitary lives in emptiness, darkness and mystery. Exploring the deep sea via submersible is a humbling and quiet experience. The seafloor rolls on and on and on, with only the occasional shell or amphipod or small fish providing any evidence for life.

“Krill swarm” by Jamie Hall – NOAA. Licensed under Public Domain via Wikimedia Commons

And yet life is there. Vast communities of krill thrive on the slowly sinking marine snow, can appear. Sperm whales dive deep into the ocean to consume the krill and emerge with the scars of fierce battles with giant squid. And when one of those great creatures dies and its carcass plummets to the seafloor, within hours it is set upon by sharks and fish, ravenous and emerging from the darkness for the unexpected feast. Within days the carcass is stripped to the bones but even then new colonizing animals arrive and thrive. Relying on bacteria that slowly tap the more recalcitrant organic matter that is locked away in the whale’s bones, massive colonies of tube worms spring to life, spawn and eventually die.

But all of these animals, the fish, whales, tube worms and amphipods, depend on oxygen. And the oceans have been like this for almost all of Earth history, since the advent of multicellular life nearly a billion years ago.

This oxygen-replete ocean is an incredible contrast to the north Atlantic Ocean during at least some of these anoxic events. Then, plesiosaurs, ichthyosaurs and mosasaurs, feeding on magnificent ammonites, would have been confined to the sunlit realm, their maximum depth of descent marked by a layer of surprisingly pink and then green water, pigmented by the sulfide consuming bacteria. And below it, not a realm of animals but a realm only of Bacteria and Archaea, single-celled organisms that can live in the absence of oxygen, a transient revival of the primeval marine ecosystems that existed for billions of years before more complex life evolved.

We have found evidence for these types of conditions during numerous events in Earth history, often associated with major extinctions, including the largest mass extinction in Earth history – the Permo-Triassic Boundary 252 million years ago. Stripping the ocean of oxygen and perhaps even pumping toxic hydrogen sulfide gas into the atmosphere is unsurprisingly associated with devastating biological change. It is alarming to realise that under the right conditions our own oceans could experience this same dramatic change. Aside from its impact on marine life, it would be devastating for us, so dependent are we on the oceans for our food.

The conventional wisdom has been that such extreme anoxia in the future is unlikely, that Cretaceous anoxia was a consequence of a markedly different geography. North America was closer to Europe and South America only completely rifted from Africa about 150 million years ago; the ancient Atlantic Ocean was smaller and more restricted, lending itself to these extreme conditions.

And yet questions remain. What was their trigger? Was it really a happenstance of geography? Or was it due to environmental perturbations? And how extensive were they? The geological record preserves only snapshots, limiting the geographical window into ancient oceans, and this is a window that narrows as we push further back in time. In one of our recent papers, we could not simulate such severe anoxia in the Atlantic Ocean without also simulating anoxia throughout the world’s oceans, a truly global oceanic anoxic event. However, that model can only constrain some aspects of ocean circulation and there are likely alternative mechanisms that confine anoxia to certain areas.

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Over the past twenty years, these questions have intersected one another and been examined again and again via new models, new geochemical tools and new ideas. And an emerging idea is that the geography of the Mesozoic oceans was not as important as we have thought.

That classical model is that ancient oceans, through a combination of the aforementioned restricted geography and overall high temperatures, were inherently prone to anoxia. In an isolated Atlantic Ocean, oxygen replenishment of the deep waters would have been much slower. This would have been exaggerated by the higher temperatures of the Cretaceous, such that oxygen solubility was lower (i.e. for a given amount of oxygen in the atmosphere, less dissolves into seawater) and ocean circulation was more sluggish. Consequently, these OAEs could have been somewhat analogous to the modern Black Sea. The Black Sea is a restricted basin with a stratified water column, formed by low density fresh water derived from the surrounding rivers sitting stably above salty and dense marine deep water. The freshwater lid prevents mixing and prevents oxygen from penetrating into deeper waters. Concurrently, nutrients from the surrounding rivers keep algal production high, ensuring a constant supply of sinking organic matter, delicious food for microbes to consume using the last vestiges of oxygen. The ancient oceans of OAEs were not exactly the same but perhaps similar processes were operating. Crucially, the configuration of ancient continents in which major basins were isolated from one another, suggests a parallel between the Black Sea and the ancient North Atlantic Ocean.

But over the past twenty years, that model has proved less and less satisfactory. First, it does not provide a mechanism for the limited temporal occurrence of the OAEs. If driven solely by the shape of our oceans and the location of our continents, why were the oceans not anoxic as the norm rather than only during these events? Second, putative OAEs, such as that at the Permo-Triassic Boundary occur at times when the oceans do not appear to have been restricted. Third, coupled ocean-atmosphere models indicate that although ocean circulation was slower under these warmer conditions, it did not stop.

But also, as we have looked more and more closely at those small windows into the past, we have learned that during some of these events anoxia was more restricted to coastal settings. And that brings us back to the Zechstein Sea. We mapped the extent of anoxia at an unprecedented scale in cores drilled by the Polish Geological Survey, and we discovered an increasing abundance of GSB molecular fossils in rocks extending from the carbonate platform and down the continental slope, suggesting that anoxia had extended out into the wider sea. But when we reached the deep central part of the basin, the fossils were absent. In fact, the sediments contained the fossils of benthic foraminifera, oxygen dependent organisms living at the seafloor, and the sediments had been bioturbated, churned by ancient animals. The green sulfur bacteria and the anoxia were confined to the edge of the basin, completely unlike the Black Sea. This is not the first such observation and this is consistent with new arguments mandating not only a different schematic but also a different trigger. And perhaps that trigger was from outside of the oceans.

If the trigger was not solely a restriction of oxygen supply then the alternative is that it was an excess of organic matter, the degradation of which consumed the limited oxygen. A likely source of that organic matter and one that is consistent with restriction of anoxia to ocean margins is a dramatic increase in nutrients that stimulated algal blooms – much like what is occurring today. And that increase in nutrients, as elegantly summarized by Hugh Jenkyns, could have been caused by an increase in erosion and chemical weathering, driven by higher carbon dioxide concentrations, global warming and/or changes in the hydrological cycle, all of which we now know occurred prior to several OAEs. And again, similar to what is occurring today.

It is likely that today’s coastal dead zones are due not to climate change but to how we use our land and especially to our excess and indiscriminate use of fertilisers, most of which does not help crops grow or enhance our soil quality but is instead washed away to pollute our rivers and coastal seas. And yet that only underscores the lessons of the past. They suggest that global warming might exacerbate the impacts of our poor land management, adding yet another pressure to an already stressed ecosystem.

Runoff of soil and fertiliser during a rain storm. Image source: Wikimedia Commons

The Zechstein Sea study is not the key to this new paradigm (and that ‘paradigm’ is far from settled). There is probably no single study that marked our change in understanding. Instead, this new model has been gradually emerging over nearly 20 years, as long as I have been studying these events. New geochemical data, such as the distribution of nutrient elements, suggest that many of these anoxic episodes, whether local or global, were associated with algal blooms. And other geochemical tools, such as the isotopic composition of trace metals, provide direct evidence for changes in the chemical weathering that liberated the bloom-fueling nutrients.

Science can move in monumental leaps forward but more typically it evolves in small steps. Sometimes, after years of small steps, your understanding has fundamentally changed. And sometimes that change means that your perception of the world, the world you love and on which you depend, has also changed. You realize that it is more dynamic than you thought – as is its vulnerability to human behaviour.
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This was originally posted on the Cabot Institute Blog and is an expanded version of an article on The Conversation.

This blog has also appeared in IFL Science and The Ecologist.