Pliocene – Cabot Institute for the Environment blog

Back to the Future ‘Hothouse’

Posted on August 16, 2018April 4, 2023 by janet.crompton

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.

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 MegTM).

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.

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This blog is written by Cabot Institute member Professor Rich Pancost, Head of Earth Sciences at the University of Bristol. This blog has been reposted with kind permission from Rich’s original blog.

Rich Pancost

Thoughts on passing 400 ppm

Posted on November 12, 2015April 12, 2023 by janet.crompton

In the next few days, the Mauna Loa atmospheric CO2 record will pass 400 ppm. This isn’t the first time that’s happened – we first crossed the 400 ppm threshold in May 2013, but the annual, saw-tooth variation in levels as the Northern hemisphere boreal forest breathes in and out has dipped us below 400 a couple of times since. This crossing is likely to be special however, as it is probably going to be the last time anybody alive today will experience an atmosphere with LESS than 400 ppm CO2.

Human emissions have been pushing up atmospheric levels by about 2.2 ppm every year in recent years, so normally we would expect the annual monthly minimum to increase to beyond 400 ppm from this year’s September minimum of 397.1 ppm, however we are in the midst of one of the largest El Nino years for over a decade, and the drought in the tropics during El Nino years slow the growth of trees relative to normal years, and increases fires. Previous strong El Nino years (like 1997) have helped to push the annual CO2 increase to a massive 3.7 ppm, and this year’s strong El Nino, coupled with increased forest burning in Indonesia, along with fossil fuel burning, have led Ralph Keeling to predict the annual rise could be as much as 4.4 ppm this year.

So why does it matter? 400 is in truth a fairly arbitrary value to get excited about, a neat quirk of our counting system and no more important as a value to the atmosphere than your car odometer ticking from 99,999 to 100,000. It doesn’t mean the car is going to collapse, but it certainly catches your attention. It’s the same with the atmosphere – it gives us pause to consider what we’ve done, and what it might mean for the climate system. For me, the most outrageous thing is that we, an insignificant population of carbon based life forms, have managed to alter the chemical composition of the atmosphere! And not just by a little – by a lot! And let’s not forget that the atmosphere is big – really big!

To me, as an Earth Scientist that leads me to think about when in Earth history the planet has experienced such high levels of CO2 before. Measuring atmospheric CO2 in the geological past is tricky – for the past ~800 thousand years we have a fantastic archive of trapped atmospheric gas bubbles in ice cores, and for the whole of that record CO2 never peaked above 300 ppm. Beyond the time for which we have the ice cores, we rely on geochemical proxies in marine and terrestrial sediments to estimate CO2 and that is the heart of my research. In a paper we published last year we showed that we have to go back to more than 2.3 Million years ago, to the very earliest Pleistocene and Pliocene to find atmospheric CO2 levels as high as we are about to permanently experience. What does that mean? Well the Pliocene was a similar world to today – the continents were in much the same place, the vegetation mix across this Earth was the same, except global temperatures were 2-3 degrees C higher than now, driven primarily by those high levels of CO2.

Another thing that strikes me today is how rapidly we’ve managed to change the atmosphere. In a little over 150 years since we started to burn fossil fuels with alacrity, we’ve gone from 280 ppm to 400 ppm. It’s hard to find geological records with the temporal precision to see changes that quick, but for sure we don’t know any time in Earth history when CO2 has changed so much, so quickly.

With COP21 in Paris just around the corner, perhaps saying goodbye to sub 400 ppm will focus minds to come up with a solution. I don’t know whether it will, or what a global solution would look like, but I hope beyond anything that we don’t do nothing.
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Cabot Institute member Dr Marcus Badger is a Research Associate in the Organic Geochemistry Group in the School of Chemistry. His research involves using biomolecules and climate models to better understand the Earth system.

Climate lessons from the past: Are we already committed to a warmer and wetter planet?

Posted on February 17, 2014April 19, 2023 by janet.crompton

Last September, the Cabot Institute and the University of Bristol hosted the 2nd International Workshop on Pliocene Climate. Following on from that, we have just released a short video describing what the Pliocene is and its relevance for understanding climate change.

The Pliocene is a geological time interval that occurred from 5.3 to 2.6 million years ago. This interval of Earth history is interesting for many reasons, but one of the most profound is that the Earth’s atmosphere apparently contained elevated concentrations of carbon dioxide – in fact, our best estimates suggest concentrations were about 300 to 400 ppm, which is much higher than concentrations of 100 years ago but lower than those of today after a century of intensive fossil fuel combustion.

Image by NASA

Consequently, the Pliocene could provide valuable insight into the type of planet we are creating via global warming. Our video release happens to coincide with pronounced flooding across the UK and focussed attention on our weather and climate. There is little doubt that increased carbon dioxide concentrations will cause global warming; instead, the key questions are: how much warming will there be and what are the consequences of that warming? One way to study that is to examine previous intervals of Earth history also characterised by high carbon dioxide concentrations. The comparisons are not perfect, of course; for example, during the Pliocene the continents were in roughly but not exactly the same positions that they are in today. But it can serve as another piece of the puzzle in predicting future climate.

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, an interval with detailed temperature reconstructions and carbon dioxide records from ice core gas bubbles. During that time, and through multiple ice ages, climate sensitivity was about 2.5 to 3°C warming for a doubling of carbon dioxide, which is in the middle of the model-based range of predictions.

Ice core sampling.
Image by NASA ICE (Ice Core Vitals) [CC-BY-2.0]
Wikimedia Commons

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. 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 in the absence of ice cores is admittedly more challenging. Instead of directly measuring the concentration of carbon dioxide in gas bubbles, we must rely on indirect records – proxies. For example, carbon dioxide concentration influences the number of stomata on plant leaves, and this can be measured on ancient leaf fossils. Alternatively, there are a number of geochemical tools based on how carbon dioxide impacts the pH of seawater or how algae assimilate carbon dioxide during photosynthesis; these are recorded by the chemical composition of ancient fossils.

These estimates come with larger error bars, but they provide key insights into climate sensitivity on a warmer Earth. Recent research indicates a convergence of Pliocene carbon dioxide estimates from these various proxies and gives us more confidence in deriving climate sensitivity estimates. In particular, it appears that an increase of carbon dioxide from about 280 parts per million (the modern value before the industrial revolution) to about 400 parts per million in the Pliocene results in a 2°C warmer Earth. Accounting for other controls, 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.

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 deeper into the past. The climate change we are inducing is a problem for humans and society, not our planet.

However, the Pliocene was a rather different world. For example – and importantly, given current events in the UK – these higher global temperatures were associated with a climate that was also wetter* than present. That provides important corroborating evidence for models that predict a warmer and wetter future.

Image by w:en:User:Ivan and licensed as GFDL

Perhaps most striking, sea level 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 and will be exacerbated by sea level rise of 12 centimetres over the last 100 years and a further 5 to 7 centimetres 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 such hysteresis does stabilise our current ice sheets, then we should not assume a planet with 400 ppm of carbon dioxide will necessarily have sea level 20 metres higher than that of today. But if hysteresis is rather weak, then the question is not whether we will see massive sea level change but rather how long it will take (Note: It is likely to take centuries or millennia!).

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

* See Brigham-Grette, J., Melles, M., Minyuk, P., Andreev, A., Tarasov, P., DeConto, R., Koenig, S., et al., 2013. Pliocene Warmth, Polar Amplification, and Stepped Pleistocene Cooling Recorded in NE Arctic Russia. Science 340 (6139), 1421-1427. doi: 10.1126/science.1233137 and Salzmann, U., Haywood, A.M., Lunt, D.J., 2009. The past is a guide to the future? Comparing Middle Pliocene vegetation with predicted biome distributions for the twenty-first century. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 367 (1886), 189-204.

This blog is by Prof Rich Pancost, Director of the Cabot Institute. Rich will be giving a public lecture on how biogeochemical cycles have regulated the global climate system throughout Earth’s history on 25 February in Bristol. The event is free and open to all, do come along to learn more.

To learn more about the Pliocene – and palaeoclimate research, in general – you can watch Professor Gerald Haug’s public lecture, Climate and Societies, recorded at the Cabot Institute as part of the 2nd International Workshop on Pliocene Climate.

Prof Rich Pancost

AGU 2013: The importance of 400 ppm CO2

Posted on February 11, 2014April 19, 2023 by janet.crompton

On 1 June 2012, a concentration of 400 ppm carbon dioxide was measured in air samples in the Arctic. On 9 May 2013, Mauna Loa, the longest recording station, measured a daily average of 400 ppm carbon dioxide. Next year we may see the global average concentration reach 400 ppm and the year after that 400 ppm could be measured at the South Pole. The 400 ppm number is arbitrary, but it is a symbol of the anthropogenic climate change that scientists have been talking about for many years.

Here at the University of Bristol, the upcoming 400 ppm epoch prompted the question of what do we know about 400 ppm CO2 climates and how could it be used to galvanize action on climate change? But 400 ppm and climate change is a bigger issue than one University can take on, so we took our idea to the American Geosciences Union Fall conference. With more than 24,000 attendees each year, AGU is the perfect place to talk about what 400 ppm CO2 means in a scientific sense and what we as scientists should do about communicating it.

Two sessions were proposed: one looking at the science of 400 ppm CO2 climates, co-convened with Kerim Nisanciouglu of the University of Bergen, Norway, the other at communicating 400 ppm co-convened with Kent Peacock of University of Lethbridge and Casey Brown of UMass Amherst.

Naomi Oreskes (pictured) asked why scientists
don’t allow themselves to sound alarmed when reporting alarming conclusions from their
research.

The communication session looked at how climate science could be communicated effectively. First to speak was Naomi Oreskes, who asked why scientists don’t allow ourselves to sound alarmed when we’re reporting alarming conclusions. Citing from neuroscience research, Oreskes argued that when scientists conform to the ‘unemotional scientist’ paradigm they actually risk being less rational and sounding inauthentic. It was clear that Oreskes’ points struck the audience, as many of them queued up to ask questions.

Myles Allen made a compelling case for sequestered adequate fraction of extracted (SAFE) carbon – i.e. compulsory carbon capture and storage. Allen pointed out that people will always pay to burn carbon and argued that a carbon price is just a way to ‘cook the planet slower’. Robert Lempert took a less controversial stand and explained how uncertainty can be managed in robust decision making. Using hydrological examples, Lempert suggested that by starting with the desired outcome and working backwards, uncertainty can be dealt with. The session finished with James Hansen, talking about the right message, and how the things that people care about needs to be communicated by the best communicators. Criticising the pursuit of unconventional fossil fuels, Hansen argued the need for a carbon tax which was redistributed back to people. A lively question and answer session followed, with all the speakers engaging in a strong discussion and the audience contributing pointed questions. No problems with talking without emotion in this session!

The 400 ppm physical science session started by focussing on what information we could draw from climates in the past where CO2 is believed to have been ~400 ppm. The first speaker was Alan Haywood who summarised the work of the PlioMIP project which tries to understand the climate of the Pliocene (~3 million years ago) – what it was like and why. The Pliocene is the most recent time period in the past when atmospheric CO2 concentrations could have been as high as they are today. Two more Pliocene presentations followed. First, Natalie Burls (standing in for Chris Brierley) explained that even with CO2 set to 400 ppm in their climate model simulations they could not match the warm temperatures reconstructed by Pliocene data – suggesting that either the climate models are not sensitive enough to CO2 or that there are other dynamical processes that we do not fully understand yet. Thomas Chalk gave a comparison between different methods for reconstructing CO2 in the past, and concluded that the Pliocene concentration was indeed at around 400 ppm. The final talk in the palaeoclimate part of the session was given by Dana Royer who presented the most compelling evidence for very different climates in the past with polar forests at 80°N indicating annual mean temperatures in the Arctic that were 30°C warmer than they are today! Dana presented new CO2 reconstructions demonstrating that the CO2 concentration at the time of the polar forests could have been around 400 ppm, again suggesting that our climate models may not be sensitive enough to CO2.

The next part of the session looked at current CO2 levels with a presentation by Steven Davis about the amount of CO2 that we have already committed to putting into the atmosphere. The energy infrastructure that we have already built amounts to future CO2 emissions of 318Gt, and new global commitments are still increasing. Vaughan Pratt followed with a talk about the reasons for the recent pause in the global warming trend, separating out natural causes and anthropogenic causes using mathematical and statistical analyses. He concludes that the recent pause is of natural origin.

The final part of the session peered through the looking glass into the future. Andrew Friedman investigates the causes of the temperature asymmetry between the northern hemisphere and the southern hemisphere and how that asymmetry may alter under the future climate emission scenarios. He concluded that the asymmetry is set to increase into the next century, with the northern hemisphere warming faster than the southern hemisphere and projects that the tropical rainbelt will shift northwards as a result.

Kirsten Zickfield has found that warming in the next
millenium might amount to 1 degree globally,
concentrated at the Poles. Sea levels are projected to
rise by 0.8m.

The final talk of the session was given by Kirsten Zickfeld who examined the climate changes we might already be committed to as a result of the CO2 emissions we have already released (under the assumption that atmospheric CO2 stays at 400 ppm). She used a climate model with biogeochemical components to identify how long it would take for the climate to reach equilibrium with the present CO2 concentration of 400 ppm, what the climatic impacts of that equilibrium might be and whether it might be possible to return to CO2 levels below 400 ppm on human timescales by using negative emissions (carbon capture/storage schemes). She found that the already committed warming into the next millennium might amount to 1°C globally, concentrated at the poles. Sea levels are projected to rise by 0.8m due to thermal expansion alone and further increases of 10m due to ice melt are possible over much longer timescales. Committed changes for the ‘other CO2 problem’ – ocean acidification – are relatively small, with a pH drop of only 0.01 projected. She concludes that even if CO2 levels could drop below 400 ppm in the future, whilst air temperatures may stabilise, sea level may continue to rise due to thermal expansion alone.

Both of the sessions were recorded for access after the event and provoked a lot of debate, during the sessions and online. We hope that in some small way these sessions have helped scientists think differently about what 400 ppm means and what we can do about it.

This blog was written by T Davies-Barnard and Catherine Bradshaw, Geographical Sciences, University of Bristol.

Why the Pliocene period is important in the upcoming IPCC report

Posted on September 16, 2013April 20, 2023 by janet.crompton

Critical to our understanding of the Earth system, especially in order to predict future anthropogenic climate change, is a full comprehension of how the Earth reacts to higher atmospheric CO2 conditions. One of the best ways to look at what the Earth was like under higher CO2 is to look at times in Earth history when atmospheric CO2 was naturally higher than it is today. The perfect period of geological history is the Pliocene, which spans from 5.3 – 2.6 million years ago. During this time we have good evidence that the Earth was 2-3 degrees warmer than today, but other things, such as the position of the continents and the distribution of plants over the surface, was very similar to today.

There is therefore a significant community of oceanographers and climate modellers studying the Pliocene, many of whom were in Bristol last week for the 2nd Workshop on Pliocene climate, and one of the main points of discussion was the exact value of CO2 for the Pliocene.

80 top scientists from 12 countries gathered for the 2nd Workshop on Pliocene climate on 9-10 September 2013 at the University of Bristol

The imminent release of the first volume of the 5th assessments of the IPCC is also expected to include sections on Pliocene climate.

Today we published a paper in Philosophical Transactions of the Royal Society A which therefore represents an important contribution to the debate. Several records of Pliocene CO2 do exist, but their low temporal resolution makes interpretation difficult. There has also been some controversy about what these records mean, as some show surprisingly high variability, given what we understand about Pliocene climate.

We sampled a deep ocean core taken by the Ocean Drilling Program in the Carribean Sea. Cores such as this record the ancient envrionment as sediment collects over time like the progressive pages in a book, and by analysing the chemical composition of the layers a history of the Earth System can be discovered. The approach that Badger et al take is to use the carbon isotopic fractionation of photosynthetic algae, which has been shown to vary with atmospheric CO2.

What this study revealed is that atmospheric CO2 was actually quite low, at around 300 ppm for much of the warm period. What was also revealed was that CO2 was relatively stable, in contrast to previous work. This implies that in the Pliocene the Earth must have been quite sensitive to CO2, as small changes in atmospheric CO2 drove changes in climate. The study of Badger et al doesn’t explicitly reconstruct climate sensitivity but it does have important implications for future change.

The paper is published in a special volume of Philosophical Transactions of the Royal Society A, edited by Bristol scientists Dan Lunt, Rich Pancost, Andy Ridgewell and Harry Elderfield of Cambridge University. The volume is the result of the Warm Climates of the Past – A lesson for the Future? meeting which took place at the Royal Society in October 2011. The volume can be accessed here: http://bit.ly/PTA2001

This blog is written by Dr Marcus Badger, School of Chemistry, University of Bristol.

Marcus Badger