̽��ֱ�� of Cambridge - palaeoclimatology

Why do climate models underestimate polar warming? ‘Invisible clouds’ could be the answer

sc604 — Wed, 08 Nov 2023 16:06:47 +0000

̽��ֱ��Earth’s average surface temperature has increased drastically since the start of the Industrial Revolution, but the warming effect seen at the poles is even more exaggerated. While existing climate models consider the increased heating in the Arctic and Antarctic poles, they often underestimate the warming in these regions. This is especially true for climates millions of years ago, when greenhouse gas concentrations were very high.

This is a problem because future climate projections are generated with these same models: if they do not produce enough warming for the past, we might underestimate polar warming – and therefore the associated risks, such as ice sheet or permafrost melting – for the future.

“During my PhD, I was drawn to the fact that the climate models we are using do not represent the magnitude of warming that happens in the Arctic,” said lead author Dr Deepashree Dutta from Cambridge’s Department of Geography, who carried out the work during her PhD at UNSW. “At the same time, we knew that the majority of these models do not represent the upper layers of the atmosphere very well. And we thought this might be a missing link.”

̽��ֱ��team turned their focus to a key atmospheric element that is missing in most models — polar stratospheric clouds — and found that they can explain a large part of the missing warming in models.

Their results, published in the journal Nature Geoscience, show that there is still much to learn about the climate of the past, present and future.

Climate models are computer simulations of our global climate system that are built using our theoretical understanding of how the climate works. They can be used to recreate past conditions or predict future climate scenarios.

Climate models incorporate many factors that influence the climate, but they cannot include all real-world processes. One consequence of this is that generally, climate models simulate polar climate change that is smaller than actual observations.

“ ̽��ֱ��more detail you include in the model, the more resources they require to run,” said co-author Dr Martin Jucker from UNSW. “It’s often a toss-up between increasing the horizontal or vertical resolution of the model. And as we live down here at the surface of the earth, the detail closer to the surface is often prioritised.”

In 1992, American paleoclimatologist Dr Lisa Sloan first suggested that the extreme warming at high latitudes during past warm periods may have been caused by polar stratospheric clouds.

Polar stratospheric clouds form at very high altitudes (15-25 km above the Earth's surface), and at very low temperatures (over the poles). They are also called nacreous or mother-of-pearl clouds because of their bright and sometimes luminous hues, although they are not normally visible to the naked eye.

These polar stratospheric clouds have a similar effect on climate as greenhouse gases – they trap heat that would otherwise be lost to space and warm the surface of the Earth.

“These clouds form under complex conditions, which most climate models cannot reproduce. And we wondered if this inability to simulate these clouds may result in less surface warming at the poles than what we’ve observed in the real world,” said Dutta.

Thirty years after Sloan’s research, Dutta wanted to test this theory using one of the few atmospheric models that incorporates polar atmospheric clouds, to see if it might explain the disparities in warming between observational data and climate models.

“I wanted to test this theory by running an atmospheric model that includes all necessary processes with conditions that resembled a time period over 50 million years ago, known as the early Eocene. It was a period of Earth’s history when the planet was very hot and the Arctic was ice-free throughout the year,” said Dutta.

̽��ֱ��Eocene was also a period characterised by high methane content, and the position of continents and mountains was different to today.

“Climate models are far too cold in the polar regions, when simulating these past hot climates, and this has been an enigma for the past thirty years,” said Jucker. “ ̽��ֱ��early Eocene was a period in the Earth’s climate with extreme polar warming, so presented the perfect test for our climate models.”

̽��ֱ��team found that the elevated methane levels during the Eocene resulted in an increase in polar stratospheric cloud formation. They found that under certain conditions, the local surface warming due to stratospheric clouds was up to 7 degrees Celsius during the coldest winter months. This temperature difference significantly reduces the gap between climate models and temperature evidence from climate archives.

By comparing future simulations to simulations of the Eocene, the researchers also discovered that it isn’t just methane that was needed to produce polar stratospheric clouds. “This is another key finding of this work,” said Dutta. “It’s not just methane, but it's also the Earth’s continental arrangement, which plays an important role in forming these stratospheric clouds. Because if we input the same amount of methane for our future climate, we do not see the same increase in stratospheric clouds.”

̽��ֱ��research has provided some of the answers to questions about the climate of the deep past, but what does that mean for future projections?

“We found that stratospheric clouds account for the accelerated warming at the poles that is often left out of our climate models, and of course this could potentially mean that our future projections are also not warm enough,” said Jucker. “But the good news is that these clouds are more likely to form under the continental arrangement that was present tens of millions of years ago, and is not found on Earth now. Therefore, we don’t expect such large increases in stratospheric clouds in the future.”

This new research has not only helped to provide a piece of the puzzle as to why temperature recordings in the Arctic are always warmer than climate models – it has also provided new insights into the Earth’s past climate.

“Our study shows the value of increasing the detail of climate models, where we can,” said Dutta. “Although we already know a lot about these clouds theoretically, until we include them in our climate models, we won’t know the full scale of their impact.”

Reference:
Deepashree Dutta et al. ‘Early Eocene low orography and high methane enhance Arctic warming via polar stratospheric clouds.’ Nature Geoscience (2023). DOI: 10.1038/s41561-023-01298-w

Adapted from a UNSW press release.

Stratospheric clouds over the Arctic may explain the differences seen between the polar warming calculated by climate models and actual recordings, according to researchers from the ̽��ֱ�� of Cambridge and UNSW Sydney.

Our study shows the value of increasing the detail of climate models where we can

Deepashree Dutta

Cavan Images / Per-Andre Hoffmann via Getty Images

Mother of pearl clouds (nacreous clouds), Polar Stratospheric Clouds.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Drought encouraged Attila’s Huns to attack the Roman empire, tree rings suggest

ta385 — Thu, 15 Dec 2022 09:00:00 +0000

Hungary has just experienced its driest summer since meteorological measurements began, devastating the country’s usually productive farmland. Archaeologists now suggest that similar conditions in the 5th century may have encouraged animal herders to become raiders, with devastating consequences for the Roman empire.

̽��ֱ��study, published in the Journal of Roman Archaeology, argues that extreme drought spells from the 430s – 450s CE disrupted ways of life in the Danube frontier provinces of the eastern Roman empire, forcing Hunnic peoples to adopt new strategies to ‘buffer against severe economic challenges’.

̽��ֱ��authors, Associate Professor Susanne Hakenbeck from Cambridge’s Department of Archaeology and Professor Ulf Büntgen from the ̽��ֱ��’s Department of Geography, came to their conclusions after assessing a new tree ring-based hydroclimate reconstruction, as well as archaeological and historical evidence.

̽��ֱ��Hunnic incursions into eastern and central Europe in the 4th and 5th centuries CE have long been viewed as the initial crisis that triggered the so-called ‘Great Migrations’ of ‘Barbarian Tribes’, leading to the fall of the Roman empire. But where the Huns came from and what their impact on the late Roman provinces actually was unclear.

New climate data reconstructed from tree rings by Prof Büntgen and colleagues provides information about yearly changes in climate over the last 2000 years. It shows that Hungary experienced episodes of unusually dry summers in the 4th and 5th centuries. Hakenbeck and Büntgen point out that climatic fluctuations, in particular drought spells from 420 to 450 CE, would have reduced crop yields and pasture for animals beyond the floodplains of the Danube and Tisza.

Büntgen said: “Tree ring data gives us an amazing opportunity to link climatic conditions to human activity on a year-by-year basis. We found that periods of drought recorded in biochemical signals in tree-rings coincided with an intensification of raiding activity in the region.”

Recent isotopic analysis of skeletons from the region, including by Dr Hakenbeck, suggests that Hunnic peoples responded to climate stress by migrating and by mixing agricultural and pastoral diets.

Hakenbeck said: “If resource scarcity became too extreme, settled populations may have been forced to move, diversify their subsistence practices and switch between farming and mobile animal herding. These could have been important insurance strategies during a climatic downturn.”

But the study also argues that some Hunnic peoples dramatically changed their social and political organization to become violent raiders.

From herders to raiders

Hunnic attacks on the Roman frontier intensified after Attila came to power in the late 430s. ̽��ֱ��Huns increasingly demanded gold payments and eventually a strip of Roman territory along the Danube. In 451 CE, the Huns invaded Gaul and a year later they invaded northern Italy.

Traditionally, the Huns have been cast as violent barbarians driven by an “infinite thirst for gold”. But, as this study points out, the historical sources documenting these events were primary written by elite Romans who had little direct experience of the peoples and events they described.

“Historical sources tell us that Roman and Hun diplomacy was extremely complex,” Dr Hakenbeck said. “Initially it involved mutually beneficial arrangements, resulting in Hun elites gaining access to vast amounts of gold. This system of collaboration broke down in the 440s, leading to regular raids of Roman lands and increasing demands for gold.”

̽��ֱ��study argues that if current dating of events is correct, the most devastating Hunnic incursions of 447, 451 and 452 CE coincided with extremely dry summers in the Carpathian Basin.

Hakenbeck said: “Climate-induced economic disruption may have required Attila and others of high rank to extract gold from the Roman provinces to keep war bands and maintain inter-elite loyalties. Former horse-riding animal herders appear to have become raiders.”

Historical sources describe the Huns at this time as a highly stratified group with a military organization that was difficult to counter, even for the Roman armies.

̽��ֱ��study suggests that one reason why the Huns attacked the provinces of Thrace and Illyricum in 422, 442, and 447 CE was to acquire food and livestock, rather than gold, but accepts that concrete evidence is needed to confirm this. ̽��ֱ��authors also suggest that Attila demanded a strip of land ‘five days’ journey wide’ along the Danube because this could have offered better grazing in a time of drought.

“Climate alters what environments can provide and this can lead people to make decisions that affect their economy, and their social and political organization," Hakenbeck said. "Such decisions are not straightforwardly rational, nor are their consequences necessarily successful in the long term.”

“This example from history shows that people respond to climate stress in complex and unpredictable ways, and that short-term solutions can have negative consequences in the long term.”

By the 450s CE, just a few decades of their appearance in central Europe, the Huns had disappeared. Attila himself died in 453 CE.

Reference

S.E. Hakenbeck & U. Büntgen, ‘ ̽��ֱ��role of drought during the Hunnic incursions into central-east Europe in the 4th and 5th centuries CE’, Journal of Roman Archaeology (2022). DOI: 10.1017/S1047759422000332

Hunnic peoples migrated westward across Eurasia, switched between farming and herding, and became violent raiders in response to severe drought in the Danube frontier provinces of the Roman empire, a new study argues.

People respond to climate stress in complex and unpredictable ways

Susanne Hakenbeck

Stefan Lefnaer

Devínska Kobyla Forest steppe in Slovakia

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Past climate change pushed birds from the northern hemisphere to the tropics

sc604 — Mon, 10 Jun 2019 19:00:00 +0000

̽��ֱ��researchers, from the Universities of Cambridge and Oxford, applied climate and ecological modelling to illustrate how the distribution of major bird groups is linked to climate change over millions of years. However, while past climate change often occurred slowly enough to allow species to adapt or shift habitats, current rates of climate change may be too fast for many species, putting them at risk of extinction. ̽��ֱ��results are reported in Proceedings of the National Academy of Sciences.

“Palaeontologists have documented long-term links between climate and the geographic distributions of major bird groups, but the computer models needed to quantify this link had not been applied to this question until now,” said Dr Daniel Field from Cambridge’s Department of Earth Sciences, the paper’s co-lead author.

For the current study, the researchers looked at ten bird groups currently limited to the tropics, predominantly in areas that were once part of the ancient supercontinent of Gondwana (Africa, South America and Australasia). However, early fossil representatives of each of these groups have been found on northern continents, well outside their current ranges.

For example, one such group, the turacos (‘banana eaters’) are fruit-eating birds which are only found in the forests and savannahs of sub-Saharan Africa, but fossils of an early turaco relative have been found in modern-day Wyoming, in the northern United States.

Today, Wyoming is much too cold for turacos for most of the year, but during the early Palaeogene period, which began with the extinction of non-avian dinosaurs 66 million years ago, the Earth was much warmer. Over time, global climates have cooled considerably, and the ancestors of modern turacos gradually shifted their range to more suitable areas.

“We modelled the habitable area for each group of birds and found that their estimated habitable ranges in the past were very different from their geographic distributions today, in all cases shifting towards the equator over geological time,” said Dr Erin Saupe from the ̽��ֱ�� of Oxford, the paper’s other lead author.

Saupe, Field and their collaborators mapped information such as average temperature and rainfall and linked it to where each of the bird groups is found today. They used this climatic information to build an ‘ecological niche model’ to map suitable and unsuitable regions for each bird group. They then projected these ecological niche models onto palaeoclimate reconstructions to map potentially-suitable habitats over millions of years.

̽��ֱ��researchers were able to predict the geographic occurrences of fossil representatives of these groups at different points in Earth’s history. These fossils provide direct evidence that these groups were formerly distributed in very different parts of the world to where they are presently found.

“We’ve illustrated the extent to which suitable climate has dictated where these groups of animals were in the past, and where they are now,” said Field. “Depending on the predictions of climate change forecasts, this approach may also allow us to estimate where they might end up in the future.”

“Many of these groups don’t contain a large number of living species, but each lineage represents millions of years of unique evolutionary history,” said Saupe. “In the past, climate change happened slowly enough that groups were able to track suitable habitats as these moved around the globe, but now that climate change is occurring at a much faster rate, it could lead to entire branches of the tree of life going extinct in the near future.”

̽��ֱ��research was funded in part by the Natural Environment Research Council (NERC).

Reference:
Erin Saupe et al. ‘Climatic shifts drove major contractions in avian latitudinal distributions throughout the Cenozoic.’ PNAS (2019). DOI: 10.1073/pnas.1903866116

A bold response to the world’s greatest challenge
̽��ֱ�� ̽��ֱ�� of Cambridge is building on its existing research and launching an ambitious new environment and climate change initiative. Cambridge Zero is not just about developing greener technologies. It will harness the full power of the ̽��ֱ��’s research and policy expertise, developing solutions that work for our lives, our society and our biosphere.

Researchers have shown how millions of years of climate change affected the range and habitat of modern birds, suggesting that many groups of tropical birds may be relatively recent arrivals in their equatorial homes.

Climate has dictated where these groups of animals were in the past, and where they are now

Daniel Field

Daniel J. Field

L-R: Knysna Turaco, Great Blue Turaco, Knysna Turaco

Yes

Changes in ocean 'conveyor belt' predicted abrupt climate changes

sc604 — Wed, 20 Mar 2019 00:00:01 +0000

In the Atlantic Ocean, a giant ‘conveyor belt’ carries warm waters from the tropics into the North Atlantic, where they cool and sink and then return southwards in the deep ocean. This circulation pattern is known as the Atlantic Meridional Overturning Circulation (AMOC) and it’s an important player in the global climate, regulating weather patterns in the Arctic, Europe, and around the world.

Evidence increasingly suggests that this oceanic current system is slowing down, and some scientists fear it could have major effects, such as causing temperatures to dive in Europe and warming the waters off the eastern coast of the USA, potentially harming fisheries and exacerbating hurricanes.

A new study published in Nature Communications provides insight into how quickly these changes could take effect if the ocean current system continues weakening.

An international team of scientists studied one of the key sections of the AMOC –where North Atlantic water sinks from the surface to the bottom of the ocean. They confirmed that changes in the ocean conveyor belt preceded abrupt and major climatic changes during the transition out of the last ice age, referred to as the last deglaciation. ̽��ֱ��study is the first to determine the time lags between past changes to the AMOC and major climate changes.

“Our reconstructions indicate that there are clear climate precursors provided by the ocean state — like warning signs, so to speak,” said lead author Francesco Muschitiello from the ̽��ֱ�� of Cambridge’s Department of Geography, who completed the work while a postdoc at Columbia ̽��ֱ��.

Until now, it has been difficult to resolve whether past changes in the ocean conveyor belt occurred before or after the abrupt climate shifts that punctuated the last deglaciation in the Northern Hemisphere. To overcome the usual challenges, the team pieced together data from a sediment core drilled from the bottom of the Nordic Seas, a lake sediment core from southern Scandinavia, and ice cores from Greenland.

Scientists typically rely on radioactive carbon (carbon 14) dating to determine the ages of sediments. This relationship is tricky in ocean sediments, though, because carbon 14 is created in the atmosphere, and it takes time for the carbon to make its way through the ocean. By the time it reaches the organisms at the bottom of the water column, the carbon 14 could already be hundreds or thousands of years old. So the team needed a different way to date the sediment layers in the marine core.

̽��ֱ��researchers solved this puzzle by measuring carbon 14 levels from a nearby lake sediment core and matching it to the marine core layers. Next, they compared the real age of the marine sediments to the deep ocean carbon 14 measurement, giving them a record of ocean circulation patterns in this region over time. ̽��ֱ��final piece of the puzzle was to analyse ice cores from Greenland, to study changes in temperature and climate over the same time period.

Comparing the data from the three cores revealed that the AMOC weakened in the time leading up to the planet’s last major cold snap around 13,000 years ago. ̽��ֱ��ocean circulation began slowing down about 400 years before the cold snap, but once the climate started changing, temperatures over Greenland plunged quickly by about 6 degrees.

A similar pattern emerged near the end of that cold snap, transitioning out of the ice age; the current started strengthening roughly 400 years before the atmosphere began to heat up dramatically, when Greenland warmed up rapidly — its average temperature climbed by about 8 degrees over just a few decades, causing glaciers to melt and sea ice to drop off considerably in the North Atlantic.

For now it’s not fully clear why there was such a long delay between the AMOC changes and climatic changes over the North Atlantic.

It’s also difficult to pinpoint what these patterns from the past could signify for Earth’s future. Recent evidence suggests that the AMOC began weakening again 150 years ago. However, current conditions are quite different from the last time around, says Muschitiello: the global thermostat was much lower back then, winter sea ice stretched farther south than New York Harbour, and the ocean structure would have been much different. In addition, the past weakening of the AMOC was much more dramatic than today’s trend so far.

“It is clear that there are some precursors in the ocean, so we should be watching the ocean. ̽��ֱ��mere fact that AMOC has been slowing down, that should be a concern based on what we have found,” said Muschitiello.

̽��ֱ��study should also help to improve the physics behind climate models, which generally assume the climate alters abruptly at the same time as AMOC intensity changes. ̽��ֱ��model refinements, in turn, could make climate predictions more accurate. As Svensson puts it: “As long as we do not understand the climate of the past, it is very difficult to constrain the climate models needed to make realistic future scenarios.”

Reference:
Francesco Muschitiello et al. 'Deep-water circulation changes lead North Atlantic climate during deglaciation.' Nature Communications (2019). DOI: 10.1038/s41467-019-09237-3

Adapted from Columbia ̽��ֱ�� story.

A new study is the first to measure the time lags between changing ocean currents and major climate shifts.

There are clear climate precursors provided by the ocean state — like warning signs, so to speak

Francesco Muschitiello

Muschitiello et al

A simplified diagram of the Atlantic Meridional Overturning Circulation

Yes

Sea ice acts as ‘pacemaker’ for abrupt climate change

sc604 — Wed, 06 Mar 2019 19:00:00 +0000

An international study involving researchers from the UK, Norway, Germany Australia, South Korea and the US has confirmed that changes in sea ice cover in the Norwegian Sea played a key role in driving abrupt climate change events between 32,000 and 40,000 years ago, where global temperatures shifted as much as 15 degrees Celsius.

̽��ֱ��results, reported in the journal Science Advances, indicate that initial sea ice reduction started before the abrupt warming over Greenland, and that sea ice expansion started before the end of the warm periods in Greenland.

̽��ֱ��Arctic sea ice is a key element of the global climate system and the strong ongoing warming of the Arctic Ocean can have major impacts on the stability of the Greenland Ice Sheet, first and foremost accelerated sea level rise.

̽��ֱ��Nordic Sea system and its water column structure during the last glacial cycle is the closest analogue to the present-day Arctic Ocean, which makes it a perfect natural laboratory to understand the role of rapid disappearance of regional sea ice on abrupt warming on the Greenland Ice Sheet.

̽��ֱ��last glacial period, 10,000–100,000 years ago, was marked by repeated abrupt climate changes with global implications. Within a matter of decades, temperature shifts of as much as 15 degrees Celsius occurred around Greenland, but the mechanisms driving these changes –known as Dansgaard-Oeschger events – are not fully understood.

“One of the most challenging aspects of palaeoclimatology is to precisely resolve and reconstruct the exact timing of the events that took place across the major climate transitions of the last glacial cycle,” said co-author Dr Francesco Muschitiello, from the ̽��ֱ�� of Cambridge’s Department of Geography. “As a result, a detailed account of the temporal relationship between changes in sea-ice extent in the Nordic Seas, and North Atlantic ocean circulation and climate across Dansgaard-Oeschger events has been so far elusive.”

̽��ֱ��researchers investigated specific organic molecules in a sediment core from the southern Norwegian Sea, one of which was produced by algae that live in sea ice and others that were produced by organisms living in open, ice-free waters thousands of years ago.

̽��ֱ��new sea ice reconstruction based on organic molecules in marine sediments was also evaluated by means of results from a model simulation of past sea ice conditions.

“Our data suggest that there were substantial changes in the sea ice cover in the southern Norwegian Sea between 32,000 and 40,000 years ago,” said Henrik Sadatzki from the ̽��ֱ�� of Bergen and the paper’s first author. “Most extensive sea ice conditions occurred at the onsets and early parts of cold periods over Greenland and the most pronounced open-ocean conditions occurred at the onsets of the abrupt changes to warm periods over Greenland.”

̽��ֱ��results support that an enhanced sea ice cover contributed to insulation of the cold, high-latitude atmosphere from relatively warmer waters that were present in the Norwegian Sea beneath the sea ice lid.

In turn, sea ice reduction allowed for heat release from the exposed Norwegian Sea waters to the atmosphere, which was a prime ingredient in shaping the abrupt warming of the Dansgaard-Oeschger climate events in Greenland.

̽��ֱ��Dansgaard–Oeschger climate events have stirred interest in documenting that the climate system contains mechanisms that may lead to abrupt and surprising climate changes.

These findings clarify the series of events taking place in the high-latitude North Atlantic across the abrupt Dansgaard–Oeschger cycles of the last glacial period. However, further work is needed to ultimately identify the physical mechanisms linking the current sudden demise of Arctic sea ice to abrupt Greenland Ice Sheet changes.

̽��ֱ��research was funded in part by the European Research Council.

Reference:
Henrik Sadatzki et al. ‘Sea ice variability in the southern Norwegian Sea during glacial Dansgaard-Oeschger climate cycles.’ Science Advances (2019). DOI: 10.1126/sciadv.aau6174

Adapted from a ̽��ֱ�� of Bergen press release.

Substantial variations in past sea ice cover in the Norwegian Sea were instrumental for several abrupt climate changes in large parts of the world, researchers have found.

United Nations Photo

Icebergs in Ilulissat Icefjord, Greenland

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Antarctic Ice Sheet study reveals 8,000-year record of climate change

sjr81 — Mon, 12 Dec 2016 16:02:17 +0000

Results of the study, co-authored by Michael Weber, a paleoclimatologist and visiting scientist at the ̽��ֱ�� of Cambridge, along with colleagues from the USA, New Zealand and Germany, are published this week in the journal Nature.

Global climate models that look at the last several thousand years have failed to account for the amount of climate variability captured in the paleoclimate record, according to lead author Pepijn Bakker, a climate modeller from the MARUM Center for Marine Environmental Studies at the ̽��ֱ�� of Bremen in Germany.

̽��ֱ��researchers first turned their attention to the Scotia Sea. “Most icebergs calving off the Antarctic Ice Sheet travel through this region because of the atmospheric and oceanic circulation,” explained Weber. “ ̽��ֱ��icebergs contain gravel that drop into the sediment on the ocean floor – and analysis and dating of such deposits shows that for the last 8,000 years, there were centuries with more gravel and those with less.”

̽��ֱ��research team’s hypothesis is that climate modellers have historically overlooked one crucial element in the overall climate system. They discovered that the centuries-long phases of enhanced and reduced Antarctic ice mass loss documented over the past 8,000 years have had a cascading effect on the entire climate system.

Using sophisticated computer modelling, the researchers traced the variability in iceberg calving (ice that breaks away from glaciers) to small changes in ocean temperatures.

“There is a natural variability in the deeper part of the ocean adjacent to the Antarctic Ice Sheet that causes small but significant changes in temperatures,” said co-author Andreas Schmittner, a climate modeller from Oregon State ̽��ֱ��. “When the ocean temperatures warm, it causes more direct melting of the ice sheet below the surface, and it increases the number of icebergs that calve off the ice sheet.”

Those two factors combine to provide an influx of fresh water into the Southern Ocean during these warm regimes, according to Peter Clark, a paleoclimatologist from Oregon State ̽��ֱ��, and co-author on the study.

“ ̽��ֱ��introduction of that cold, fresh water lessens the salinity and cools the surface temperatures, at the same time, stratifying the layers of water,” he said. “ ̽��ֱ��cold, fresh water freezes more easily, creating additional sea ice despite warmer temperatures that are down hundreds of meters below the surface.”

̽��ֱ��discovery may help explain why sea ice is currently expanding in the Southern Ocean despite global warming, the researchers say.

“This response is well-known, but what is less-known is that the input of fresh water also leads to changes far away in the northern hemisphere, because it disrupts part of the global ocean circulation,” explained Nick Golledge from the ̽��ֱ�� of Wellington, New Zealand, an ice-sheet modeller and study co-author. “Meltwater from the Antarctic won’t just raise global sea level, but might also amplify climate changes around the world. Some parts of the North Atlantic may end up with warmer temperatures as a consequence of part of Antarctica melting.”

Golledge used a computer model to simulate how the Antarctic Ice Sheet changed as it came out of the last ice age and into the present, warm period.

" ̽��ֱ��integration of data and models provides further evidence that the Antarctic Ice Sheet has experienced much greater natural variability in the past than previously anticipated,” added Weber. “We should therefore be concerned that it will possibly act very dynamically in the future, too, specifically when it comes to projecting future sea-level rise.”

Two years ago Weber led another study, also published in Nature, which found that the Antarctic Ice Sheet collapsed repeatedly and abruptly at the end of the Last Ice Age to 19,000 to 9,000 years ago.

An international team of researchers has found that the Antarctic Ice Sheet plays a major role in regional and global climate variability – a discovery that may also help explain why sea ice in the Southern Hemisphere has been increasing despite the warming of the rest of the Earth.

̽��ֱ��Antarctic Ice Sheet has experienced much greater natural variability in the past than previously anticipated.

Michael Weber

Iceberg in the Weddell Sea

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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Taking the long view on climate change

bjb42 — Sat, 01 Sep 2007 15:53:42 +0000

̽��ֱ��Earth’s climate has always changed and no doubt always will. However, this alone does not tell us very much about the climate system: we need to be able to say exactly how and why climate can change. Today this requirement has been brought to the fore by the prospect of human-induced global climate change resulting largely from greenhouse gas emissions that arise from our massive and growing appetite for fossil fuels. Thousands of scientists are now striving to predict how a sharp rise in greenhouse gas concentrations will affect the entire climate system, including the ecosystems and societies that it supports. But how can we be sure about our theories of climate change, let alone our theories of ecosystem or market response? Just how important are greenhouse gases in controlling global climate? And what are the timescales and thresholds of climate adjustment? These are just some of the urgent questions that have been raised by the prospect of anthropogenic climate change.

To help answer such questions we can look to the past, at how the Earth’s climate evolved prior to the relative stability that human society has so far enjoyed. Researchers in the Department of Earth Sciences are taking up this challenge, using marine sediments as their lens into the past, and as a guide to the future.

Palaeoclimatology

̽��ֱ��study of past climate change – palaeoclimatology – aims to reconstruct what has happened in the past, in the oceans, on the land, in the atmosphere and in ecosystems, and to infer how the global climate system works ‘as a whole’. In the last 20 years of palaeoclimate research, three major questions have emerged that are particularly relevant to modern climate change. First, how did changes in solar radiation (insolation) and atmospheric carbon dioxide (CO2) conspire to trigger massive global climate upheavals such as the glacial–interglacial (‘ice-age’) climate cycles? Second, what regulates atmospheric CO2 concentrations under changing climatic conditions, and what roles can we ascribe to marine biological productivity or ocean circulation changes in particular? And third, how abruptly can regional climate change and with what repercussions for the rest of the world?

All of these questions are interconnected of course, although each bears on a different aspect of the climate system’s ability to pace and amplify climate perturbations through sensitive ‘feedback’ processes.

Past climate by proxy

A central aspect of palaeoclimate reconstructions is the ‘proxy’ character of our observations. Because scientists cannot measure past ocean temperatures directly, they must measure the impacts of past temperature changes instead, usually based on temperature-sensitive organisms or temperature-sensitive chemical constituents in their shells or skeletons.

As a palaeoceanographer, Dr Luke Skinner specifically makes use of marine sediments as a window into the past. Among the many advantages of using marine sediments are that they can be obtained from nearly two thirds of the Earth’s surface, they generally provide unbroken and often very high-resolution records of past conditions, and they contain a diversity of constituents that can be analysed, from tiny fossil shells to grains of sand dropped by passing icebergs.

To reconstruct past climate change, Dr Skinner collects and studies the fossil calcite shells of foraminifera – single-celled blobs of protoplasm – that have accumulated on the sea floor. Using the shells of these tiny creatures, Dr Skinner has been able to generate detailed records of temperature change, both at the sea surface and in the ocean interior. In combination with ice-rafted debris and oxygen- and carbon-isotope records, these reconstructions have helped to demonstrate that the North Atlantic region experienced very intense and abrupt climate swings in the past, involving massive glacier surges as well as drastic changes in the deep ocean circulation system and the Gulf Stream. It has also been possible to show that these same changes in the Atlantic Ocean’s circulation were accompanied by a ‘see-saw’ in temperatures across the hemispheres, with heat pooling in the South to the extent that it was not efficiently delivered to the North. Based on records such as these it is now clear that global change can be heterogeneous and can occur too suddenly to be presaged by obvious warnings.

Perspectives on the future

Although it is clear that no previous climate period can really serve as a blueprint for the future, important lessons can still be learned from the study of the past. One important example is the use of palaeoclimate data to guide the improvement of our climate simulation models. Because numerical and statistical models provide our only means for predicting future climate, it is imperative that they be as general as possible. Studies like those described here are helping to achieve this, by revealing the feedbacks, thresholds and characteristic timescales for climate adjustment, across a wide range of climatic contexts.

In the future, global CO2 levels will only be stabilised if we either drastically cut our emissions or identify, trigger or create a process that ‘mops up’ exactly as much CO2 as millions of consumers are able to produce each day (the basis of carbon capture). ̽��ֱ��history of climate change tells us that we are going to need as many one-way fluxes out of the atmosphere as we can muster if are going to compete with the ‘leak’ we have created in the Earth’s largest standing carbon reservoir, the solid Earth. We have much to learn about the climate system, both for our own sake and for the sake of knowledge itself.

For more information, please contact the author Dr Luke Skinner (luke00@esc.cam.ac.uk) at the Department of Earth Sciences.

Cambridge Earth Scientists are contributing to our understanding of the climate system by studying the history of climate change recorded in sediments deposited on the sea floor.

Studies like those described here are helping to achieve this, by revealing the feedbacks, thresholds and characteristic timescales for climate adjustment, across a wide range of climatic contexts.

Irargerich from Flickr

Melting is your destiny

Carbon capture

Whenever fossil fuel (coal, oil or gas) is burnt, carbon is released as CO2 into the atmosphere, where it traps the Sun's heat. Can we counteract this build-up by capturing and storing CO2? Any solution would require storage of many millions of tonnes reliably and possibly for up to 10,000 years. Compressing and injecting CO2 into deep geological formations could provide the answer. ̽��ֱ��presence of oil, gas and natural CO2 trapped in reservoirs underground for millions of years demonstrates that storage of CO2 is feasible. At the Sleipner Oil Field in the Norwegian sector of the North Sea, CO2 is already being separated from natural gas and re-injected at about 1 km depth below the sea surface. ̽��ֱ��CO2 rises through the sandy earth before spreading out below a series of thin mudstones beneath the thick overlying mudstone. A collaborative research project between Professor Mike Bickle in the Department of Earth Sciences and Professor Herbert Huppert in the Institute of Theoretical Geophysics has been modelling the spread of these accumulations to work out how much CO2 is trapped and to understand the flow of CO2 in the reservoir. A particular challenge is to predict the behaviour of the stored CO2 over time to determine the safety of long-term CO2 storage in this way. ̽��ֱ��benefits are clear, as Professor Bickle explains: 'CO2 storage is a feasible, politically achievable and relatively inexpensive way for dealing with the problem of increasing atmospheric CO2 levels. For more information, please contact Professor Mike Bickle (mb72@esc.cam.ac.uk) at the Department of Earth Sciences.

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