Alternate histories back unique modern warmth claims

Tree rings have a light-colored band, or earlywood, that forms in the spring and a dark-colored band, or latewood, that forms in the summer. The width of the band tells how much the tree grew during that period and therefore can be used as a proxy for the climate during that season. That approach has some uncertainties, but Martin Tingley and Peter Huybers have reduced their impact on telling if any year is the warmest. Credit: thaths via Flickr Creative Commons license

If you build a temperature record going back in time to judge modern warming against, how certain can you be of your answer? That’s a big question for scientists making such records from effects temperatures have had on the natural world. And figuring out if today’s heat is unique is too great a challenge for the methods scientists normally use to calculate uncertainty, according to Harvard University’s Martin Tingley.

But Martin and Peter Huybers have shown the precise chances that northern areas of the world are warmer than any time in rebuilt records reaching back to the year 1400. They have worked out that there’s less than one chance in 20 that 2005, 2007, 2010 and 2011’s northern summers weren’t the warmest in that time. They also find that summer 2010 has a 99% chance of being the warmest western Russia has seen. There have already been lots of claims made over the unusualness of recent warmth, Martin pointed out, but his and Peter’s are the most robust yet. “We put these estimates on a much sounder statistical footing,” he told me.

Saying one year’s summer is uniquely warm across a long period is difficult for subtle reasons that Martin explained through his height. “I’m a tall guy, 6 foot 4 inches,” he said. “I’ve never met you, but I’m going to bet I’m taller than you. What’s the intuition behind my bet? We have a sense of the distribution of heights. I’m aware I fall pretty far out on the tail, so the chances are if I meet an average person they don’t fall further out than I do. What if I’m in a room with 1,000 people I’ve never met before? Am I still likely to be the tallest in the room? Probably not.”

Temperature patterns produce perplexing Pliocene puzzle

Lafayette College’s Kira Lawrence and her teammates have used ocean bed sediment cores, like this one, to produce a 5 million year climate record. © Intergrated Ocean Drilling Program

US, UK and Hong Kong Researchers have produce a unique ‘movie’ of climate reaching back 5 million years, by bringing together data drilled from ocean beds. It reveals three important temperature patterns during the warm early part of the Pliocene period that they couldn’t recreate together in climate models using existing explanations. That’s important because scientists hope the Pliocene could help us know what the future of a warmer Earth might be like. And having uncovered another layer to the Pliocene puzzle, team member Kira Lawrence from Lafayette College in Easton, Pennsylvania, underlined the value of finding its solution.

“Our community of scientists think of the Pliocene as though it was about 3°C warmer than modern temperatures with CO2 concentration about where we are right now,” Kira told me. “But we haven’t recognised before that the pattern of temperature was a lot different. If that’s where we’re headed in the not too distant future, if the temperature and precipitation patterns change in that way, we should have some significant things to think about.”

The Pliocene period started 5.3 million years ago, during which primates made important evolutionary steps towards humanity. Since 2000, there has been a climate data explosion reaching back through this era. Around the world, international drilling expeditions have pierced ocean beds kilometres below sea level, reaching hundreds of metres into sediment to bring back ‘core’ samples. Tiny fossils within that rock and mud can tell scientists temperatures through history, which can give climate scientists real data to test their models against.

Projected warming set to exceed civilisation’s experience

Oregon State University’s Shaun Marcott has built a climate record reaching back 11,300 years, showing that today’s temperatures are warmer than at least 70% of that period. Credit: Shaun Marcott

The world is headed for average surface temperatures warmer than it has seen in at least 11,300 years. That’s one conclusion US researchers have reached after bringing together 73 studies of ancient climate from across the world into a single global record. Their work supports previous records for the past 2,000 years built mainly from tree ring data, explained Shaun Marcott from Oregon State University, and gives a much broader view.

“We can put today’s global temperature into context against the entire Holocene period,” Shaun told me. “That’s when human civilisation was born, developed and progressed to today.” Modern temperatures are higher than in around three-quarters of that period, which reaches back to the end of the last ice age. And their comparison against forecasts for 2100 made in models used by the Intergovernmental Panel on Climate Change (IPCC) is even starker. “If those scenarios come to fruition, we’ll be well outside anything human civilisation has seen,” Shaun warned. “We won’t have even have been close.”

Knowing climate’s history helps understand its present, and so researchers have puzzled out temperatures on the Earth’s surface from proxy, or indirect, records for the last 2,000 years. In particular, bringing together measurements from tree rings, ice and coral has showed a sharp recent temperature rise often referred to as the ‘hockey stick’. Meanwhile, studies scattered across the world had reached back across the 11,300 years since the beginning of the Holocene. But they can be influenced by regional effects, and no one had pieced them into a global view that would overcome that.

Evidence rethink puts CO2 and ancient warming back in sync

A thin layer of ice from an area of the Antarctic where ancient ice records are collected, in polarized light that reveals ice crystals. Rethinking how ice crystal formation affects ancient data collection is helping to solve an outstanding climate puzzle. © Frédéric Parrenin

A different way to dig up links between past levels of CO2 in the air and temperatures could solve a troubling question over the historical climate. Previously, data collected from long cylinders drilled from Antarctica’s ice sheet seemed to show temperatures rising hundreds of years before CO2 levels did. If ancient warming came before a CO2 rise, then the greenhouse gas seemingly couldn’t have caused the warming. Climate skeptics have used this to argue  that the CO2 we produce today isn’t causing global warming.

Now, Frédéric Parrenin at the French National Centre for Scientific Research in Grenoble and his teammates have used a different method on these cylindrical ice cores. They say that their approach shows CO2 and temperature rises happened together during the last ‘deglaciation’, when ice sheets retreated during an abrupt warming period 20,000-10,000 years ago. “This makes it possible that CO2 was actually a cause of warming corresponding to the last deglaciation,” Frédéric told me.

Scientists have been using Antarctic ice cores, and bubbles of air from the time the ice formed trapped inside, to study climate history for over 30 years. The time capsule-like bubbles show what chemicals were in the air. Meanwhile, the amounts of different forms, known as isotopes, of elements like hydrogen, carbon and oxygen in the ice reveals the temperature it formed at. And finally, scientists figure out how old the ice and bubbles are from how deep they are in the core – and that’s where Frédéric found problems.

Cave deposits reveal permafrost concern

University of Oxford’s Anton Vaks explores a cave, where he could find stalactites and stalagmites that reveal when the soil above was permafrost. Credit: University of Oxford

Stalagmite and stalactite deposits in Siberian and Mongolian caves have revealed the most accurate permafrost history yet, suggesting that a global 1.5°C temperature rise could trigger a widespread thaw. “The finding shows how vulnerable the permafrost is,” said Anton Vaks from the University of Oxford. “Russian gas facilities in north-western Siberia are located close to the boundary of the continuous permafrost and rely on it as hard ground. Thawing of the permafrost may cause damage both to Russia, as well as its gas trade partners, like the European Union. The melting permafrost may also release part of the organic carbon currently trapped in it as greenhouse gases, CO2 and methane, enhancing global warming.”

Anton first used cave deposits’ power to study climate history during his PhD to build a 350,000 year record for the northern margin of the Saharan-Arabian Desert. “Stalagmites and stalactites grow only when rain or snowmelt water seep into the cave through the ceiling,” Anton explained. “Therefore each layer of growth of stalagmites and stalactites records a humid event in the desert.” He realised that these deposits’ ability to track water flow could equally measure melting of previously permanently frozen soil known as permafrost. “Cave deposits cannot grow when the rock above the cave is frozen,” Anton said. “Thus, each growth layer in a stalagmite forms during warm periods, whereas growth breaks represent cold periods with permafrost. Past periods that were warmer than now are especially important, because they can show what may happen to the permafrost in the future warmer world.”

Warming weakens deep freeze on Arctic islands

Pictures from William D'Andrea's August 2012 expedition to Svalbard. There are 24 slides in this series - apologies for the poor formatting. Credit: The Earth Institute/Columbia University

Normally 6°C wouldn’t be very warm – but in the Norwegian islands of Svalbard it’s a sultry modern summer, unlike anything seen for at least 1,800 years. That’s what sediments taken from an Arctic lake have told William D’Andrea from Columbia University in New York and a US team. It’s even warmer than a medieval warm period when parts of the northern half of the planet were as hot as, or hotter, than today. And while the record they’ve made reflects just this one site, it adds to the picture showing how unique today’s climate is. It’s also another step towards understanding how climate has changed through history, William told me.

Climate dynamics are extremely complex, and cooling in some locations can happen at the same time as warming in others, or increased precipitation in some places along with drought in other places,” he said. “These are the fingerprints we are trying to map and understand by generating such reconstructions.”

The fingerprints slowly become clearer as scientists collect more historical records, often as tubes of ice drilled from glaciers, or of mud and rock drilled from sea and lake beds. The tubes, or cores, cut through layers of mud or ice built up year after year. Scientists can then use fossils and chemicals to date and work out what conditions were like when they were laid down.

Global view answers ice age CO2 puzzle

Paleoclimate researcher Jeremy Shakun. Credit: Harvard University

Previous data suggesting that the world started warming out of the last ice age before CO2 levels in the atmosphere started rising don’t show the full picture. That’s according to US, French and Chinese scientists who have added to those Antarctic measurements with more taken from 80 locations across the globe. Harvard University’s Jeremy Shakun and colleagues show the greenhouse gas rises before temperature, supporting the case that CO2 drove climate change then, as it is now. “This provides a very tangible example of what rising CO2 can mean for the climate over the long term,” Jeremy said.

In the 1980s, researchers began building the history of CO2 in the atmosphere from cylinders of ice drilled from the Antarctic. Bubbles in the ice contain air from the time they formed, which researchers can measure. They can also figure out how old the ice holding the bubbles is from how deep it is in the core. And finally they can also work out temperature from the amount of the different forms, known as isotopes, of elements like hydrogen, carbon and oxygen in the ice. That’s because the temperature at which the snow that eventually became the ice formed affects how much of each it contains. And because some isotopes are radioactive and decay to a more stable isotope with time, studying them gives scientists another way to check the ice’s age.

The 800,000 year record of atmospheric CO2 from Antarctic ice cores, and a reconstruction of temperature based on hydrogen isotopes in the ice. The current CO2 concentration of 392 parts per million (ppm) is shown by the blue star. Credit: Jeremy Shakun/Harvard University

Such methods show temperature and CO2 levels rising and falling together for 800,000 years, Jeremy told journalists over the phone on Tuesday. “The question is: Which is the cause and which is the effect?” he asked. “If you look up close you see temperature changed before CO2 did. This is something the global warming skeptics have jumped on to say, ‘Obviously CO2 doesn’t cause warming because it came after the warming in these records’. But these ice cores only tell you about temperatures in Antarctica. For the same reason that you don’t look at just one thermometer from London or New York to prove or disprove global warming, you don’t want to look at just one spot in the map to reconstruct the past either.”

Ice age data cools down warming rate

The maximum ice distribution on the northern hemisphere during the last ice age, a period which was simulated by Andreas Schmittner and his colleagues to determine how much the world should warm for each doubling in concentration of CO2 in the Earth's atmosphere. Credit: Dr. Martin Jakobsson, Stockholm Geo Visualization Lab.

The expected rate of global warming in response to increasing CO2 levels in the atmosphere has been over-estimated, scientists claimed last week. Andreas Schmittner from Oregon State University and his colleagues have used information from the cold peak of the last ice age to recalculate “climate sensitivity”. This is a standard measure of the change in worldwide average air temperature at the Earth’s surface when the concentration of the greenhouse gas CO2 in the atmosphere doubles.

At the moment the United Nations Intergovernmental Panel on Climate Change (IPCC) considers that climate sensitivity is between 2 and 4°C per doubling, with 3°C the best estimate. “The biggest surprises were that our best estimate was considerably lower than that from the IPCC, and how narrow the possible range was,” Schmittner told Simple Climate. “We find a relatively low range of possible values – about 1-3°C – and a best estimate of 2.3°C. Particularly important is the fact that very high values can be excluded.”

Schmittner was inspired to look back to when the world reached the iciest stage of its last ice age after talking to the public about climate change. His talks referred to this period, known as the last glacial maximum, or LGM, that occurred 21,000 years ago. Then, ice sheets covered much land where people currently live in the northern half of the globe, and CO2 concentrations in the atmosphere were around 185 parts per million (ppm). That compares with concentrations around 280 ppm before the industrial revolution, and 389 ppm in 2010.

“I showed a figure of the ocean surface temperature reconstructions at the Last Glacial Maximum,” Schmittner explained. “It fascinated me because these data showed a surprisingly small cooling of only 2°C in the global average. I thought that this rich dataset could potentially be used to improve estimates of climate sensitivity, but data from land were lacking.”

Climate change threatens slow-moving species

The Andes, an example of a region with highly varied terrain, low climate-change velocity, and high concentrations of endemic species. Credit: Dennis Pedersen

Fast rates of warming pose the greatest threat to species that occupy small areas of similar climate, researchers in Denmark and the UK showed last week. Brody Sandel from Aarhus University and his colleagues compared where amphibians, birds, and mammals can be found today with how fast climate has changed over the past 21,000 years. They found that fewer “endemic” species, which only inhabit tightly defined locations, live in areas where climate has changed fastest since the peak of the last ice age. “As we embark on these upcoming decades of rapid climate change, we can expect that it should continue to have major consequences on weakly-dispersing species and species living in flat regions,” Sandel said.

Sandel and his colleagues set out to try and understand how greatly species’ current homes are determined by events that happened long ago or more recently. “People often talk about historical effects, but it’s been difficult to measure exactly relative to the effect of modern climate, so we wanted to be able to put those two things side-by-side,” he explained. To do this, the scientists studied a property they call climate-change velocity, that brings together the rate at which warming creeps over the planet with landscape shape.

“If you’re standing at a point on the Earth’s surface, you can think of that climate as moving over you,” Sandel said. “For example, at one point the mean annual temperature might be 10°C, and 100 years later that climate condition will have moved a certain distance. You can take that distance that it moved and divide it by the time it took to move there, and that gives you a velocity measurement. It describes the minimum migration rate that a species would have to obtain to keep up with climate as it changes over it.”

Food links climate change and social unrest

The Hanging, by Jacques Callot, depicts a scene from the 30 Years War that occurred in Europe between 1618 and 1648, during which time the continent was going through a cold period. Usually thought of as being triggered by disputes between Protestant and Catholic Christians, David Zhang and his colleagues now point to the climate as an ultimate cause for upheaval during this period.

People’s struggle to feed themselves has led to economic chaos and war through history, and the most vicious struggles can be traced directly back to climate change. That’s the conclusion of an ambitious attempt to work out exactly how climate and human social crises are linked by David Zhang from the University of Hong Kong and his colleagues. “Climate change has been scientifically proven to be the ultimate cause of significant human crises in pre-industrial Europe and the Northern Hemisphere,” Zhang stated boldly on Tuesday.

Many studies have shown that civilizations have collapsed in times of climate change, typically in periods when the world cooled. Zhang has been especially active in this area. “Over the last 7 years I have published over 20 papers on climate change and social responses,” he told Simple Climate. But, he noted, other researchers’ work had been criticised for a lack of evidence that definitively showed that climate causes upheaval in human societies. Zhang’s response, in a paper published in Proceedings of the National Academy of Sciences of the USA on Monday, calls on great volumes of data to show that there are, in fact, strong statistical links.

The researchers from Hong Kong and China brought together measurements on 16 different climate, agricultural, and social factors between 1500 and 1800 AD. “The great challenge is collecting and analysing this huge amount of data,” Zhang said. “The datasets come from different disciplines, and we have read over a thousand pieces of literature. For the same reason, we invited other economists, geographers and anthropologists to join us in the research to make sure there were no mistakes.”