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.

CO2 casts off shackles to power up Atlantic hurricanes

NOAA’s GOES-13 satellite captured this visible image of Hurricane Sandy battering the U.S. East coast on Monday, Oct. 29 at 9:10 am EDT. Sandy’s center was about 310 miles south-southeast of New York City. Tropical Storm force winds are about 1,000 miles in diameter, and are set to intensify in the 21st century. Credit: NASA GOES Project

Changes in greenhouse gases and other air pollution will likely make Atlantic storms that could hit the Caribbean and Eastern US more intense through this century. That’s according to research from Gabriel Vecchi at the US National Oceanographic and Atmospheric Administration (NOAA) in Princeton, New Jersey, and Gabriele Villarini at the University of Iowa. They’ve found that more greenhouse gases strengthen these storms but other pollutants known as aerosols or particulates, which include soot, do the opposite. Increases in both types of pollution through the 20^th century therefore cancelled each other out. But with more recent efforts to limit aerosol pollution succeeding, Atlantic storms now look set to become more destructive. “Both reductions in particulate pollution and increases in greenhouse gases are going to co-operate, we think, to give us more intense hurricanes in the Atlantic,” Gabriel said.

Gabriel has long studied Atlantic storms, and together with Gabriele recently found that how often they happen will likely only increase during the first half of the 21^st century. “The number of storms in a season is only part of the story,” Gabriel told me. “A big question for society is the intensity.” So it was natural, he added, to follow on by looking at how strong and long-lasting they are. Scientists have already looked at their intensity for narrow “time-slices”, for example from 1985-2005 and then predicting from 2080 to 2100. “People haven’t explored how we go from the late 20^th century to the late 21^st century,” Gabriel said.” That’s because to do this research they need complex and very detailed ‘high resolution dynamical’ climate models, which use up scarce time on the world’s most powerful computers. For the same reason, previous studies only look at a few possible scenarios for how much of the greenhouse gas CO2 humans will produce by burning fossil fuels.

Could pollution be stopping warming’s impact on rain?

A brown cloud of pollution over Phoenix, Arizona. Brown clouds of aerosol pollutant particles could be overwhelming the expected changes in rainfall arising from increasing greenhouse gas levels in the air. Credit: Flick/Flickr

Contrary to previous predictions and measurements, rain patterns have got more uniform as the world has warmed over the past 70 years. So say Michael Roderick and his teammates from Australian National University, Canberra, who’ve developed an ‘accounting system’ that looks closely at where and when rain fell. And the reason could be aerosols – clouds of pollutant particles – produced by humans. “The existing dogma is that increasing greenhouse gas concentrations in the atmosphere have raised rainfall variability,” Michael told me. “In that context, our results emphasise the importance of taking a whole system approach in trying to understand how something complex, like rainfall, is changing in different places.”

When scientists want to understand how climate has been changing over large areas, they usually look at maps of long-term average data that ignore patterns of change in time, Michael explained. When they want to look at how it’s changed over time, they usually either look at a single place or a worldwide average, which ignores patterns in where the changes are. But Michael, along with fellow scientists Fubao Sun and Graham Farquhar, wanted to find a way to link place and time.

To do this Fubao started from a common statistical test called Analysis of Variance or ANOVA. Normally it’s used to compare the effect of different “treatments” – such as a variety of temperatures – on the yield of a crop, for example. In such cases each treatment must be repeated more than once, giving different “replicates”, for the test to be valid. ANOVA can be used to give a value for variance – a measure that shows how spread out an experiment’s measurements are.

Developed countries duck warming responsibility

Beijing Normal University’s John Moore, Xuefeng Cui and their collegues assessed the relative impact on future warming if developed and developing countries follow the pledges to cut CO2 emissions they made at the UN climate change conference in Cancún, Mexico, in November 2010, shown here. Credit: UNclimatechange/Flickr

Disputes between leaders of rich and poor countries currently mean little comes from meetings where they’re meant to draw up plans to slow and stop climate change. But developed countries’ existing promises would achieve just 1/3 of any warming slowdown, even though we’re responsible for more than 2/3 of CO2 emissions before 2005. That’s according to a team of mainly Chinese researchers who have tried to settle these fights using “earth system” models, considering both natural and human factors. “Developed countries need to take more responsibility in climate mitigation by cutting more carbon emissions and helping developing countries to control carbon emission while maintaining economic development,” said Xuefeng Cui from Beijing Normal University (BNU).

At the United Nations’ Climate Change Conference in Cancún, Mexico, in November 2010, leaders agreed to try and limit the global temperature rise to 2°C higher than pre-industrial levels. They also agreed that doing this needs deep, but fair, cuts in the amount of warming-causing greenhouse gases humans emit. But they still argue about how to share those cuts. That prompted Cui and his team to make an unusual effort to use science to show what is fair.  “The arguments in the IPCC process demand some fact-based reasoning rather than just the ‘blame game’,” team member John Moore told Simple Climate. “Our study is the first interdisciplinary study by climate, social, economic, and ecological scientists and policy makers to look at the historical responsibilities and effect of future mitigation by applying state-of-art earth system models,” Xuefeng added.

Getting such a broad view meant that the team had to develop entirely new methods for their research, published online in the scientific journal Proceedings of the National Academy of Sciences of the USA on Monday. “Most scientists are interested in the real impacts rather than assigning responsibilities,” Moore said. “These are more abstract philosophical and moral points than they tend to consider.” It took a 37-strong team of scientists to develop the approach, and one of the two earth systems models, they used. Whole earth system models are needed to understand the effects plants, animals, land and oceans have on climate.

Volcano cloud over tree-ring temperatures clears

Pennsylvania State University’s Michael Mann thinks he has found the reason behind key outstanding disagreements between the historical temperature record based on tree rings and climate models for the same period. Credit: Pennylvania State University

The sudden chills violent volcano eruptions cast over the world centuries ago effectively erased themselves from the historical climate record produced by examining tree-rings. So suggests a team led by Michael Mann from Pennsylvania State University, who famously used 1,000 years of tree-ring measurements in the “hockey stick” graph showing how unusual today’s temperatures are. Michael warns the skipped years could affect scientists’ estimates of how much the world warms in response to greenhouse gases in the atmosphere, known as its climate sensitivity. But other than the volcano years, the scientist notes that tree-ring data is a remarkably accurate match with the climate models they used for comparison. “Interestingly, the effect has little influence on long-term trends, including conclusions about how previous temperatures compares to modern ones,” he told me. “Instead, it appears only to have implications for how strong past short-term cooling events were.”

A tree’s age can usually be told from the rings that form across its trunk representing each year’s growth. How thick each ring is shows how much the tree grew in the year in question, which is influenced by the temperatures that tree experienced. That means examining the thickness of rings in old trees can provide a way to tell temperatures back through history. Many challenges have already been overcome in turning this simple-sounding idea into a history of the world’s temperature, but Michael was still troubled by one particular detail.

Greenhouse focus confirms humans’ warming role

Reto Knutti, from the Institute for Atmospheric and Climate Science at the Swiss Federal Institute of Technology Zurich, and his colleague Markus Huber, have developed a new model to demonstrate humans' responsibility for recent warming. Credit: Swiss Federal Institute of Technology Zurich

At least three-quarters of the climate warming observed since the mid-twentieth century is extremely likely to be caused by human activities. That’s what Reto Knutti and Markus Huber from the Swiss Federal Institute of Technology, Zurich, said last week, confirming previous similar conclusions made using a different method. They say that it is “extremely unlikely” that the temperature changes we have seen are caused by natural variation, even if that variation is much stronger than current models suggest.

The Swiss researchers are clearly not the first to find humans are very likely to be responsible for global temperature rises. However, as a contributor to previous reports by the UN Intergovernmental Panel on Climate Change (IPCC), Knutti felt that the methods used to establish responsibility could be better. “Attribution of past changes was largely presented as a result of “optimal fingerprinting”, and that is a black box for most people,” he told Simple Climate. “It’s statistically complex, makes a number of assumptions and is not physically intuitive.”

Tension simmers over climate link to plant growth

A misty canopy at dawn in the Amazon forest, where calculations of plant growth from satellite measurements that differ from direct measurements have come under criticism. Image courtesy of Peter van der Steen

A surprise finding that plants are growing less quickly when, with current temperatures and CO2 levels, we might expect the opposite has come under fire from two independent groups of scientists. In August 2010, Maosheng Zhao and Steven Running from the University of Montana, Missoula showed that a measure of plant growth speed had slowed slightly since 2000. That’s even though it had accelerated in the 1980s and 1990s, a situation that fits the higher levels of CO2 in the atmosphere available to help plants grow through photosynthesis. But now, a year later, scientists from the US and Brazil have complained that this does not match what they’ve seen directly in Amazonian forests. Meanwhile, an Australian scientist adds to these objections with claims that Zhao and Running have over-estimated the effect that temperature has had on growth rates.

The disagreement focuses on how Zhao and Running calculated the measure of plant growth they use – net primary productivity (NPP) – from satellite data. “Measuring growth of a single tree is easy, however, at the global level, for billions of trees and plants, measurement of growth is only possible with data from satellites,” Zhao told Simple Climate. “Our model uses vegetation greenness information observed from satellites and daily global weather data to calculate vegetation growth of each kilometre over 110 million square kilometres of vegetated land surface.” The downside in this approach is that the view of Earth’s surface is often blocked, for instance by clouds and smoke. When that happens, the Montana researchers use the data from before and after the days the satellite can’t see the surface to fill in the missing measurements. They can then calculate NPP from those vegetation greenness measurements.  

Chinese pollution postpones temperature rises

Sulphur emissions, which contribute to acid rain that can damage soil, plants and trees like these, have also slowed temperature rise between 1998 and 2008 from the more rapid warming seen between the mid-70s and mid-90s. Credit: Wikimedia Commons

The full climate impact of China’s massive industrialisation between 1998 and 2008 has yet to be felt, thanks to its reliance on coal, US and Finland-based researchers said this week. Using this fuel for energy generation did release large volumes of the greenhouse gas CO2 that will warm the planet in the long term. However, it also emitted pollutants derived from the element sulphur that oppose this warming effect in the short term, explained Boston University’s Robert Kaufmann. “That let natural variations in that decade really predominate,” Kaufmann told Simple Climate.

These findings help answer a long-standing climate question, which stumped Kaufmann when he was speaking about global warming to the public in New Jersey in 2008. “A member of the audience said that he had heard that global temperatures hadn’t risen for about 10 years,” the researcher explained. “He asked me why not, and I must admit that I was at a loss to explain it.” Climate scientists have conceded that there has technically been no warming in this time, even though the US National Oceanographic and Atmospheric Administration (NOAA) said 2005 was then the warmest year on record. Kaufmann found that in fact there hadn’t yet been any satisfactory explanation why this had happened, partly due to the tools used by climate scientists. Most “general circulation” models (GCMs) used to simulate processes in the atmosphere and on the Earth calculate climate patterns from the laws of physics. While these are good at modelling changes in the long term, they are much less accurate over periods of just a few years, Kaufmann said.

Soot and methane cuts promise threefold benefits

Vehicles are a significant source of black carbon and other pollutants in many countries. Credit: Caramel/flickr

Limiting methane and soot emissions would save lives and keep farming output high, as well as playing an important role in fighting global warming. That’s according to some 70 scientists who have reviewed the available research on these substances for the United Nations Environment Partnership (UNEP). Such cuts were also surprisingly feasible, with just 16 ways of limiting emissions providing about 90 percent of the possible climate benefit from a list of 2000 control measures.

“We estimate that adoption of the 16 control measures we considered would save about 2 million lives a year and save 50 million tons of crops a year,” said NASA’s Drew Shindell, who led the project. “For climate, putting control measures in place could eliminate about half the warming we’ll otherwise face over the next 40 years.”

Atlantic cold tongue tells of humans’ climate impact

Wind waves depend on wind speed and have long been logged by ship crews. Ocean wind data corrected with wind-wave heights suggests a slow down of the southeast trade winds in the equatorial Atlantic. Credit: Image courtesy of Ship DAVID STARR JORDAN, NOAA Photo Library.

Humanity has significantly altered the climate in the tropical Atlantic between 1950 and 2009, gradually weakening the area’s trade winds, University of Hawaii at Manoa researchers say. That effect is caused by burning fossil fuels, Hiroki Tokinaga and Shang-Ping Xie explain. However, it is more directly driven by the aerosols of soot and sulfur-based particles this releases than global warming caused by the resulting CO2 greenhouse gas emissions.

Falls in aerosol emissions now look set to reverse this trend, Tokinaga notes, showing how government actions can influence the environment. “Human-produced aerosol emissions had continuously increased until the 1970s, but then they started to decrease because of legislation in North America and Europe,” he told Simple Climate. “Increased greenhouse gas forcing contributes to a broader warming of the tropical Atlantic. Another climate shift might happen when the increased greenhouse gas forcing gets stronger than the aerosol forcing in the next few decades.”