Scientists aim to improve and localize their predictions
Modern explorers have pushed into nearly every nook and cranny of the globe, from polar Antarctica to the depths of the Amazonian jungle. Yet there’s land still to explore, and regularly comes news of unexpected and wondrous findings — a mongooselike carnivore spotted in Madagascar, a massive waterfall discovered in Peru.
Such is the state of climate science today. In some respects its territory has been thoroughly probed. Despite vigorous questioning of the premises and conclusions of research into climate change, reviews of the evidence consistently confirm the basic findings of the United Nations’ Intergovernmental Panel on Climate Change: The Earth’s rising temperature cannot be explained by natural processes alone. Emissions of greenhouse gases from human activities, like burning fossil fuels, must be included to account for the observed warming. Policy makers will be building on this firm foundation during meetings in Cancun, Mexico, in late November and early December to hammer out the next global climate-control agreement.
But unexplored corners of global climate remain. The most recent IPCC report, from 2007, acknowledged this terra incognita: “There is still an incomplete physical understanding of many components of the climate system and their role in climate change.” Such unknowns are fertile territory for scientists.
They are now pushing, for example, into perhaps the least understood aspect of observational climate science: how tiny particles called aerosols influence climate. Some types of aerosols cool the planet, while others warm it. They also affect how many clouds form and where, further redrawing the planetary climate picture. After decades of effort, researchers are finally starting to disentangle this particular unknown, in part thanks to fleets of airplanes, satellites and other instruments deployed to monitor aerosols.
Understanding the physical processes that drive the climate system today is one thing; figuring out how changes in the system will influence people’s lives in the future is another problem altogether. Here, scientists focus on regional climate prediction — zooming their computer models down to ever-smaller scales to understand how climate change might alter local environments. Such work is the only way to know how drought will affect Phoenix or floods affect Pakistan. While just beginning to map out the thorniest problems in regional predictions, such as how to accurately model short-term circulation patterns in the atmosphere and ocean, researchers are inking in the details using some promising approaches.
By the time the next IPCC report is released, in 2013 or 2014, scientists should have shortened — or at least sharpened — their list of unknowns. “There’s a whole lot of climate science that needs to be done,” says climate researcher Noah Diffenbaugh of Stanford University. And while the new information might not pacify climate change skeptics, it will clarify key areas of uncertainty in the climate system. It will also help fill in the last details of the geography of climate, giving society a guide for navigating the shoals of future changes.
Aerosols, tiny as they are, have an outsized influence on climate. These particles range from fractions of a micrometer to several micrometers across and come from both artificial and natural sources — including power plants, biomass burning, sea spray, volcanic eruptions and even wildfires (SN: 11/6/10, p. 28).
Most aerosols cool the planet, by acting as a sunscreen to reflect the sun’s incoming rays back into space. The 1991 eruption of Mount Pinatubo in the Philippines, for instance, spewed enough sulfur and other particles to circle the globe and drop temperatures worldwide by about half a degree Celsius.
Climate scientists agree that without aerosols, the Earth would have warmed more than it has in recent decades. Exactly the extent of the cooling effect, though, is a matter of dispute. Even less well understood are the indirect effects of aerosols: They can change the size and distribution of clouds around the globe.
Aerosols serve as tiny seeds on which water vapor can condense. So more aerosols mean more “cloud condensation nuclei” for clouds to form around; long thin clouds known as ship tracks, for example, trace a ship’s airborne emission like a jet contrail. More seed particles mean that clouds become thicker, whiter and more reflective, cooling things down even more.
Many scientists consider this indirect effect to be the largest single uncertainty in understanding the interactions between aerosols and climate. Over the last decade and more, researchers have launched a flotilla of observing equipment in more than a dozen field campaigns to measure the distribution and effects of aerosols in regions from Asia to North America. Typically, airplanes will fly a gridlike pattern gathering air samples, while researchers on the ground use vehicles or ships to measure patterns of aerosol emission. (NASA’s Glory mission, scheduled for launch in February, will continue this full-out push.)
Such detailed work is now paying off. Antony Clarke and Vladimir Kapustin of the University of Hawaii at Manoa analyzed more than 1,000 vertical slices of the atmosphere, taken mostly around the Pacific in various campaigns since 1995. The researchers report in the Sept. 17 Science that there were more cloud condensation nuclei, along with other measures of aerosols, above regions with a lot of human activity compared with more pristine areas. The work “provides a more revealing picture of combustion influences over global scales,” says Clarke.
Another paper in the same issue of Science scrutinized how atmospheric chemistry over the Amazon naturally produces aerosols and clouds there. A team led by Ulrich Pöschl of the Max Planck Institute for Chemistry in Mainz, Germany, found that the number of clouds above the rainforest was limited by the natural supply of aerosols. In regions with more pollution, the limiting factor on cloud formation was how fast winds could lift aerosols high into the atmosphere. The study underscores how the complex relationship between aerosols and clouds can differ over pristine and polluted areas.
Hot plates in the sky
One kind of aerosol is particularly capturing researchers’ attention these days — black carbon, better known as soot. These particles come from fossil fuel–burning power plants, fires started to clear land for agriculture and, to a lesser extent, from coal- and wood-burning cookstoves in developing nations.
Once thought to be a minor player in global warming, black carbon is now a major focus of interest. Unlike other aerosols that have net cooling effects, black carbon particles absorb solar radiation and warm the atmosphere, the way wearing a black T-shirt on a sunny day warms the body. Studies have fingered these tiny hot plates in the sky as a major contributor to Arctic warming seen in recent decades (SN: 11/21/09, p. 5), and some researchers think soot may be an important factor in glaciers melting in the Himalayas.
Still, scientists don’t fully understand how much black carbon there is, where it comes from and how it gets removed from the atmosphere over time. In one ongoing project, researchers fly repeatedly from the Arctic to the Antarctic, traveling high into the atmosphere and dipping down nearly to touch the ocean. The National Science Foundation project, called HIAPER Pole-to-Pole Observations or simply HIPPO, collects air samples to test for many atmospheric gases, notably carbon dioxide, but it also collects black carbon.
During its first flights, in January 2009, the HIPPO plane measured unexpected concentrations of black carbon along its path. That could be because computer models don’t accurately estimate how well rain would remove the particles from the air, says Joshua Schwarz, an atmospheric scientist at the NOAA Earth System Research Laboratory in Boulder, Colo. Different air masses also contained surprisingly similar sizes of black carbon particles, no matter where that air came from. “We don’t have a reasonable explanation for that yet,” Schwarz says.
Spurred by prodding from scientists, policy makers are increasingly looking at ways to cut black carbon emissions. The concept is politically palatable, since it involves cleaning up pollution in a way that helps people’s health. Black carbon is also an easy target because soot falls out of the atmosphere in a matter of weeks, yielding measurable results in a short period of time. Strict pollution measures introduced in Beijing before the 2008 Olympics, for instance, temporarily cut the city’s black carbon emissions by about 25 percent.
New research, however, suggests that the climate benefits of cutting black carbon might come with a flip side. In May in Geophysical Research Letters, a team led by Wei-Ting Chen of the Jet Propulsion Laboratory in Pasadena, Calif., modeled the climatic outcome of slashing black carbon emissions in half. The cooling benefits of reducing emissions, her team found, were partially offset by the fact that less soot meant fewer cloud condensation nuclei and thus fewer clouds to cool things down. Still, says team member John Seinfeld of Caltech, “on the whole, black carbon reduction is still good for climate, and good for human health as well.”
Targeting particular sources of black carbon might be important in reducing it efficiently. Aerosol plumes dominated by black carbon from fossil fuel burning in China were roughly twice as efficient at absorbing solar radiation and heating things up as were black carbon plumes from biomass burning, Veerabhadran Ramanathan, a black carbon specialist at the Scripps Institution of Oceanography in La Jolla, Calif., and colleagues reported in August in Nature Geoscience.
With other scientists, Ramanathan has launched Project Surya, which aims to replace polluting cookstoves across India with low-cost, cleaner alternatives. Women who receive the new stoves are required to remove and photograph the filter that strips out black carbon particles. “Just seeing the black filter makes them say, ‘Is this what we are breathing?’” says Ramanathan.
Even as researchers push into unexplored territory in aerosols, others are looking to cover some ground closer to home — understanding how climate change will affect people’s lives. In this case, scientists aim to improve their predictions of the regional impacts of climate change. Many local effects are already apparent. A major U.S. government report last year, for instance, detailed how climate change has led to heavier winter rain in the Northeast, more intense hurricanes in the Southeast and smaller snowpacks in the Rocky Mountains.
Global temperatures are projected to increase by 2 to 4 degrees Celsius by the end of this century, but locally the effects could be much stronger. For instance, computer models that incorporated a global temperature rise of 2 degrees Celsius found that large parts of Europe, North America and Asia could experience heat waves with temperatures up to 6 degrees Celsius higher than normal. Local factors such as plant root depth and forest cover could partly account for the differences, Robin Clark and colleagues at the Met Office Hadley Centre in Exeter, England, wrote in September in Geophysical Research Letters.
Heat waves could also become more common in the United States over the next three decades, suggests new work by Stanford’s Diffenbaugh and Moetasim Ashfaq of Oak Ridge National Laboratory in Tennessee. In their models, published in August in Geophysical Research Letters, the researchers found that limiting global mean temperature below a 2-degree rise still led to heat waves over much of the western United States, especially between the years 2030 and 2039.
Devastating heat waves, like the one that killed an estimated 35,000 people in Europe in 2003, aren’t the only change in store. “There’s a huge suite of potential climate change impacts,” Diffenbaugh says. The problem with knowing what exactly to expect is that regional climate predictions are less sophisticated than global climate models. Researchers would like to tell officials exactly how hot, cold, wet or dry it will be in their city or state three decades from now. But they can’t.
To get a good look at local climate, modelers “downscale” the results from global models, using the coarse data as a starting point to make finer-scale local calculations. A typical global model has a resolution of 300 kilometers, meaning that any smaller feature isn’t detected and analyzed in the model’s calculations. By contrast, resolution in a regional climate model can be 50 kilometers or even smaller. Climate scientists take a global model and stitch a piece of regional model into it, like photoshopping in a more detailed image of a face in a crowd.
In some instances, this approach is starting to yield consistent results. Reporting in June in Nature Geoscience, Erich Fischer and Christoph Schär of ETH Zurich show how different regional climate models come up with similar patterns of future heat waves in Europe. Over the next century, extreme heat events will be longest and most common in southern Europe and most severe in the north, the researchers write.
But other aspects of regional climate modeling lag far behind. Many regional models have yet to incorporate basic land-use information, like details on urbanization, forest clearing or irrigation, says Jim Hurrell, a climate modeler at the National Center for Atmospheric Research in Boulder. And long-term forecasts of how local precipitation might change are particularly poor — making it difficult to anticipate droughts.
Regional models are also only as good as the global approach that they are fit into. If the global model contains errors, “that puts major question marks around the fidelity of your regional simulations,” Hurrell says. In particular, global modelers have yet to get a good handle on “decadal variability,” or internal factors within the climate system that vary on a time frame of 10, 20 or 30-odd years.
Such factors include the El Ni±o Southern Oscillation, a warming pattern that occurs in the tropical Pacific every few years, and the Pacific Decadal Oscillation, a similar but longer-term pattern associated with cooling in the central North Pacific. Global climate models don’t include these phenomena in detail, but will need to if researchers want to accurately predict climate in a particular area decades out.
Only over the last couple of years have scientists gathered enough relevant data, such as ocean temperatures from a global array of buoys called ARGO, to be able to contemplate how to add decadal variability into global models. “It’s an extremely challenging problem,” Hurrell says.
Global modelers probably won’t be able to solve that problem in time for the next IPCC report, but the ongoing work should help improve regional efforts over the next few years, says Diffenbaugh. “We’re on the precipice of knowing a lot more,” he says.
That information couldn’t come soon enough for local officials and other community stakeholders. The U.S. government is pushing to establish a national climate service, similar to the National Weather Service, to provide local climate information (SN: 3/13/10, p. 32). Many states and cities, including California, Phoenix and New York City, have also started their own regional study efforts.
How well these efforts work — or don’t — will become clear as the next IPCC report takes shape. By the time the final publication is out, the uncertainties surrounding regional climate predictions, as well as aerosols, will be smaller than today. And climate’s terra incognita will shrink.
Uncertainty in the climatic landscape
While not as dire as the state of understanding on aerosols and regional climate, other areas of climate science also have some unanswered questions.
Carbon cycle feedback
Uncertainties remain in knowing exactly how carbon cycles through the ocean, atmosphere and land. Rising temperatures, for instance, can melt permafrost and send more carbon into the atmosphere — but at what rate, scientists don’t quite know.
The great ice sheets atop Antarctica and Greenland behave in unpredictable ways as they warm, and individual glaciers advance and retreat in unexpected ways as well. Arctic sea ice is shrinking and thinning overall, but its exact behavior is a surprise every season.
Closely related to the problem of aerosols, clouds exhibit various poorly understood feedbacks on climate, some of which computer models completely miss. Scientists agree that clouds are likely to increase in a globally warmed world but don’t know how strong a climate effect they might exert.
Future changes in rain or snowfall will depend largely on how much global atmospheric temperature rises compared with ocean and land temperatures. Researchers are still working to tease out the details.
Oceanic conveyor belt
Circulation patterns within the North Atlantic carry warm water to the north and east, where it cools, sinks and returns south in a giant conveyor-like loop. Some models of abrupt climate change have gone so far as to suggest that this conveyor could shut down altogether, affecting climate across Europe.
Clearing land for agriculture or other reasons affects the amount of carbon the ground can store. But this effect is poorly understood and not incorporated into many models of future climate.
— Alexandra Witze
Icons: E. Feliciano
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