UCAR Communications


staff notes monthly

September 2003

In the thick of climate change

NCAR research sheds light on warming global temperatures and likely regional impacts

When Tim Killeen taught a class on climate change at the University of Michigan a decade ago, he stressed scientific uncertainties. Now the NCAR director says he would teach the class quite differently, emphasizing the growing sophistication of computer models that track worldwide climate changes as well as observations of melting permafrost, receding glaciers, and other phenomena that indicate a warming trend.

“We’re much more sure of the science and the anthropogenic effects,” he says. “Scientists have made major strides.”

Thanks to the power of supercomputers and the increased scope of climate models, scientists know more than ever about the impact of human-emitted greenhouse gases on the world’s climate. Just in the last year, NCAR scientists—often working with colleagues at other institutions—have

• Determined why global temperatures warmed during the beginning of the 20th century, then leveled off or even cooled during the middle of the century before warming dramatically in recent decades
• Produced evidence that the atmosphere is warming several miles above Earth
• Successfully simulated temperature variations over the past 1,000 years
• Found evidence that climate change will have different impacts on frost days on the western sides of continents than on the eastern sides.

Trees play an important role in the climate system by storing carbon.

Such work is taking on increasing relevance. Countries are facing unusual weather patterns that may be signs of global warming, including the extraordinary heat wave in Europe this summer that claimed thousands of lives. “It’s almost as if this whole arena of science is being born as we watch,” Tim says, “and NCAR’s role is right at the leading edge of it.”

To mark Boulder’s emergence from an unusually hot summer (although certainly not as severe as Europe’s), Staff Notes Monthly is highlighting a few key NCAR research developments, largely in the Climate and Global Dynamics Division. This is by no means meant to be a comprehensive synopsis of CGD—let alone the rest of NCAR, where several divisions, including the Environmental and Societal Impacts Group, are looking into the effects of climate change. But it provides, at least, a small sampling of the organization’s work.

The recent (and more distant) past

Jerry Meehl and his CGD colleagues have long been intrigued by zigzagging temperature patterns over the past 100 years. The first part of the 20th century saw a distinct warming trend, followed by little warming or even slight cooling from the 1940s to the 1970s and then dramatic warming toward the end of the century (the El Niño year of 1998 set a global record for heat). Skeptics of global warming have long pointed to this inconsistent pattern as evidence that Earth is simply going through natural cycles of warmer and colder years. It was difficult to imagine why Earth would have warmed in the early 20th-century when greenhouse gas emissions were barely increasing, and then temporarily stopped warming right when greenhouse gas levels began rapidly rising.

Jerry Meehl

But Jerry and other NCAR scientists (CGD’s Warren Washington, Caspar Ammann, Julie Arblaster, and Tom Wigley working with Claudia Tebaldi of ESIG and RAP) have now determined the role of pollution on the 20th-century climate pattern. The team used the NCAR and U.S. Department of Energy Parallel Climate Model (PCM) to run a large series of simulations looking at two natural factors, or forcings, that affect climate (volcanic activity and solar radiation) and three forcings associated with human activity (sulfate emissions, greenhouse gas emissions, and ozone change).

The results indicate that increasing solar radiation played a major role in the early part of the century, warming the planet by about 0.4°C (0.7°F) between the late 1800s and the 1940s. As industrialization boomed in the decades after World War II, sulfates in the atmosphere blocked solar radiation, producing a slight cooling effect. Beginning in the 1970s, carbon dioxide and other greenhouse gases that were pumped into the atmosphere by power plants, cars, and other sources emerged as a dominant climate factor, warming the planet by several tenths of a Celsius degree by the end of the century.

“The beauty of this research is you can put each of the forcings by itself into the model over the 20th century, and you can also combine the forcings to see the impacts,” Jerry says. “What this shows is that the combination of natural and anthropogenic forcings has produced what we’ve observed.”

Warming temperatures in the 20th century. This chart illustrates the effects of natural and human-related factors on climate, using results generated by multiple runs of the NCAR and U.S. Department of Energy Parallel Climate Model. The orange line incorporates only natural climate variations (the influence of volcanic and solar activity), whereas the broken line incorporates both natural variations and human-related factors (emissions of sulfates and greenhouse gas emissions). The shaded areas represent the temperature range across the model runs, while the lines represent the means. The black line represents observations. Although early-century warming can be accounted for by natural factors, only by adding the effects of human-related emissions can the PCM simulate the observed late-century warming. (Illustration courtesy Jerry Meehl, Warren Washington, and Julie Arblaster.)

Peering farther into the past, Caspar and Fortunat Joos, a UCAR affiliate scientist from the University of Bern, used the Climate System Model at NCAR to simulate climate over the past 1,000 years. The model produced temperatures that closely correlated with climate data collected from the field—a strong indication that sophisticated models are accurately capturing the impacts of solar and volcanic activity, as well as of industrial emissions. The simulation also confirmed that temperatures in the past few decades are the highest in at least 1,000 years, exceeding natural variations and even surpassing temperatures in the so-called Medieval Warm Period at the beginning of the millenium.

What’s happening in the lower atmosphere?

One of the great mysteries of climate change is how temperatures in the troposphere—the lowest level of the atmosphere—are being affected. Over the past 25 years, a series of instruments aboard 12 U.S. satellites has provided a unique temperature record extending several miles above Earth. The data had indicated that the lower atmosphere was not warming much, an apparent contrast with the distinct warming trend in the average air temperature near Earth. Skeptics pointed to the lower-
atmosphere data to question the very existence of global warming, even though a National Academy of Sciences report concluded that the lower atmosphere and the surface layer would have different temperature trends.

In May, however, Science published an article indicating the lower atmosphere, indeed, was warming. The authors included Tom Wigley, Jerry Meehl, Caspar Ammann, Julie Arblaster, and Warren Washington, as well as Tom Bettge of SCD. (The lead author was Ben Santer of the Lawrence Livermore National Laboratory.) The new findings drew on a reanalysis of the raw satellite data by a group based at Remote Sensing Systems in Santa Rosa, California, that accounted for the effects of heating on the radiation sensor itself and adjusted for the drifting orbit of each satellite. The new results indicated a global temperature rise in the lower atmosphere of about one-third of a degree Fahrenheit between 1979 and 1999, which tracked with results from the PCM.

Providing further evidence of climate change, Ben, Tom, and several of the same researchers published another study in Science in July—this time showing that human activity is largely responsible for an increase in the height of the tropopause. The bitterly cold tropopause provides a unique window into atmospheric temperatures because it’s the transition zone between the
troposphere, where temperatures cool with increased altitude, and the stratosphere, where temperatures warm with increased altitude.

Observations have shown that the tropopause has risen by hundreds of feet since 1979, but no one was able to say precisely why until this year. Using newly available results from the PCM, the research team determined that 80% of the rise could be attributed to greenhouse gas emissions (which warm the troposphere) and ozone depletion (which cools the stratosphere).

“Determining why the height of the tropopause is increasing gives us insights into the causes of the overall warming of the lower atmosphere,” Tom explains. “Although not conclusive in itself, this research is an important piece in the jigsaw puzzle.”

Should we plant more crops?

One of the most intriguing aspects of climate change is the role played by vegetation. Although somewhat counterintuitive, croplands actually cool Earth more than shady forests. That’s because crops—as a whole, lighter in color than forests—reflect more sunlight back into space, reducing the impact of solar warming. CGD’s Gordon Bonan estimates that present-day farmland may be providing a cooling effect on U.S. temperatures in July by as much as 1°C to 2°C (1.8°F to 3.6°F).

Corn and other crops can have a cooling impact on climate.

Does that mean farms will save us from global warming? The answer, unfortunately, is no. In the United States and many countries in the midlatitudes, areas that used to be agricultural are being converted back to forests (this trend is particularly pronounced in New England). The opposite is happening in the tropics, where forests are being cleared for plantations. But, as luck would have it, tropical farmland has a warming impact—both because clearing the dense forests releases a significant amount of carbon into the air and because the change from tall trees to shorter crops reduces vertical air movements and leads to warming temperatures.

Add to the mix the fact that boreal forests are likely to advance over the next century into areas that are now tundra, reducing the amount of solar radiation that arctic regions reflect back into space, and the result is a recipe for accelerated warming. “The loss of agriculture in the midlatitudes, the growth of agriculture in the tropics, and the movement of forests is all pushing us toward a warming planet,” Gordon warns.

Not all the news is bad, however. Trees store carbon, so larger forests mean more carbon dioxide is taken out of the atmosphere. We should know much more about the carbon cycle in a few years, as CGD’s Dave Schimel is overseeing a project that will greatly increase observations of carbon sinks and sources, and CGD’s Peter Thornton is producing a model of the carbon cycle that can be integrated into the Community Climate System Model at NCAR.

Gordon Bonan

A coming deluge?

A major unknown about climate change is how it will affect precipitation. Dave points out that a warmer and wetter world can spur the growth of many plants, but a warmer and drier world could prove devastating to vegetation.

CGD’s Kevin Trenberth, in analyzing the global hydrological cycle over the past few years, has become intrigued with a paradoxical effect involving rainfall and the lack of it. He says the key question is not how much total precipitation is likely to fall—that may remain more or less the same—but rather whether the character of the precipitation will change in such a way that we’re likely to have long dry spells interrupted by an occasional deluge.

Why would that happen? In a warmer world, greater evaporation will lead to far more moisture in the atmosphere, which in turn may create more intense storms. Kevin recently wrote an article on this topic with CGD’s Aiguo Dai, RAP’s Roy Rasmussen, and ATD’s Dave Parsons for the Bulletin of the American Meteorological Society. “As climate warms, the amount of moisture in the atmosphere…is expected to rise much faster than the total precipitation amount,” the authors wrote. “This implies that the main changes to be experienced are in the character of precipitation: increases in intensity must be offset by decreases in duration or frequency of events.”

NOAA’s Tom Karl and colleagues have already confirmed a similar pattern for 20th-century rainfall in the United States, with an increasing percentage of our rain coming in heavier spurts. Tom Wigley has also researched this topic, working with a colleague to analyze model runs that indicate more extreme precipitation events are likely to occur.

Such a change in precipitation would lead to both floods and droughts. The heavy rains would also cause more runoff, with less moisture actually soaking into the soil to sustain crops and other plants. “This rainfall pattern would be less easy for societies to manage,” Kevin says.

Hotter in the west?

Jerry Meehl remembers an interesting question being raised during climate discussions a couple of years ago: how will climate change affect the number of frost days? The assumption was that warmer temperatures would simply lead to fewer days with overnight frosts, but Jerry studied the question in greater detail by taking advantage of the large number of climate change simulations at NCAR.

The resulting research, which he conducted with Claudia Tebaldi and CGD’s Doug Nychka, yielded a surprising result. True, the number of frost days will decline almost everywhere in a future warming climate. But a statistical study indicated the impacts would be far greater on the western sides of continents throughout the world, with the eastern sides of continents less affected.

Climate change is expected to have complex impacts on frost.

The reason, it turns out, has to do with a change in regional atmospheric circulation patterns, which are expected to bring in air from differing directions. Increasingly, warm air will be circulated from the south on the western sides of continents, leading to warmer nighttime temperatures and significantly fewer frosts. The eastern sides of continents, however, will tend to get more air from the north—and, therefore, their nighttime temperatures will not rise as significantly.

Interestingly, this summer may have provided a harbinger of that pattern, as high-pressure ridges funneled hot, dry air from the south over both western North America and western Europe, producing record-breaking temperatures. In contrast, eastern North America and eastern Europe enjoyed comparatively cool summers.

New directions

The study on frost days is the first of a series of research projects associated with the new NCAR climate change strategic initiative that will focus, in part, on extreme events such as heat waves and floods. “Weather and climate extremes have maybe the biggest impact on societies and ecosystems,” Jerry explains.

Indeed, one of the most important trends in climate change research is not just to model rising global temperatures. Scientists want to look into finer-grained impacts such as frost days and precipitation patterns to help societies adjust to the changing climate of the 21st century.

To that end, Claudia Tebaldi and other scientists are using Bayesian statistical methods (a highly regarded approach to probability assessments) to combine the forecasts of the world’s leading computer models. This work sheds light on how regional temperatures may change in coming decades, and it also quantifies the margin of error of the combined forecasts.

One of the team’s studies integrated the predictions of nine of the world’s leading climate models, based on a pessimistic scenario of continuing greenhouse gas emissions. The analysis concluded that the rate of warming will be most pronounced in the eastern Canadian Arctic by the end of the century, with a 95% probability that average winter temperatures will increase by 5°C to 9°C (9°F to 16°F). In contrast, Southeast Asia can expect a winter temperature increase that will likely be limited to the range of 1°C to 4°C (2°F to 7°F).

As a major source of greenhouse gases, cars and other vehicles appear to be be contributing to a warmer climate.

Linda Mearns (ESIG) and Warren Washington, Doug Nychka, and Jerry Meehl are helping to oversee the NCAR initiative to assess the impacts of weather and climate, including climate change. A cross-divisional group of NCAR scientists will explore uncertainty, extreme weather and climate events, and the interactions between climate and health.

As Tim puts it: “Climate change is an area where society needs sound science. The work is incredibly important.” •David Hosansky

Global warming controversy

When Senate Commerce Chairman John McCain wanted information about global warming, one of the experts he turned to was UCAR president Rick Anthes. In a 28 July letter, the Arizona Republican asked whether a scientific consensus exists that warming temperatures can be linked to greenhouse gas emissions.

The UCAR president responded in the affirmative. He wrote the senator, “There is strong agreement among the vast majority of climate scientists that Earth has been experiencing a warming trend in the last century and that this global climate change would not be expected from the natural variability such as that experienced in past millennia.”

Rick Anthes

Not all policy makers accept that view, however. Some contend that warming temperatures may be resulting from natural variability, and they worry that steps to reduce greenhouse gas emissions could set back the nation’s economy.

How do UCAR and NCAR navigate the dangerous crossings where policy and climate change science intersect?

NCAR director Tim Killeen says the organization’s role is not to act as an advocate for one side or another, but rather to produce the best possible science on the subject. “We have to be true to the science,” he says. “We can’t shy away from the field. My hope is we’ll be looked at as a credible source of objective scientific information on this issue.”

Warren Washington—a senior scientist in the Climate and Global Dynamics Division, head of the National Science Board, and former science advisor to several U.S. presidents—seconds that view. “Obviously, climate change research has its ups and downs with respect to the policy makers,” he says. “We should continue to concentrate on getting the best possible answers to the important scientific questions.”

Warren acknowledges that the policy debate over global warming can be frustrating to some scientists. But, he says, “These things go in cycles. Over time the research will have an impact.”

Rick Anthes’s letter to Sen. John McCain

Also in this issue...

UCAR quilters stitch for babies

Up-the-Hill 2003

Time for a Realignment

Recollections from Steve Dickson

Random Profile

Will tomorrow's cities have clean air?

Delphi Question: Nap room


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