In the thick of climate change
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.
Were 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 worlds climate. Just in the last year, NCAR
scientistsoften working with colleagues at other institutionshave
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. Its
almost as if this whole arena of science is being born as we watch,
Tim says, and NCARs role is right at the leading edge of it.
To mark Boulders emergence from an unusually hot summer (although
certainly not as severe as Europes), 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 CGDlet 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
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.
But Jerry and other NCAR scientists (CGDs 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 weve observed.
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 fielda 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.
Whats happening in the lower atmosphere?
One of the great mysteries of climate change is how temperatures in the
tropospherethe lowest level of the atmosphereare 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-
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 Julythis
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 its the transition
zone between the
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. Thats because cropsas
a whole, lighter in color than forestsreflect more sunlight back
into space, reducing the impact of solar warming. CGDs 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
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 impactboth 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 CGDs Dave Schimel is overseeing a project that will greatly increase observations of carbon sinks and sources, and CGDs Peter Thornton is producing a model of the carbon cycle that can be integrated into the Community Climate System Model at NCAR.
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.
CGDs 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 fallthat may remain more or less the samebut rather whether the character of the precipitation will change in such a way that were 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 CGDs Aiguo Dai, RAPs Roy Rasmussen, and ATDs 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.
NOAAs 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.