Fingerprints in the Sky
For a physicist and atmospheric scientist, Ben Santer of Lawrence Livermore National Laboratory has an unusual specialty—he is a fingerprint expert.
Like a police detective, Santer looks for unique identifying marks that can be used to determine responsibility. But instead of looking for criminals, Santer is looking for specific, quantitative evidence of human influence on changes in the Earth’s climate. The “fingerprints” he tries to match are patterns of data from observations of the atmosphere and from computer simulations.
“We examine the output from the computer models and compare this with observations,” Santer says. “In the model world (unlike the real world!) we have the luxury of being able to change one factor at a time, while holding the others constant. This allows us to isolate and quantify the effect of individual factors.”
One of the most dramatic applications of this “fingerprinting” technique recently answered the question of why the tropopause has been rising for more than two decades. The tropopause is the boundary between the lowest layer of the atmosphere—the turbulently mixed troposphere—and the more stable stratosphere. The “anvil” often seen at the top of thunderclouds is a visible marker of the tropopause, which lies roughly 10 miles above the Earth’s surface at the equator and 5 miles above the poles.
The average height of the tropopause rose about 200 meters between 1979 and 1999. In their first comparison of observed data with computer models, Santer and his colleagues concluded that the increase in tropopause height was driven by the warming of the troposphere by greenhouse gases and the cooling of the stratosphere by ozone depletion1. But to quantify the influences (or “forcings” in climate jargon) even further, they considered three anthropogenic forcings—well-mixed greenhouse gases, sulfate aerosols, and tropospheric and stratospheric ozone—as well as two natural forcings—changes in solar irradiance and volcanic aerosols—all of which are likely to influence tropopause height. 2
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Figure 1 |
The observational data were compared with seven simulation experiments using the Parallel Climate Model (PCM). In the first five experiments, only one forcing at a time was changed. In the sixth experiment, all five forcings were included simultaneously. The seventh experiment used only the two natural forcings. To improve the reliability of the results, each experiment was performed four times using different initial conditions from a control run. This methodology allowed the researchers to estimate the contribution of each forcing to overall changes in atmospheric temperature and tropopause height.
The results of the analysis showed that in the mid-1980s, the combined five forcings (ALL) diverged from the natural forcings (SV), showing the dominance of human influences (Figure 1A). Of the five forcings, greenhouse gases (G) and ozone (O) played the biggest role (Figure 1B). Major volcanic eruptions can be seen to lower tropopause height sharply by cooling the troposphere and warming the stratosphere, but this effect is short-lived. The analysis concludes that about 80% of the rise in tropopause height is due to human activity. The model-predicted fingerprint of tropopause height changes was statistically detectable in two different observational data sets, NCEP and ERA (Figure 2).
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| Figure 2 |
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Michael Wehner of Berkeley Lab’s Computational Research Division, a
co-author of the Science paper, points out that it would have been very difficult
to estimate the relative contributions of different forcing mechanisms without
the very large ensembles of climate model experiments conducted by DOE-funded
researchers at the National Center for Atmospheric Research and made possible
by NERSC and other DOE and NSF supercomputing facilities. Wehner manages the
largest single collection of publicly available climate model output, the
DOE Coupled Climate Model Data Archive (http://www.nersc.gov/projects/gcm_data/),
which is stored in NERSC’s HPSS system.
Looking at the tropopause data as part of the bigger climate picture, Santer said, “Tropopause height is an integrated indicator of human-induced climate change. It reflects global-scale changes in the temperature structure of the atmosphere. It is also consistent with results from other studies that have identified anthropogenic fingerprints in a range of different climate variables, such as ocean heat content, surface temperature, sea-level pressure patterns, and Northern Hemisphere sea-ice extent.”
“The challenge for the future,” Santer added, “is to take a closer look at the internal consistency of all these climate change variables. Our present work suggests that this story is a consistent one, but there are lots of details that need to be worked out.”
Santer received the Department of Energy’s 2002 Ernest Orlando Lawrence Award for “his seminal and continuing contributions to our understanding of the effects of human activities and natural phenomena on the Earth’s climate system.”
Research funding: BER, NOAA
1B. D. Santer, R. Sausen, T. M. L. Wigley, J. S. Boyle, K. AchutaRao, C. Doutriaux, J. E. Hansen, G. A. Meehl, E. Roekner, R. Ruedy, G. Schimdt, and K. E. Taylor, "Behvaiour of tropopause height and atmospheric temperature in models, reanlyses, and observations: Decadal changes," J. Geophys. Res. 108, 4002 (2003).
2B. D. Santer, M. F. Wehner, T. M. L. Wigley, R. sausen, G. A. Meehl, K. E. Taylor, C. Ammann, J. Arblaster, W. M. Washington, J. S. Boyle, and W. Brüggemann, "Contributions of anthropogenic and natural forcing to recent tropopause height changes," Science 301, 479 (2003).