July 19, 2004

The Data Weigh In

New research confirms that temperatures in the lower atmosphere are not behaving as climate models have projected—and are warming far less than expected.

Universally, climate models that are run with increasing atmospheric concentrations of greenhouse gases produce some degree of warming at the earth’ surface, but even more warming above the surface, especially in the layer from 5,000 to 30,000 feet. Models project this warming aloft to be especially strong in the tropical half of the planet (less so for a very small region around the poles). Actual observations of temperature trends in the lower atmosphere, however, don’t confirm these model results, instead showing that warming trends in general decline with altitude.

Why is this important? The atmosphere is an integrated whole, and temperatures aloft are an important determinant of temperatures at the surface. If the models have this wrong upstairs, but right for the area near the surface, they’ve been pretty lucky, or, some might say, pretty “adjusted.”

This discrepancy in models vs. observations forms the crux of one of the major arguments against over-dependence on models for projections of future climate conditions—if the models can’t accurately portray the present, they cannot be relied upon to predict the future.

The hypothesis that the models are still getting it wrong is strongly supported by the new results from a research effort led by David Douglass of the University of Rochester and published in a pair of articles in Geophysical Research Letters (appearing online on July 9, 2004). Two scientists involved in this effort include Paul Knappenberger and Patrick Michaels, familiar to readers of these alerts.

In their first paper, Douglass and colleagues compared the temperature trends from the surface upwards through the lower atmosphere as projected by three state-of-the-art climate models with several different sets of actual temperature observations made at various heights in the atmosphere. The three climate models (Hadley CM3, DOE PCM, and GISS SI2000) were each run with the historic values of a combination of natural (solar variability, volcanic eruptions) and anthropogenic (greenhouse gases, aerosols) climate forcing agents. The results for the past two decades (the period of greatest anthropogenic influences) were compared against observations made during the same time. The observations consist of sets of radiosonde data (collected from instruments attached to weather balloons as they ascend through the atmosphere) as well as temperature data collected by satellites.

Douglass found that while the modeled and observed trends matched well at the surface, everywhere else there was considerable discrepancy with the observations, which in many cases indicated a cooling trend, where the models predicted warming to be occurring (Figure 1).

Temperature Trends

Figure 1. Modeled (dotted lines) and observed (solid lines) temperature trends (10-3K/decade) in the lower atmosphere during two recent decades in four different regions. While the modeled and observed trends match well at the surface, in nearly every case, the modeled trends above the surface are greater than the observed trend (source: Douglas et al., 2004a).

This represents clear evidence that climate models are failing to capture the actual workings of the atmosphere. Since weather systems derive their characteristics (size, strength, precipitation efficiency, etc.) largely from temperature contrasts in the atmosphere, models which fail to accurately portray observed trends there also fail to accurately portray observed weather patterns. This fact applies not only to current climate patterns, but also to the model projections of future ones.

In their second paper, the Douglas research team investigated whether these same set of observations could shed light on the hypothesis that the satellite measurements of temperature trends in the earth’s lower atmosphere are contaminated by temperature trends higher up in the stratosphere. Recently, Fu et al. (2004) suggested in a highly publicized paper in Nature magazine that the reason that satellite temperature measurements of the lower troposphere (as compiled by researchers at the University of Alabama-Huntsville, UAH) showed only about half the warming trend as observations collected at the earth’s surface during the past 25 years, was that in formulating the trends of the lower atmosphere, the UAH researchers failed to take into account a cooling influence from the stratosphere (the atmospheric layer just above the troposphere). Fu concluded that had the stratospheric data been properly handled, the temperature trends in the lower atmosphere would match the surface (and modeled) trends much more closely. We showed, in an earlier World Climate Reprto why this argument was not valid (see http://www.co2andclimate.org/wca/2004/wca_17a.html for details). Douglass adds more proof.

Two completely independent temperature measuring systems are employed to monitor temperature in the troposphere—direct observations made by thermometers carried aloft by weather balloons, and indirect observations made by satellite-borne instruments that record the temperature-dependent microwave emissions from oxygen molecules. These two datasets provide the ability to crosscheck the results against one another. When the temperature trend calculations are done thoroughly and carefully (best accounting for all known data quality issues), the results are remarkably similar—the observed warming in the lower troposphere is about half of that observed at the earth’s surface. As described in Douglass’s first paper, this result runs contrary to climate model projections.

But, the fact that temperature trends from weather-balloon data and from satellite data match well is not new news—we have highlighted this fact for years (e.g. http://www.co2andclimate.org/wca/2004/wca_15f.html). However, no one really knows why the lower atmospheric trends are different from the trends measured at the surface, or why the observed lower atmospheric trends differ from model projections. Fu’s suggestion that it results from unaccounted for stratospheric influences isn’t physically reasonable. Douglass’ work points to other possible causes.

Douglass’s team focused on surface temperatures derived from the balloon-based lower atmospheric observations of temperature and moisture. They then compared the derived surface temperatures with the observed surface temperatures. They found that the trends in the surface temperatures derived from the weather balloons more closely matched the satellite temperature trends than they did the observed surface temperature trends from ground-based weather stations (Figure 2).

Since the derived surface temperatures are largely free of local effects such as urbanization, industrialization, land-use changes, and myriad other data quality issues which plague the surface measurements (see http://www.co2andclimate.org/wca/2004/wca_18c.html for more details), Douglass concluded that these non-climatic influences likely play a large role in the discrepancy between observed surface temperature trends and observed lower atmospheric temperature trends. This also means that climate models are overestimating the warming from atmospheric composition changes. These results confirm similar work that focused only on temperatures in the United States (Kalnay and Cai, 2003, see http://www.greeningearthsociety.org/wca/2003/wca_1b.html for more details)

Temperature Trends

Figure 2. Temperature trends by 5º latitude bands (where data were available) in three different data sets—observed surface data (red circles), derived surface data (cyan circles), satellite-based lower tropospheric data (blue circles). All of these data are mostly independent from each other. Notice how the trends from the derived surface data more closely match the trends from the satellite data than from the observed surface data (source: Douglass et al., 2004b).

Together, the results from these two papers provide strong evidence for three important points:

1) The discrepancy between temperature trends measured at the earth’s surface and those measured in the earth’s lower atmosphere is real.

2) A large part of this discrepancy is likely caused by local, non-climatic influences on surface thermometers (and not by stratospheric contamination of the lower tropospheric data).

3) Climate models including the observed changes to known climate forcing agents (both natural and anthropogenic) are unable to replicate the observed behavior of the temperatures in the lower atmosphere. Furthermore, if local, non-climatic influences are largely responsible for the surface temperature trends, then the climate models are getting the surface trends right for the wrong reasons—indicating a failure at that level as well.

These findings should give pause to anyone relying on climate model results to guide their actions.

References:

Douglass, D.H., Pearson, B.D., Singer, S.F., 2004a. Altitude dependence of atmospheric temperature trends: Climate models versus observation. Geophysical Research Letters, 31, doi:10.1029/2004GL020103.

Douglass, D.H., et al., 2004b. Disparity of tropospheric and surface temperature trends: new evidence. Geophysical Research Letters, 31, doi:10.1029/2004GL020212 .

Fu, Q., et al., 2004. Contribution of stratospheric cooling to satellite-inferred tropospheric temperature trends. Nature, 429, 55-58.

Kalnay, E., and Cai, M., 2003. Impact of urbanization and land-use change on climate. Nature, 423, 528-531.




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