TCS Daily


The Air Up There - Is It Hotter?

By Sallie Baliunas - November 21, 2003 12:00 AM

If human activities are having a dramatic effect on globally-averaged temperature, then the temperature in the low atmosphere would be rising at a rate faster than at the Earth's surface. A flurry of recent studies continues to round out the picture and suggests that alarmism about catastrophic anthropogenic global warming is more hype than scientific fact.

The best analysis of air temperature over the last 25 years is based on measurements made from satellites and checked with information from weather balloons. That work, conducted by J. Christy and R. Spencer at the University of Alabama at Huntsville (UAH), shows a small global warming trend. Even if the small trend were entirely human-caused -- an unlikely possibility because temperature exhibits many naturally-caused changes -- it contradicts the forecasts of extreme, human-made global warming.

 

It's not surprising, then, that the satellite measurements are intensely studied and debated.

 

Let's start with some background. There is concern that the air's increasing content of carbon dioxide and other greenhouse gases, mainly from the burning of fossil fuels, may cause substantial global warming. Because naturally-caused temperature changes have always occurred and will continue to occur, the human effect must be searched for against that varying temperature backdrop, preferably in areas of the climate system especially sensitive to human-caused warming.

 

Nearly all computer simulations of climate say that the air layer at a height from just above the surface to about five miles -- called the troposphere -- is very sensitive to human-made global warming. Thus, one important test of the human effect -- a stronger warmer trend, especially in the low troposphere compared to the surface -- should be obvious in reliable balloon and satellite observations that have been made from 1978.

 

Specifically, near the surface, the globally-averaged temperature constructed from thermometer readings scattered across the globe rose about 0.4 C during the last 25 years. That is the period in which the air's concentration of human-produced greenhouse gases has been rapidly increasing. The low troposphere should show a greater warming trend -- about 0.5 C over the last 25 years -- if the human-made global warming effect is pronounced.

 

Three new analyses of troposphere temperatures have appeared in the publications Science and Journal of Oceanic and Atmospheric Technology. They all start with the same set of measurements made from satellites, but find different results. Because not one but a series of satellites has collected the data, corrections need to be made to the measurements from each instrument to produce a precise record of temperature going back over two decades. How to find the best result?

 

The first of the three recent analyses of the satellite measurements appeared in Journal of Oceanic and Atmospheric Technology. It continues the UAH work by Christy and Spencer. It is solidly-based because its results were checked by careful comparison to good measurements from the weather balloons. The UAH team finds a small warming trend of approximately 0.07 C per decade in the low troposphere, with a few hundredths degree C uncertainty in the last 25 years. In an independent study by NOAA researchers published earlier this year in Journal of Climate, good weather balloon information agrees with the UAH satellite analysis.

 

That observed trend is much cooler than estimates of the human-made trend, according to the computer simulations.

 

The remaining two studies consider the same satellite measurements and find results consistent with computer-based forecasts of globally-averaged human warming. But those two studies also produce contradictory results, indicating the small temperature trend from UAH is the most reliable.

 

The second of the three analyses of satellite data, developed by a team led from Lawrence Livermore Labs and appearing in Science, claims to find a substantial rise in the height of the top of the troposphere, called the tropopause. The increase in height of the tropopause nearly matches that predicted by computer simulations for human-made global warming resulting from a significant warming in the troposphere below.

 

But that second analysis contains within itself its own counterargument, which wasn't mentioned in the study. The temperature of the troposphere -- presumed to be the cause of the observed tropopause rise -- is an easier and more direct measurement to make than the difficult-to-measure (and, for the computer simulations, to estimate) changing height of the tropopause. The temperature trend derived from the second analysis of the satellite data shows a small warming trend in tropospheric temperature, which agrees with UAH's trend within the uncertainties. The conclusion is that the measurement and modeling of changes in the tropopause height may be too uncertain to use in the question of evaluating the size of the human-made warming trend.

 

The third analysis of the satellite data, made by a team led from the University of Maryland and also published in Science, claims to see a significant warming trend in the troposphere, consistent with forecasts of a human-made enhanced greenhouse effect. But it, too, seems uncertain, as evidenced by the following.

 

The satellite instruments sample the air temperature often enough to see afternoon, sun-heated warmth and evening, sun-absent cooling. This third analysis incorrectly shows cool temperatures in the early to mid-afternoon and warm temperatures after sunset. That odd result seems to arise from the omission of an important temperature correction owing to calibration errors among satellite instruments, noted five years ago by the UAH team, and independently verified by Remote Sensing Systems in California, whose work is just appearing in Journal of Climate. Also, no independent balloon comparisons for cross-checking results were provided to readers in that second Science paper.

 

Thus, the UAH analysis, which has been thoroughly scrutinized by many independent researchers and measurements, shows a small temperature trend in the low troposphere. It has been checked by good weather balloon measurements, and may be the most reliable indicator of the temperature of the low troposphere of the last 25 years.

 

The small troposphere temperature trend indicates that the human-made part of the warming trend at the surface has been exaggerated by at least a factor of two to three. Adjusting forecasts downward by the same amount suggests a human-made global warming trend of less than 1 degree C over the next 100 years, an amount that would be lost in the background of natural change, thereby answering panic with scientific facts.

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