Accurate "Thermometers" in Space
October 2, 1997
Just how accurate are space-based measurements of the temperature of the Earth's atmosphere? In a recent edition of Nature, scientists Dr. John Christy of the University of Alabama in Huntsville, and Dr. Roy Spencer of NASA/Marshall describe in detail just how reliable these measurements are.
The question is very important, as these temperature measurements from satellites in space are one of our most important windows into measuring and understanding the phenomenon of Global Warming.
Unlike the surface-based temperatures, global temperature measurements of the Earth's lower atmosphere obtained from satellites reveal no definitive warming trend over the past two decades. The slight trend that is in the data actually appears to be downward. The largest fluctuations in the satellite temperature data are not from any man-made activity, but from natural phenomena such as large volcanic eruptions from Mt. Pinatubo, and from El Niño. So the programs which model global warming in a computer say the temperature of the Earth's lower atmosphere should be going up markedly, but actual measurements of the temperature of the lower atmosphere reveal no such pronounced activity.
Recent claims to the contrary by Hurrell and Trenberth have been shown to be false for a number of reasons, and are laid to rest in the September 25th edition of Nature (page 342). The temperature measurements from space are verified by two direct and independent methods. The first involves actual in-situ measurements of the lower atmosphere made by balloon-borne observations around the world. The second uses intercalibration and comparison among identical experiments on different orbiting platforms. The result is that the satellite temperature measurements are accurate to within three one-hundredths of a degree Centigrade (0.03 C) when compared to ground-launched balloons taking measurements of the same region of the atmosphere at the same time.
See what the satellites reveal about climate and temperature at your house over the past two decades!!
- First, the influence of a man-made doubling of the amount of carbon dioxide in the atmosphere is small compared to the Earth's natural cooling rate, on the order of only a percent.
- Second, there is a much more important greenhouse gas than carbon dioxide, namely water vapor. Water vapor over the Earth is extremely variable, both in space and in time.
- Third, the ways in which clouds and water vapor feed back and ultimately influence the temperature of the Earth are, at best, poorly understood.
- Fourth, while the whole Earth is indeed in a state that scientists describe as "radiative equilibrium," where the incoming sunlight equals the outgoing infrared radiation to provide a roughly constant overall temperature, the surface is far from this radiative balance condition. Evaporation and convection processes in the atmosphere transport heat from the surface to the upper troposphere, where it can be much more efficiently radiated into space since it is above most of the greenhouse-trapping water vapor. So in short, it is this convective overturning of the atmosphere - poorly represented in computer models of global warming - that primarily determines the temperature distribution of the surface and upper troposphere, not radiation balance.
A computer model is only as reliable as the physics that are built into the program. The physics that are currently in these computer programs are still insufficient to have much confidence in the predicted magnitude of global warming, because we currently don't understand the detailed physical processes of clouds that will determine the extent and nature of water vapor's feedback into the Earth's temperature.
Images of the Earth, such as this one in the infrared, tell us much about the distribution of water vapor. Areas within the Earth's atmosphere that are extremely dry, especially in the tropics, can act as large "chimneys" that allow energy to freely radiate into space, enhancing the cooling of the Earth. The effects of the tropical dry troposphere are poorly understood, and currently are not well-incorporated into computer models of global warming.
Learn more about efforts to study
the global distribution of
water vapor and its effects
on weather and climate
- How dry is the Tropical Free Troposphere?
- Is the Earth's Temperature Up or Down or Both?
- Global Climate Monitoring: The Accuracy of Satellite Data
- Today from Space: Near Real-Time Data and Imagery of the Earth's Atmosphere
- Studies of Climate and Weather from the NASA/Global Hydrology and Climate Center in Huntsville, Alabama