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.
Why is it important?
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.
Over the past century, global measurements of the temperature at the Earth's surface have indicated a warming trend of between 0.3 and 0.6 degrees C. But many - especially the early - computer-based global climate models (GCM's) predict that the rate should be even higher if it is due to the man-made "Greenhouse Effect". Furthermore, these computer models also predict that the Earth's lower atmosphere should behave in lock-step with the surface, but with temperature increases that are even more pronounced.
What is the "Controversy"?
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.
How do we know the Satellite Data are Correct?
In theory, one could argue that the computer models are accurate, and that the real measurements have some problem. However this is not the case. An incredible amount of work has been done to make sure that the satellite data are the best quality possible. 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.
So What is Going On?
The atmosphere is extremely complex in its behavior. Because of this, finding the correct explanation for the behavior we observe is complex as well. Virtually all scientists will agree that a doubling of the amount of carbon dioxide in the Earth's atmosphere should have some effect on the temperature of the Earth. But it is much less certain how or if we will recognize the effects of this increase. There are several reasons:
- 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.
The Answer Lies Partly in a Better Understanding of Water's Role
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.
And the Intergovernmental Panel on Climate Change (IPCC) agrees:
``Feedback from the redistribution of water vapour remains a substantial uncertainty in climate models...Much of the current debate has been addressing feedback from the tropical upper troposphere, where the feedback appears likely to be positive. However, this is not yet convincingly established; much further evaluation of climate models with regard to observed processes is needed."
Climate Change 1995, IPCC Second Assessment
Images of the Earth, 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.
More Complex Than We Had Thought
Improving our understanding of the potential magnitude and extent of any man-made global warming will require a significant amount of critical scientific investigation, both in space and on Earth, using both observational and computational analysis techniques. It is clear that if we've learned anything in the past two decades, it's that the response and dynamics of the Earth as a complex, interconnected machine are far more detailed, intricate, and complicated than we first envisioned. Through NASA's Earth Observing System, researchers will continue to improve our ability to monitor the Earth system so that we may understand the subtleties of variations in the global atmosphere. NASA's continued direct observations of the Earth will help enable us to sort out the complicated issues of climate variability and change that affect the planet.