Unraveling Two Riddles Of Global Warming
Washington Post, Feb. 21, 2000
Of all the troublesome questions in the study of global warming and potential climate change, none is more forbidding, or more profound, than these:
- Is the observed increase in the worldwide average temperature - around 1.1 degrees Fahrenheit over the past 100 years--genuinely abnormal, or is it well within the bounds of natural variability?
- And is the apparent super-warming of the past two decades - about 0.5 degree - actually a mistake, in view of the fact that upper-air temperature measurements over the same 20 years show no warming at all?
Both questions got closer to definitive answers last week. In the Feb. 17 issue of the journal Nature, Henry N. Pollack of the University of Michigan and others took on the issue of multicentury climate change, which has an aggravating built-in difficulty: Accurate measurements of surface temperatures have been taken only since the end of the 19th century.
So to discern how hot it was before 1850, scientists have to rely on what are called "proxy" indicators of temperature change: width of tree rings, chemical makeup of ice cores, type and amount of pollen in sediments, growth rates of coral and the like. Pollack's group used a different and completely independent proxy indicator: temperature at various depths in 616 boreholes on six continents. These holes, typically 2 to 4 inches wide and a quarter of a mile deep, originally were drilled for geological research, oil exploration or other reasons. Now they provide a handy means of taking the Earth's past temperature. Heat moves two ways through the ground. One is from the surface down, As thermal energy from the planet's sun- and air-warmed crust seeps downward through rock. The other is from the bottom up, as heat from the Earth's molten innards travels upward. By lowering thermometers into these holes, and subtracting the bottom-up signal from the readings, scientists can find evidence of past temperature shifts as they propagate down through the rock. It's a slow process; in 1,000 years, heat pulses penetrate only about 1,600 feet.
"By looking at these temperature disturbances," Pollack said, "we can reconstruct surface conditions" from centuries past by combining results from many sites. In all, 479 of the 616 holes showed average warming of 1.8 degrees F since 1500 (a bit higher than what other proxy measures indicate), with 80 percent of that since 1800, and more than half since 1900. Interestingly, the new five-century findings - which confirm a smaller study the group published in 1998 - suggest that natural temperature variation may be larger than previously thought. Any warming detected during the 16th, 17th and 18th centuries was almost certainly not caused by human society. It is likely to be the product of natural processes - perhaps a recovery from the "Little Ice Age," a cold snap that gripped Europe from 1400 to 1850. Atmospheric carbon dioxide levels did not rise appreciably until the early 19th century. As for the ultra-hot 1980s and '90s, surface measurements from thousands of land-based stations and ships show an increase of about half a degree. But two other, independent sources - weather balloon instruments and satellites that measure the temperature of the lower atmosphere - show no net warming during that period.
This contradiction suggests that either the surface measurements or the satellite/balloon figures are simply wrong. Moreover, it casts doubt on the computerized global climate simulations, or "models," that are used to assess the threat of warming. In general, the models predict that enhanced greenhouse warming should heat the air in the lower troposphere - the layer of the atmosphere closest to the Earth, and the one sampled by balloons and satellite readings. That disparity became so vexing that the National Research Council (NRC) appointed an expert panel to consider the issue. Its report, issued last month, indicates that maybe both trends are right: Something different is happening at the Earth's surface from what is going on only a few thousand feet above.
In the Feb. 18 issue of Science, several of the NRC panel members confirmed and expanded that idea. Physicist Benjamin D. Santer of Lawrence Livermore National Laboratory and colleagues conducted a new analysis showing that "it really is plausible to have temperature trend differences between the surface and the troposphere in the '80s and '90s," as Santer noted. His group compensated for the fact that the surface data are not globally complete, as are the satellite data. Then, they factored into the climate models certain events peculiar to that period--such as the effects of stratospheric ozone depletion and the umbrella-like aerosol clouds blown aloft by volcanoes El Chichon and Mount Pinatubo. With those adjustments, "the predicted change in the troposphere is very much in line with observations," Santer said. "The discrepancy is quite small." In another paper in Science, research meteorologist Dian J. Gaffen of the National Oceanic and Atmospheric Administration and co-authors used a highly respected data set from weather balloons to show that "there has been a greater warming at the surface than aloft in the tropics" since 1979, when the first satellite measurements began. But when they extended their analysis to the 1960s and '70s, "we find that the discrepancy doesn't hold farther back in time," Gaffen said. The results indicate that either "we shouldn't make too much of differences over short periods" such as two decades, or "there may have been some different things going on in the atmosphere since 1979," such as volcano and ozone effects, Gaffen said.