New evidence of global warming
new 300-site survey of borehole temperatures spanning four conti- nents and five centuries has confirmed what most scientists already believe the Earth is getting warmer and the rate of warming has been accelerating rapidly since 1900.
"In terms of climate change, the 20th century has not been just another century," said Henry N. Pollack, University of Michigan professor of geological sciences. "Subsurface rock temperatures confirm that the average global surface temperature has increased about 1 degree C. (1.8 degrees F.) over the last five centuries with one-half of that warming taking place in the last 100 years. The 20th century is the warmest and has experienced the fastest rate of warming of any of the five centuries in our study."
Pollack presented temperature readings from 300 underground boreholes in Europe, North America, Australia and South Africa at the American Geophysical Union meeting held last month.
Pollack is one of several geologists who take the Earth's temperature by lowering sensitive thermometers into boreholes drilled from the surface. Because subsurface rocks preserve a record of actual surface temperature changes over time, boreholes are an important data source for scientists studying global climate change. Short-term changes, such as seasonal variations, penetrate only a few meters underground. Long-term changes on scales of hundreds of years are preserved at greater depths. Since meteorological data has been recorded globally only for the last 100 years or so, borehole temperatures are especially important in determining surface temperature for previous centuries.
Individual borehole temperatures can be skewed by local topography or climate conditions, so Pollack and assistant research scientist Shaopeng Huang merged the readings into continental data ensembles to balance out local effects and let regional trends come through. They then combined all four regions to get a global average. Because meteorologists track long-term climate changes in 100-year intervals, Pollack and Huang also looked for century-long trends in borehole data.
When they compared the average worldwide borehole temperature change with global meteorological records over the last century, they found both recorded a 0.5 degree C. average global temperature increase since 1900. "The ground says the same thing the air says," Pollack explained.
According to Pollack, 80 percent of the total 1 degree C. warming recorded in borehole readings from 1500 to the present occurred after 1750 when people began large-scale burning of coal, wood and other fossil fuels during the Industrial Revolution. Since most warming has taken place after 1750, Pollack believes it is likely a direct result of human activity, rather than a natural climate fluctuation.
"If the upward trend of greenhouse gas emissions continues, we can expect another 1 degree C. increase in average global temperature by 2050," Pollack said. "This estimate is not based on model computations, but a projection of actual data. Our results agree with the estimates of global climate warming issued by the United Nations' Intergovernmental Panel on Climate Change (IPCC) and are fully consistent with the conclusion of the IPCC's scientific panel that human activity is a significant driving force behind global warming."
Pollack's study has been funded by the National Science Foundation and the Czech-USA Cooperative Science Program.
Just to make things more confusing, new computer modeling, also from the University of Michigan, suggests that global warming might not be a product of human activity. Ironically, argues Joyce Penner, professor of atmospheric, oceanic and space sciences, carbon and sulfur emissions can have the reverse effect, serving to cool down the planet.
Penner, an expert in computer simulations of climate change, recently found that, whereas greenhouse gases have led to a warming of 2.5 Watts per square meter (W/m2), aerosols like soot particles and sulfuric acid reflect nearly twice as much energy under certain conditions.
"This effect clouds our understanding of climate change over the last 100 years, but still cannot protect us from the larger increases in greenhouse gases expected in the future," Penner said. "If further research serves to uphold these initial findings, the warming we've seen over the last 100 years may simply be due to natural variability," she said. Penner is presented the work at last fall's meeting of the American Geophysical Union in San Francisco.
When floating freely in air, carbon aerosols from fossil fuel emissions add between .16 and .20 W/m2 to the atmosphere, and thus heat it. But, according to the latest simulations by Penner and her colleagues at the Lawrence Livermore National Laboratory, in Livermore, Calif., and the Centre des Faibles Radioactivites, in Gif sur Yvette, France, carbon aerosols trapped in clouds may cool the earth's atmosphere by as much as -4.4 W/m2 or a net decrease of roughly 0.7 to 2.1 degrees Celsius, provided nothing changed in the future.
Penner said that the models are uncertain because they rely on poorly known estimates for natural sources of aerosols, so the actual number for negative forcing could be as low as -2.4 W/m2. Even so, she said, the results are startling. "I had not expected to get such a large negative forcing from carbon aerosols in clouds. If these results hold up, we are going to have to do a lot more work to understand how climate might change in the future."
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