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Ed Brook is a geologist. But he has a lot to say about the effect of greenhouse gases on the earth’s climate. All he needs to confirm that effect is glacial ice, basic stratigraphic techniques, and some sophisticated analytical equipment. Glacial ice provides Brook a direct way to measure atmospheric gases from the past 400,000 years. That’s because as the ice forms, it traps air bubbles, and those air bubbles are the equivalent of sealed vials holding samples of the atmosphere. The samples not only show what gases were present in the atmosphere at the time the ice was formed, but also provide information about global and regional climates and atmospheric circulation. These samples can help us understand past climate change and the effects of our current activities on present and future climates. All of the greenhouse gases act like the glass in a greenhouse, hence their name. They allow visible light energy to penetrate the atmosphere and reach the Earth. There the light is converted to infrared energy, which is trapped by the atmosphere. “If human activity has changed the climate, increases in greenhouse gases are certainly a major reason,” says Brook, whose particular interest is the greenhouse gas methane. While not as plentiful as the two major greenhouse gases, water vapor and carbon dioxide, methane’s particular characteristics allow scientists to learn things from its history that they cannot learn from the other two. Methane does not react with the ice surrounding it, but remains relatively unchanged over the thousands of years it’s trapped in the ice. It is produced only in land habitats, not in oceanic, so it provides information specifically about what was happening on land. Because it lasts about 10 years in the atmosphere, variations can be seen in the amount produced from decade to decade. The methane Brook analyzes comes from glacial ice cores from Greenland and three locations in Antarctica. In his lab, small cubes taken from the cores are melted to release the gas bubbles, which are then analyzed in a mass spectrometer or gas chromatograph. Although Brook’s work is still in its early years, the ice cores already have delivered two important messages. The first is that climate is capable of changing rapidly. During the last Ice Age there were times when the Earth’s temperature rose or fell abruptly in only 10 years. “If we can’t understand how that happened, we can’t say we don’t have to worry about it happening in the future because we have a stable climate today,” says Brook. He points out that human civilization evolved during a period of stable climate. If rapid climatic change happened now, how would we cope? The second message is that humans really have changed the atmosphere. Since the turn of the century the amount of carbon dioxide in the atmosphere has almost doubled, and the amount of methane has tripled. The amount of each is larger than at any time in history. “We’ve managed to do something in 100 years that the Earth could not do in 400,000,” says Brook. Historically, the major source of methane was anaerobic bacteria living in northern and tropical wetlands. Higher atmospheric methane was usually the result of large scale global warming. Currently, however, a variety of human activities also result in the production of methane. Rice paddies provide an ideal habitat for anaerobic bacteria, which also thrive in the stomachs of many of our agricultural animals. Normal levels of carbon dioxide in the atmosphere are the result of life on land and in the ocean. The excess that is responsible for the higher levels we have now is essentially all due to the burning of fossil fuels. Brook is using historical data about atmospheric methane and global temperatures for several studies. First, comparing the amount of methane in Arctic and Antarctic ice cores can be used to precisely time past climate events and determine whether they were worldwide or regional in nature. It also helps correlate atmospheric temperature data to climate information gathered from other sources such as pollen, corals, and marine sediments. Second, although the amount of atmospheric methane can be used to synchronize ice cores, there are subtle differences between concurrent samples from the northern and southern hemispheres. These differences indicate the source of increased methane production. Without knowing the source, it’s difficult to understand the relationship between changes in methane amounts and changes in climate. Finally, Brook will look at the relationship between temperature changes and methane concentrations. It’s obviously important to know whether a rise in methane has always followed a rise in temperature, or whether the reverse is true. Although scientists readily admit they do not know everything about how the global climate system works, the rapid climate changes of the past suggest that there may be a threshold level of greenhouse gases that, if exceeded, results in dramatic climate changes. The work on ice cores shows us how significant human impact on the atmosphere has been. It also shows us that the Earth’s climate is capable of functioning in a different and more variable way than it has over recorded human history. “These lessons clearly tell us that the time to reduce greenhouse gas emissions is now,” says Brook. |