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Dear Tom,
What causes global warming?
Thad Lagorinsky, Chicago

Dear Thad,
Many complex issues are involved, but it is believed that increases in atmospheric concentrations of “greenhouse gases” resulting from human activity have accelerated the rate of global warming in the last 170 years. However, the most recent ice age peaked about 12,000 years ago and global temperatures have risen irregularly since then. Earth’s surface temperatures have risen about 11 degrees in that period and most of that rise was accomplished before the effects of human activities became significant. Global warming involves issues not just in meteorology and climatology but in biology, geology and astronomy as well. We are only now becoming aware of and understanding those issues.