THE NIGHT BEFORE HIS FINAL LECTURE of the semester last spring, Michael Mann drove home from New York City in a blinding downpour. As his car sluiced through a foot of water on the Cross Bronx Expressway, record-setting rainfall totals were being reported up and down the East Coast. (An average April's worth of rain fell on New York in a single day.) No single weather event can be blamed on global warming, but the deluge perfectly fit the sort of extreme storm that climate scientists like Mann predict will become more common in a warming world.
Mann, a distinguished professor of meteorology and director of Penn State's Earth System Science Center, had just given a speech at Fordham University on the underlying physics
of climate change. His harrowing trip back to State College gave him an ideal real-world example to work into his final lecture, which focused on how rising levels of carbon dioxide in the earth's atmosphere are driving changes in agriculture, ocean currents, and weather patterns across the globe.
"Suddenly, over the past 24 hours, people have been talking about this again," Mann told his students. The unusually heavy spring flooding was triggered by very warm air from the Gulf
of Mexico encountering cold air drawn from the Arctic--a mixture that many climate models predict will become more common, resulting in similar downpours. ''A warmer atmosphere
holds more moisture," Mann explained, as several of his students, heads down, typed away on their laptops.