Why Global Warming is Undeniable

The last half century was warmer than anytime in the last 1,300 years; 11 of the past 12 years are the hottest on record since reliable record-keeping began; glaciers and polar ice are vanishing; concentrations of carbon dioxide are 35 percent higher than preindustrial levels. 

What's the probability that global warming is mostly caused by humans? In just five years, the Intergovernmental Panel on Climate Change has raised its estimate of that probability from 66 percent to 90 percent. Better climate models, better science, and more and better data essentially prove that, as Walt Kelly's Pogo put it on Earth Day in 1971, "We have met the enemy, and he is us."

William Collins of Berkeley Lab's Earth Sciences Division, Robert Colman of the Australian Bureau of Meteorology, James Haywood of the United Kingdom's Met Office, Martin R. Manning of the National Oceanic & Atmospheric Administration, and Philip Mote, Washington State Climatologist, were participants in the scientific working group section of the IPCC's 2007 report. The five authors summarize the group's evidence and conclusions in the August edition of Scientific American, whose editors also summarize the report's sections dealing with the consequences of warming and what needs to be done.

Scientists model interactions of variables as diverse as clouds and sea ice, volcanoes, wildfires, and fossil-fuel soot, the warming of the oceans and the atmosphere, and many other factors. Today's different models not only do a good job of "predicting" past climate but essentially agree on the future: "Even if emissions were immediately reduced enough to stabilize greenhouse gas concentrations at current levels, climate change would continue for centuries."

There will be change, but how bad? Decisions we make now, or fail to make, will lead to discomfort at best, or to disaster. To assess the effects of any decisions requires better climate models, based on better climate science. Collins is establishing a new Department of Climate Science based in the Earth Sciences Division to meet this challenge.

— Paul Preuss