The Physical Science Behind Climate Change By William Collins, Robert Colman, James Haywood, Martin R. Manning and Philip Mote The authors were participants in Working Group I of the 2007 IPCC assessment. William Collins is a professor in residence in the department of earth and planetary science at the University of California, Berkeley, and a senior scientist at Lawrence Berkeley National Laboratory and the National Center for Atmospheric Research in Boulder, Colo. Robert Colman is a senior research scientist in the Climate Dynamics Group at the Australian Bureau of Meteorology Research Center in Melbourne. James Haywood is the manager of aerosol research in the Observational Based Research Group and the Chemistry, Climate and Ecosystem Group at the Met Office in Exeter, England. Martin R. Manning is director of the IPCC WG I Support Unit at the NOAA Earth System Research Laboratory in Boulder, Colo. Philip Mote is the Washington State climatologist, a research scientist in the Climate Impacts Group at the University of Washington, and an affiliate professor in the department of atmospheric sciences. Why are climatologists so highly confident that human activities are dangerously warming the earth? Here some of the participants in the most recent and comprehensive international review of the scientific evidence summarize the arguments and discuss what uncertainties remain. For a scientist studying climate change, “eureka” moments are unusually rare. Instead progress is generally made by a painstaking piecing together of evidence from every new temperature measurement, satellite sounding or climate-model experiment. Data get checked and rechecked, ideas tested over and over again. Do the observations fit the predicted changes? Could there be some alternative explanation? Good climate scientists, like all good scientists, want to ensure that the highest standards of proof apply to everything they discover. And the evidence of change has mounted as climate records have grown longer, as our understanding of the climate system has improved and as climate models have become ever more reliable. Over the past 20 years, evidence that humans are affecting the climate has accumulated inexorably, and with it has come ever greater certainty across the scientific community in the reality of recent climate change and the potential for much greater change in the future. This increased certainty is starkly reflected in the latest report of the Intergovernmental Panel on Climate Change (IPCC), the fourth in a series of assessments of the state of knowledge on the topic, written and reviewed by hundreds of scientists worldwide. The panel released a condensed version of the first part of the report, on the physical science basis of climate change, in February. Called the “Summary for Policymakers,” it delivered to policymakers and ordinary people alike an unambiguous message: scientists are more confident than ever that humans have interfered with the climate and that further human-induced climate change is on the way. Although the report finds that some of these further changes are now inevitable, its analysis also confirms that the future, particularly in the longer term, remains largely in our hands—the magnitude of expected change depends on what humans choose to do about greenhouse gas emissions.