Benjamin Santer

Human-caused climate change is not a hypothetical future event. It is real, and we are experiencing it in our lifetimes. Despite compelling evidence of human effects on global climate, there is a continuing need for scientists to explain “how we know it’s us.” The first part of this session will summarize the scientific underpinning for the “discernible human influence” conclusions of the Intergovernmental Panel on Climate Change and the U.S. National Academy of Sciences. It will cover natural and human influences on the climate system, observations of climate change in the atmosphere and oceans, and the "fingerprint" methods scientists have used to study the causes of climate change. This part of the session will show that the climate system is telling us an internally—and physically—consistent story. The message in this story is that observed changes in many different (and independently-measured) aspects of the climate system cannot be explained by natural causes alone.

Studies of the causes of climate change typically rely on computer models of the climate system. Such models are the only tools we have for attempting to help us understand the size (and geographical and seasonal distribution) of the climate changes we are likely to experience over the 21st century. But not all computer models are equally adept at capturing key features of the present-day climate. How do we confront models with reality and evaluate model skill? Should models with better performance in reproducing today’s climate be regarded as more trustworthy predictors of 21st century climate change? Is it easy to identify the “top 10” climate models in the world? How should decision-makers—and scientists interested in studying the impacts of climate change—use and interpret information on the strengths and weaknesses of different climate models? Can we find clever ways of reducing uncertainties in projections of future climate change? Can scientists make informed statements about human contributions to changes in the frequency and/or intensity of extreme events? These are a few of the questions that will be addressed in the second and final part of the session.