This colloquium was given in celebration of Earth Day on April 22, 2009. Abstract - Largely as a result of our dependence on fossil fuels, Earth's climate is changing. Regional shifts are now evident in precipitation patterns, in storms, in diminished land and ocean ice, in rising sea level, and in the distributions of plants and animals. Advances in technology and advances in science have led both to our current condition and our understanding of its implications. As we look to the future, the wise use of technology and scientific understanding that allows us to decarbonize our energy systems will give us a choice among alternative futures. About the Speaker - James J. McCarthy is Chair of the AAAS and the Alexander Agassiz Professor of Biological Oceanography at Harvard University. He received his Ph.D. from Scripps Institution of Oceanography and B.S. in biology from Gonzaga University. He teaches courses in ocean and climate science and oversees Harvard’s program in Environmental Science and Public Policy. His research interests relate to marine plankton, biogeochemical cycles and climate. He has served on and led many national and international groups charged with planning and implementing studies of global change, including chair of the international scientific committee that establishes research priorities and oversees implementation of the International Geosphere - Biosphere Program from 1986 to 1993; founding editor for the American Geophysical Union’s Global Biogeochemical Cycles; co-chair of the Intergovernmental Panel on Climate Change (IPCC), Working Group II, which had responsibilities for assessing impacts of and vulnerabilities to global climate change for the Third IPCC Assessment (2001); lead author of the Arctic Climate Impact Assessment; and vice-chair of the Northeast Climate Impacts Assessment. He has been elected a fellow of AAAS and the American Academy of Arts and Sciences and a foreign member of the Royal Swedish Academy of Sciences.