Dear Mr. President,

You take office for the second time at a critical moment. As you may know, the U.S. has just recorded the hottest year in its history, beating the old mark by a full degree; the same year that saw the deep Midwest drought, and the fury of Hurricane Sandy, also witnessed the rapid and unprecedented melt of the Arctic ice pack.

If we are to restrain the rise in the planet’s temperature, it will require strong action from, among others, the planet’s sole superpower. Some of that work will be difficult, requiring the cooperation of Congress. But other steps are relatively easy.

Eighteen months ago some of us wrote you about the proposed Keystone XL tar sands pipeline, explaining why in our opinion its construction ran counter to both national and planetary interests. Nothing that has happened since has changed that evaluation; indeed, the year of review that you asked for on the project made it clear exactly how pressing the climate issue really is.

We hope, as scientists, that you will demonstrate the seriousness of your climate convictions by refusing to permit Keystone XL; to do otherwise would be to undermine your legacy.

Thank you,

James Hansen
Research Scientist
The International Research Institute for Climate and Society
The Earth Institute, Columbia University
Ralph Keeling 
Scripps CO2 Program Scripps Institution of Oceanography
John Harte
Professor of Ecosystem Sciences
University of California
Jason E. Box
Byrd Polar Research Center
John Abraham
Associate Professor, School of Engineering
University of St. Thomas
Ken Caldeira
Senior Scientist. Department of Global Ecology
Carnegie Institution
Michael MacCracken
Chief Scientist for Climate Change Programs
Climate Institute
Michael E. Mann
Professor of Meteorology
Director, Earth System Science Center
The Pennsylvania State University
James McCarthy
Alexander Agassiz Professor of Biological Oceanography
Harvard University
Michael Oppenheimer
Albert G. Milbank Professor of Geosciences and International Affairs
Woodrow Wilson School and Department of Geosciences
Princeton University
Raymond T. Pierrehumbert
Louis Block Professor in the Geophysical Sciences
The University of Chicago
Richard Somerville
Distinguished Professor Emeritus and Research Professor
Scripps Institution of Oceanography
George M. Woodwell
Founder, Director Emeritus, and Senior Scientist
Woods Hole Research Center
Mauri Pelto
Department of Environmental Science
Nichols College
David Archer
Professor, Department of Geophysical Sciences
The University of Chicago
Dr. Ted Scambos
Lead Scientist, National Snow and Ice Data Center
University of Colorado at Boulder
Terry L. Root
Senior Fellow
Stanford University
Alan Robock, Professor II 
Distinguished Professor, Department of Environmental Sciences
Rutgers University