Alternative Hypotheses About Climate Change

A distinguished group of nineteen AGU Fellows (names below) have published an article in EOS (PDF, AGU membership required) in which they present three competing hypotheses about climate change, only one of which can be true. Which one do you think is correct?

Humans are recognized as having a major role in influencing environmental variability and change, including their influence on the climate system. To advance scientists’ understanding of the role of humans within the climate system, there remains a need to resolve which of the following three hypotheses is correct:

Hypothesis 1: Human influence on climate variability and change is of minimal importance, and natural causes dominate climate variations and changes on all time scales. In coming decades, the human influence will continue to be minimal.

Hypothesis 2a: Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and involve a diverse range of first- order climate forcings, including, but not limited to, the human input of carbon dioxide (CO2). Most, if not all, of these human influences on regional and global climate will continue to be of concern during the coming decades.

Hypothesis 2b: Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and are dominated by the emissions into the atmosphere of greenhouse gases, the most important of which is CO2. The adverse impact of these gases on regional and global climate constitutes the primary climate issue for the coming decades.

These hypotheses are mutually exclusive. Thus, the accumulated evidence can only provide support for one of these hypotheses. The question is which one?

In a coming post, I'll give their answer and why they think it matters. Meantime, what do you think?

The authors are:

ROGER PIELKE SR., University of Colorado, Boulder;
KEITH BEVEN, Lancaster University, Lancaster, UK;
GUY BRASSEUR and JACK CALVERT, National Center for Atmospheric Research, Boulder, Colo.;
MOUSTAFA CHAHINE, Jet Propulsion Laboratory, California Institute of Technology, Pasadena;
RUSSELL R. DICKERSON, University of Maryland, College Park;
DARA ENTEKHABI, Massachusetts Institute of Technology, Cambridge;
EFI FOUFOULA- GEORGIOU, University of Minnesota, Minneapolis;
HOSHIN GUPTA, University of Arizona,Tucson;
VIJAY GUPTA, University of Colorado;
WITOLD KRAJEWSKI, University of Iowa, Iowa City;
E. PHILIP KRIDER, University of Arizona;
WILLIAM K. M. LAU, NASA, Greenbelt, Md.;
JEFF MCDONNELL, Oregon State University, Corvallis;
WILLIAM ROSSOW, City College of New York, New York;
JOHN SCHAAKE, U.S. National Oceanic and Atmospheric Administration, Silver Spring. Md.;
JAMES SMITH, Princeton University, Princeton, N. J.;
SOROOSH SOROOSHIAN, University of California, Irvine;
ERIC WOOD, Princeton University