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AGU revises position on climate change

WASHINGTON A statement released on January 24 by the worlds largest scientific society of Earth and space scientiststhe American Geophysical Union, or AGUupdates the organizations position on climate change: the evidence for it, potential consequences from it, and how to respond to it.

The statement is the first revision since 2003 of the climate-change position of the AGU, which has a membership of 50,000 researchers, teachers, and students in 137 countries. The society adopted the statement at a meeting of AGUs leadership body, the AGU Council, in San Francisco, California, on 14 December 2007. AGU position statements expire in four years, unless extended by the Council.

Following is the text of the revised statement (also available online at

Human Impacts on Climate

The Earth's climate is now clearly out of balance and is warming. Many components of the climate systemincluding the temperatures of the atmosphere, land and ocean, the extent of sea ice and mountain glaciers, the sea level, the distribution of precipitation, and the length of seasonsare now changing at rates and in patterns that are not natural and are best explained by the increased atmospheric abundances of greenhouse gases and aerosols generated by human activity during the 20th century. Global average surface temperatures increased on average by about 0.6C over the period 19562006. As of 2006, eleven of the previous twelve years were warmer than any others since 1850. The observed rapid retreat of Arctic sea ice is expected to continue and lead to the disappearance of summertime ice within this century. Evidence from most oceans and all continents except Antarctica shows warming attributable to human activities. Recent changes in many physical and biological systems are linked with this regional climate change. A sustained research effort, involving many AGU members and summarized in the 2007 assessments of the Intergovernmental Panel on Climate Change, continues to improve our scientific understanding of the climate.

During recent millennia of relatively stable climate, civilization became established and populations have grown rapidly. In the next 50 years, even the lower limit of impending climate changean additional global mean warming of 1C above the last decadeis far beyond the range of climate variability experienced during the past thousand years and poses global problems in planning for and adapting to it. Warming greater than 2C above 19th century levels is projected to be disruptive, reducing global agricultural productivity, causing widespread loss of biodiversity, andif sustained over centuriesmelting much of the Greenland ice sheet with ensuing rise in sea level of several meters. If this 2C warming is to be avoided, then our net annual emissions of CO2 must be reduced by more than 50 percent within this century. With such projections, there are many sources of scientific uncertainty, but none are known that could make the impact of climate change inconsequential. Given the uncertainty in climate projections, there can be surprises that may cause more dramatic disruptions than anticipated from the most probable model projections.



Contact: Peter Weiss
American Geophysical Union

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