CHICAGO--The Earth and its life may be entering a "new era where natural forces are being overwhelmed" by human influences on climate and habitat,  AAAS President James J. McCarthy said at the 2009 AAAS Annual Meeting.

Delivering the annual AAAS President's Address on Thursday, on the eve of the 200th anniversary of Charles Darwin's birth, McCarthy said the world explored by the great naturalist is disappearing, replaced by a planet where human activity has left an indelible mark.

Today, the tools of science--from space exploration to oceanography--clearly show the significant impact that humans have had on the planet. Although some predictions for the Earth's future remain dire, the sobering story assembled by the researchers is resonating with a public eager for action, said McCarthy, an influential climate-change scholar and the Alexander Agassiz Professor of Biological Oceanography at Harvard University.

Political leaders at all levels, from mayors to heads of state, have recognized that "we are now at the moment where we must act on this," he said.

While scientists in the United States have been optimistic that U.S. President Barack Obama will renew efforts to address climate change, others have worried that the growing economic crisis will shift focus and funds away from the climate issue. McCarthy urged Obama to look to the example of another U.S. president--Abraham Lincoln--as a guide to action.

"That was a time when an extraordinary leader was thoroughly distracted, by a terrible war, but he realized that there were other things that were in the nation's interest, that must be pursued, that required scientific inquiry," said McCarthy.

The signs of a changed world are already apparent, McCarthy noted. Since 1950, the incidence of flooding and wildfires has risen dramatically on each continent. Decades-old slabs of sea ice are being replaced by thinning sheets that disappear from year to year. From the poles to the equator, climate change is threatening the livelihoods of some of the world's most vulnerable populations.

For 800,000 years, levels of heat-trapping atmospheric carbon dioxide never rose above 300 parts per million--"until now," said McCarthy. And, he added, the consensus of mainstream researchers and world science organizations is that the trend is driven by human activity.

By coincidence, the roots of the modern climate crisis are also having an anniversary year, said McCarthy. While 1859 may be best known in scientific circles for the publication of Darwin's On the Origin of Species by the Means of Natural Selection, it also marks the debut of the first commercial oil well, the precursor of the modern internal combustion engine, and the first demonstration that carbon dioxide is a greenhouse gas.

The technologies introduced in the 1850s "are those that today look to have significantly altered the planet," said McCarthy at a Thursday morning gathering of international journalists attending the Annual Meeting.

In reflecting on the anniversaries, McCarthy said researchers know more about the mechanisms of evolution than in Darwin's day. "But in addition, we know much more about how the inanimate world, the environment, has changed over time and how inextricably life and its surroundings have co-evolved," he noted.

"Obama's science team is without equal," said McCarthy, "but these people will need all of our support as this new administration moves aggressively to solve the economic and energy security problems our nation faces, and at the same time assume a new role as an international leader in global efforts to curb anthropogenic climate change."

McCarthy has served on and led many national and international groups charged with planning and implementing studies of global change. He served as 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). He also served as lead author of the Arctic Climate Impact Assessment, and vice-chair of the Northeast Climate Impacts Assessment. He was founding editor for the American Geophysical Union's Global Biogeochemical Cycles.

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.

At the end of the 2009 meeting, McCarthy will turn over presidential duties to AAAS president-elect Peter C. Agre and begin a one-year term as chairman of the AAAS Board of Directors. Agre, a 2003 Nobel Prize laureate, is the director of the Johns Hopkins Malaria Research Institute and professor at the Bloomberg School of Public Health.

Visit the AAAS 2009 Annual Meeting site to watch a video of McCarthy's plenary address.