Photo:National Snow and Ice Data Center/World Data Center for Glaciology; Photos by William O. Field [top] and Bruce F. Molnia [bottom]

RETREAT: Alaska's Muir Glacier pulled back 12 kilometers and thinned more than 800 meters between August 1941 [top] and August 2004 [bottom]. The global average temperature increased about 0.1 °C per decade in that period.

In the last century or so, humankind has rescripted its role in the natural world. We have learned to treat many dread diseases, feed billions of people, cut canals between continents, and harness the power of the atom. We've bent much of nature to our will.

But we still can't do a darn thing about the weather.

Though we are clearly learning to adapt to extreme weather, it still killed 19 000 people per year on average between 2000 and 2004, according to data gathered at the Center for Research on the Epidemiology of Disasters at the Université Catholique de Louvain, in Brussels. In the United States, just one huge storm, Hurricane Katrina in 2005, killed at least 1600 people and caused damage estimated at US $81.2 billion. Climate change, the alteration in the established weather patterns, is likely to bring still more economic, political, and social havoc. Even the less extreme climate-change scenarios predict stronger and more frequent storms leading to more deaths and greater damage to property. In the more severe scenarios, cities—even entire nations—could disappear under rising oceans; once-productive farmlands parch; and vast swathes of ocean become increasingly acidic, sending out ripples of extinction.

Overwhelming scientific evidence indicates that the Earth has warmed noticeably over the past century and a half. Eleven of the last 12 years were among the warmest since global records began in 1850. The global average temperature is up almost 1°C since that time. Sea level is rising by about 1 centimeter every three years, in part because the oceans absorb much of the increased heat and expand.

What is less clear is the extent of humankind's role in the change. The latest consensus of climate scientists, summarized in last February's report of the Intergovernmental Panel on Climate Change, is that by far the biggest component of the forces currently warming the Earth is the increasing concentration of carbon dioxide, and the source of that carbon dioxide is us.

Even if you differ with the panel's conclusion, you undoubtedly agree that variations in climate don't occur without a reason. Briefly, the observed global warming prior to 1950 is best explained by natural variations in the sun's brightness and volcanic activity. In contrast, the warming since 1950 defies explanation by any known natural cause. Yet it fits quite closely with what we would expect from the well-documented human contribution to increased carbon dioxide. Among the strongest evidence is that the climate is changing with a geographic and altitude-specific pattern consistent with explanations based on greenhouse gases but not with other possible explanations-including such oft-suggested alternatives as variations in the sun's brightness and in the intensity of cosmic rays.

Our influence on climate may be inadvertent, but it is a milestone in civilization's progress. We have, for the first time, the technological capacity to noticeably alter climate on a global basis within a person's lifetime. History suggests that our expanding population and increasing technological ability will cause this capacity to grow with time, not decline. If not because of greenhouse gas emissions, it will be because of something else, such as changes in land coverage or the acidification of the ocean. The question now is: Should we strive to channel this capacity to our benefit, or should we struggle perpetually to avoid having any impact, for better or worse?

I believe the choice is clear. Whether we start today or in a decade, it is inevitable that we will begin to apply our newfound capabilities to actively manage—even engineer—climate. In fact, it could be argued that our limited efforts to reduce greenhouse gases through the Kyoto Protocol represent a primitive form of engineering. It may be many decades before we have sufficient confidence in our skills to apply them more broadly, but there are moral as well as practical reasons to begin doing so. We are wise to invest in technologies that will help us adapt to a changing climate. But by themselves, they will still leave us vulnerable. Engineering the climate could help transform the remaining risks into benefits: increasing global crop yields through longer and more predictable growing seasons, altering large-scale weather patterns to deliver rainfall where it is needed, and limiting the frequency and magnitude of deadly floods and other natural disasters. Such engineering might also mitigate the natural climate change that has been a large and sometimes destructive force in human history—such as the Little Ice Age that is linked to many famines in Europe between the 14th and 19th centuries. Providing food and water to a growing global population and shielding them to the greatest extent possible against the ravages of severe weather is both a moral and a practical obligation. If society has the tools to do this within acceptable risk levels, it should apply them.