Still, you may think the idea of intentionally modifying the climate is frightening or even repellent. If so, you're not alone—it would be a high-risk endeavor and raises the ire of environmentalists and atmospheric scientists alike. Some of our feelings about intentional modification—climate management—stem from the noble but naive expectation that all human influence on the climate can be eliminated. In truth, today we have likely already introduced irreversible ecological changes around the world. Among other things, as warming drives species northward, those in the polar regions have nowhere to go and may be long extinct before our efforts can cool the planet enough for them to survive.

The real debate is not how to eliminate all human influence—that is an unrealistic and perhaps even meaningless endeavor. The more relevant issue is how much human influence our planet and its inhabitants can tolerate. Surprisingly, the "less is better" conventional wisdom on this point is overly simplistic.

Suppose we accept, as many climate experts have, that a realistic goal for the next few decades is to halt the growth of greenhouse gas levels rather than reduce it. In choosing the carbon dioxide level to stop at, it is quite possible that we will find that an atmospheric carbon dioxide level of 550 parts per million—about twice the preindustrial amount and a level we're likely to surpass before the end of this century—will alter precipitation patterns over some huge swath of the globe to the relative benefit of farmlands, population centers, and globally important ecosystems, as compared to a little less or a little more carbon dioxide, say, 500 ppm or 600 ppm. Or we might find something else entirely. The point is, we won't know unless we ask the question and do the research.

Becoming climate managers will be one of the most difficult things human beings have ever done. It took relatively simple technology—cars and fossil-fuel power plants—to get to this point. It will take far more sophisticated stuff to get us to where we can confidently start deliberately altering climate. Such "geoengineering" technologies are mostly just fantasy now. The roster of possibilities ranges from the aeronautical to the agricultural: putting up space shields that cover billions of square meters; using chemicals to reflect sunlight or increase Earth's cloud cover; stimulating massive growth of phytoplankton in the oceans; and huge reforestation projects [see illustration, "Nine Ways to Cool the Planet"]. These sound farfetched, but as is true of most multigenerational technology development efforts, our early ideas probably offer only a glimpse of what will eventually unfold.

There is now a small but growing will to support geo engineering research, despite entrenched reservations about the idea. Yet, as difficult as it would be to develop geoengineering technologies, deploying them would be a hugely more challenging affair, requiring not just the engineering technologies but breakthroughs in climate forecasting, systems management, global politics, economics, and social sciences.

Our tools and skills are clearly insufficient for climate management today. The consequences of mismanaging climate would certainly be global and could be catastrophic. We lack the scientific understanding to accurately predict the results of intentionally modifying the climate. And today we are missing the political system even to decide what sort of climate we want to strive for. But now is the time to commit to developing the tools we'll need. One of the hidden dangers of delaying such a commitment is that gaining too much knowledge about the coming climate may be an impediment to making a better one. Our current scientific understanding is just beginning to support reliable climate predictions at spatial scales needed to determine which nations will benefit overall from the climate we are heading for, and which will suffer.

But within a decade or two—perhaps sooner—advances in our understanding of climate and the inevitable increase in computational power will likely let us predict who wins and who loses in considerable detail. At that point, those nations that believe they are winners will lose the incentive to support climate management efforts that lessen their advantage. Any international consensus for action will dissolve. And progress toward global solutions will cease. The upcoming 10- or 20-year time frame may well provide us our last opportunity to establish a long-term international agreement on how to manage climate. It is a grace period we mustn't squander.