By BJøRN LOMBORG

This time a year ago, passionate climate activists told us that we had just weeks left to save the planet. The looming Copenhagen climate change summit in December 2009 was, they claimed, our “last chance” to avert catastrophic global warming.

How things change. We are now just weeks ahead of this year’s United Nations climate change summit in Cancun, Mexico, yet few people would be presumptuous enough to believe that the gathering will make any real difference to rising temperatures. Copenhagen’s failure dashed hopes of any comprehensive agreement to limit greenhouse gas emissions. After flocking to last year’s meeting and being embarrassed, most global leaders will steer clear of Cancun.

Yet, some things stay depressingly the same. Attendees in Cancun will be singing the same tune that they did last year: Nations must commit themselves to drastic, immediate carbon cuts. This ignores both economic reality and 20 years of experience that tell us that this policy choice is incredibly expensive, utterly ineffective and ultimately politically unsellable.

How did we get to the point where we have fixated on a response to climate change that would do so little good for such a high cost? This goes back much further than last year’s summit in Copenhagen.

Many environmental activists blame so-called “deniers” for halting action on global warming. It is true that the heated discussion about the reality of global warming has created more heat than illumination—and that it has distracted us from having a constructive discussion about the best policy response to global warming.

But environmental activists themselves must accept responsibility for helping block sensible solutions to global warming. They have engaged in alarmist rhetoric and ignored the economic science that shows that carbon cuts are a deeply flawed policy response.

Nearly 20 years after the so-called Earth Summit in Rio de Janeiro (which produced the first international agreement to limit greenhouse gas emissions), no leading industrialized power will find the political will to impose the draconian carbon taxes or order the substantial carbon cuts it would take to markedly lower carbon emissions. This is for a very good reason.

View Full Image EPA How did we get to the point where our response to climate change would do so little good for such a high cost?

Fully implementing the Kyoto Protocol—the last comprehensive carbon cut treaty that the world had—would have cost hundreds of billions every year in lost economic growth. And even if it had been fully implemented across the century—a far shot from what has actually happened—it would only have reduced temperatures by less than one-third of one degree Fahrenheit in 100 years.

The reason for this is that alternative energy technologies are far from ready to take over from fossil fuels. If green technology is not ready to take up the slack, then forcing carbon cuts through taxes will simply hurt growth and development—particularly painful to developing nations.

World-wide public spending on research and development for clean energy technologies is a paltry $2 billion a year. Increasing this to $100 billion a year could be a game-changer. Not only would it be almost twice as cheap as the $180 billion a year cost of fully implementing Kyoto, but the effect of this kind of spending would be hundreds of times greater. But this should not be our only response to global warming. We should also invest considerably more in adaptation to global warming’s effects, and research geo-engineering technologies as a potential backstop.

I was hopeful a year ago that Copenhagen’s failure might be a blessing in disguise, because policy makers might wake up to reality. Instead, it turns out that the chief lesson that they learned 12 months ago was to send bureaucrats rather than global leaders to Cancun in order to avoid another PR fiasco.

Mr. Lomborg is director of the Copenhagen Consensus, a think tank, and author of “Cool It: The Skeptical Environmentalist’s Guide to Global Warming” (Knopf, 2007). His new film, “Cool It,” opens in U.S. theaters nationwide today.