Global warming means more people will die from the heat. Sea levels will rise, and there'll be more malaria, starvation and poverty. Concern has been great, but humanity has done very little that will actually prevent these outcomes. Carbon emissions have kept increasing, despite repeated promises of cuts.
We all have a stake in ensuring that climate change is stopped. We turned to climate scientists to tell us about global warming. Now we need to turn to climate economists to enlighten us about the benefits, costs and possible outcomes from different responses to this challenge.
World leaders are meeting in Copenhagen in December to forge a new pact to tackle global warming. Should they continue with plans to make carbon-cutting promises that are unlikely to be fulfilled? What could be achieved by planting more trees, cutting methane or reducing black soot emissions? Is it sensible to focus on a technological solution to warming? Or should we just adapt to a warmer world?
Much of the policy debate remains focused on cutting carbon, but there are many ways to go about repairing the global climate. Our choices will result in different outcomes and different costs.
The optimal combination of solutions will create the biggest impact for the least money. A groundbreaking paper by economists Eric Bickel and Lee Lane is one of the first - and certainly the most comprehensive - study of the costs and benefits of climate engineering. Deliberately manipulating the Earth's climate seems like something from science fiction. But as President Barack Obama's science adviser, John Holdren, has said, it has "got to be looked at." Many prominent scientists agree.
Dr. Bickel and Mr. Lane offer compelling evidence that a tiny investment in climate engineering might be able to reduce as much of global warming's effects as trillions of dollars spent on carbon-emission reductions.
Climate engineering has the advantage of speed. There is a significant delay between carbon cuts and any temperature drop - even halving global emissions by mid-century would barely be measurable by the end of the century. Making green energy cheap and prevalent will also take a long time. Consider that electrification of the global economy is still incomplete after more than a century of effort.
Many methods of atmospheric engineering have been proposed. Solar radiation management appears to be one of the most hopeful. Atmospheric greenhouse gases allow sunlight to pass through but absorb heat and radiate some down to the Earth's surface. All else being equal, higher concentrations will warm the planet. Solar radiation management would bounce a little sunlight back into space. Reflecting only 1 per cent to 2 per cent of the total sunlight that strikes the Earth could offset as much warming as that caused by doubling pre-industrial levels of greenhouse gases.
When Mount Pinatubo erupted in 1991, about a million tons of sulphur dioxide were pumped into the stratosphere, reacting with water to form a hazy layer that spread around the globe and - by scattering and absorbing incoming sunlight - cooled the Earth's surface for almost two years. We could mimic this effect through stratospheric aerosol insertion - essentially launching material such as sulphur dioxide or soot into the stratosphere.
Another promising approach is marine cloud whitening, which sprays sea-water droplets into marine clouds to make them reflect more sunlight. This augments the natural process, where sea salt from the oceans provides water vapour with the cloud condensation nuclei.
It is remarkable to consider that we could cancel out this century's global warming with 1,900 unmanned ships spraying sea-water mist into the air to thicken clouds. The total cost would be about $9-billion, and the benefits of preventing the temperature increase would add up to $20-trillion. That is the equivalent of doing $2,000 worth of good with every dollar spent.
Many of the risks of climate engineering have been overstated. Marine cloud whitening would not lead to permanent atmospheric changes, and could be used only when needed. Turning sea water into clouds is a natural process. The biggest challenge is public perception. Many environmental lobbyists oppose even researching climate engineering. This is startling, given the manifold benefits. If we care about avoiding warmer temperatures, it seems we should be elated that this simple, cost-effective approach shows so much promise.
Climate engineering could remain a backup option in case of necessity. Or we could put it on the agenda today. In either case, there's a commanding case for its serious consideration. We're on track to being the generation that wasted decades bickering over carbon-emission cuts and failed to stop the harmful effects of warming. That would be a shameful legacy - one that could be avoided by rethinking climate policy.
Bjorn Lomborg is author of The Skeptical Environmentalist and Cool It: The Skeptical Environmentalist's Guide to Global Warming. He is director of the Copenhagen Consensus Center and adjunct professor at the Copenhagen Business School