Nearly every plan for dealing with disastrous climate change depends on there being enough time to boost energy efficiency, redesign urban systems, change to cleaner transportation, and so forth, all to reduce greenhouse gas emissions before it's too late. If we don't make it, we likely hit a climate transition point where the global climate system finds a new stable state -- a "tipping point," if you will, into a climate this planet hasn't seen in a million years or more. The answer is simple, if daunting: we need to make sure that we start reducing anthrogenic carbon emissions before we get to that tipping point.

Probably the best example of what a reasonable greenhouse reduction plan looks like comes from Robert Socolow of Princeton University. His "stabilization wedges" idea has gotten some attention of late due to a brief appearance in An Inconvenient Truth, but he's been writing about the idea for several years. With Socolow's plan, no one solution dominates. Well-understood technologies and practices like energy efficiency, renewable energy, biofuels, urban redesign, carbon sequestration, nuclear power, and so forth, all play a role. The stabilization plan takes fifty years to carry out, but starts to reduce greenhouse gases in relatively short order.

But what if we don't have ten years to get started and fifty years to make it happen?

The latest word from NASA's Goddard Institute for Space Studies (which specializes in studying the Earth's environmental systems) is that the Earth is currently a degree or so away from its million year temperature peak, and that another degree of increase could hit the climate tipping point. How long would that take? We've seen temperature increases of 0.2-0.3 degrees per decade over the past thirty years, so -- if NASA is right, and if nothing changes -- we'd hit the point of no return by 2040-2050 (or, to be technically correct, no return for tens of thousands of years).

But there's a twist: nothing we do will have an immediate result. The slow pace at which the planet's temperature adjusts to perturbations, or Earth's thermal inertia, means that we're only now seeing the temperature results from twenty or thirty years ago. Even if we were to stop putting out any anthrogenic greenhouse gases today, thermal inertia would guarantee that we'd still see temperature increases for at least the next twenty years.

This means that we're looking at an increase of at least another half degree or so, guaranteed. If plans like stabilization wedges take another decade to get going, that's roughly another quarter-degree increase. And if the emissions reduction plans don't work as well as intended or see technical or political delays, or if we have faster-than-expected temperature increases, we may end up hitting that one degree increase despite taking all the right steps now.

Now, it's possible that one degree is insufficient to push us into a climate tipping point (and previous studies have suggested that an increase of three to six degrees would be necessary). It's also possible that a brief period at "tipping point" temperatures may not be enough to make the change stick -- so that we hit the danger point, but efforts long underway pay off at the right time to pull us back from the brink.

But if we do face an incipient global climate disaster, we have one last card to play: geoengineering.

Geoengineering solutions (sometimes referred to as "terraforming Earth") are the science fiction-sounding ideas that try to make large-scale adjustments to the geophysics of our home planet: blocking a fraction of incoming sunlight with a giant mirror in space: using genetically-engineered plants and microbes to remove atmospheric carbon dioxide and methane at a faster-than-natural rate; seeding the atmosphere with fine particles to reflect sunlight away; dumping iron into the oceans to stimulate the growth of carbon dioxide-devouring plankton; and more. None of these would be done easily, or cheaply; quite possibly, none of these could be done effectively, or without grossly negative results (for example, a too-large plankton bloom has the potential to render parts of the ocean sterile).

In fact, none of the likely geoengineering proposals would be devoid of possible consequences that could make an already-bad situation even worse. We still know too little about the nuances of geophysical systems to be confident in our ability to engineer changes without a large risk of disaster. This is enough to make some otherwise tech-friendly environmentalists reject the notion of geoengineering out of hand. (They also worry that consideration of geoengineering would play into the hands of "do nothing" denialists, but I doubt that many people would be swayed by the argument that spending tens of billions of dollars to put a giant mirror in space in 20 years is a wiser user of resources than shifting to renewable energy and high-efficiency vehicles now.)

But while I consider global warming-induced climate disruption the single greatest problem facing us today, and believe that it should receive far more attention and mitigation effort than it does, I'm willing to look at what we do in case our best efforts fail.

The question isn't whether geoengineering is a better approach than more conventional ideas like stabilization wedges -- it's not, and it won't be without decades more research into how the Earth works, decades that we don't have. Geoengineering solutions are drastic remedies for disastrous situations. At the very least, we should be able to weed out the geoengineering proposals that would be likely to cause even greater problems than they solve.

The early examination of options we'd rather not use is vital. If the climate collapses faster than expected, or if our efforts fail (or are blocked by recalcitrant leaders), we will see people desperate for survival trying out these kinds of last-ditch solutions. It would be a good idea if they knew what the consequences could be before they choose which one(s) to try. If the planet faces disaster, not everyone will be willing to simply throw up their hands and say, "well, we're boned!" -- some of us will do what we can to catch human civilization before it's gone completely over the cliff.