For a long time, the climate science consensus suggested that to avoid increased average surface temperatures beyond those to which our civilization could adapt, we need to reduce emissions 80 percent by 2050. (No one suggested we stop there, but that goal was advocated as a way to avoid tipping points.)

There were voices from the beginning arguing that this was too slow a phase-out. But as Joe Romm has argued, the consensus-seeking nature of the IPCC process tends to downplay and ignore real dangers. It has become obvious that we need to reduce emissions faster than the conventional wisdom of a few years ago suggested.

For example, the rate at which the oceans absorb CO2 has slowed drastically as they become saturated. This suggests another tipping point looms: when the oceans begin to release the CO2 they contain, they’ll become a source rather than a sink. At any rate, if the ability of nature to absorb our emissions has dropped, we have to cut emissions more than we would have.

Similarly, the ice caps are melting at a much faster rate than mainstream predictions suggested. Because water reflects less heat than ice, this is another cooling mechanism that has been reduced. Again, we have to cut those emissions faster than we planned.

How fast do we need to cut emissions?

One obvious answer is, “as quickly as possible,” but that is not really an answer. “Possible” is a political calculation, not just a physical, technological, or economic one. “Failure is not an option” is unfortunately an empty boast. Failure is always an option. People die every minute because failure is an option. I’m sure whoever cut down the last tree on Easter Island had some rationalization. I have a dear personal friend who has all sorts of lung problems, gets pneumonia regularly, and still has not quit smoking, in spite of many attempts. I hope she manages to quit before it kills her, but it looks to me like failure is definitely an option.

What we want with emissions cuts is the best of chance of survival. We must consider the probability that a certain reduction rate will avoid the worst consequences of unchecked climate chaos, and that it will preserve our technological civilization and our ability to continue to grow enough food for everyone.

We must also consider the chances of winning politically. Reducing emissions so slowly that we get caught in feedbacks, which nullify the first cuts, is pointless. Calling for cuts that are so fast we have zero chance of politically attaining them is also pointless. Within that range, we have to weigh how much winning a cut politically improves the probability of survival against how much calling for a certain rate of reduction lowers the chance it will be enacted.

And no, rate of reduction is not the only thing that determines chances of enactment. There is much more to political success than choosing a policy sweet spot. But a really bad policy choice does ensure failure.

We also have to consider that when it comes to emission cuts, “objects in this mirror are larger than they appear.” Consider the 80 percent emissions cut by 2050 that now appears inadequate. Population by 2050 is expected to be about 50 percent higher than at present, so this would require much larger per-capita reductions. Unless we end up with a decades-long depression (certainly possible), we can expect worldwide economic growth per-capita. An 80 percent absolute reduction by 2050 probably requires an 80 percent reduction per unit of GDP by 2030 or 2040.

Let’s look at what the scientific community says about timing. James Hansen, perhaps the world’s leading climate scientist, has suggested we need to cut not just emissions but also atmospheric concentrations down to the equivalent of 350 parts per million of CO2 (PDF). This is below current concentrations of 385 parts per million, and thus would require negative emissions. To lower concentrations back to 350 parts per million, we have to peak soon and start dropping emissions by around 2015. Because Hansen does not say how quickly we must drop down to 350 ppm, we can’t infer the other end of the schedule from this, but obviously it means faster reductions than most standard IPCC reduction scenarios.

The climate modeling group of the School of Earth and Ocean Sciences of the University of Victoria in Canada suggests that we need net emissions of zero by 2050 (PDF). (To be exact, they suggest a cumulative emissions limit that would imply this. Some of their statements quoted in the popular press suggest that this would be 90 percent met by actual reductions, and 10 percent met by sequestration.) Their calculations suggest that even this reduction would still have a 33 percent chance of a temperature rise above 2°Celsius, which is often considered the “safe threshold” level. And many question whether even 2°Celsius is safe.

If we want a better than a 66 percejt chance of survival, and we want to stay well below 2° rather than barely below 2°, it looks like we need to cut even faster. Monbiot’s Heat suggested a 90 percent reduction over 20 years. I’ve proposed a 95 percent reduction over the same time frame. If you turn back to the University of Victoria document (PDF), and turn to the graph “Probability of exceeding 2°C,” you will see that these proposals fit nicely into the space on the bottom left of the graph, which is much safer than 2°.

probability of exceeding 2deg

I have argued in the past that we could reach this target with an increase in GDP from energy and maintenance savings and from various positive externalities (other than reduced climate chaos).

Lester Brown suggests an 80 percent reduction over the course of 10 years (PDF). If we look at the graph again, this suggestion doesn’t add a great deal of safety, given the cost. Maybe Brown is using a different graph. He leaps from Hansen’s ultimate target of a 350-ppm concentration, with no end date specified, to espouse “cut 80 percent by 2020,” without any middle steps that explains why he chose 2020, rather than 2015 or 2030.

Still, an 80 percent reduction in emission rates by 2050 no longer appears adequate. To a lot of people it never did. But even those who supported that rate of reduction before have to admit that with feedbacks occurring much faster than predicted, we have to escalate the rate of reduction. We can argue about how much faster, but that we have to cut significantly faster seems difficult to dispute.

At this point I don’t think there is any doubt we need to be most of the way to zero by 2030. (Brown would say by 2020.) Maybe even more importantly, emissions have to peak and begin to drop by 2015, which means we have to start physically making changes by 2012 at latest, but better this year. If they continue to rise through 2020 as some advocate, the drop back to 350 ppm becomes almost impossible.

We are out of time. We need to move from dirty to clean tech. And if we choose to make that transition, then: “Yes, we can.”