Recently Judith Curry has been emphasizing the need to consider the possibility that natural variability may be causing a substantial part of the recent warming trend. The following comment is typical:
In the latest issue of Nature Climate Change, there is an interview with Richard Muller (behind paywall). Its a good interview, with this notable Q&A:
Q: Do you believe the global warming you see is a result of human actions?Neither Curry nor Muller has identified any natural forcings -- solar, volcanoes, cosmic rays -- that they believe have been miscounted. Nor have they identified a shifting of energy within the system -- like the El Nino effect, but over decades of warming -- that might account for the current conditions. They seem to be arguing, purely and simply, that our estimates of natural forcings and natural climate oscillations is limited, so we must, as a matter of acknowledging uncertainty, consider that we may have underestimated these natural effects, which are thought to be small:
RM: I have not a done a scientific study, but my own impression–based on reading the literature-is that some of the warming we have seen is caused by humans. To my mind, you can’t rule out half of the warming being caused by humans, but I think to conclude that most of it is–as the IPCC says–could be an overestimate. This is my personal impression; the other members of the team might feel differently.
Well said, RM.
|Gillett et al. (2012)|
Without reopening the discussion of what Hansen said/meant, I have a question specifically for you, Dr. Curry, if you don’t mind. You have argued that it is premature to attribute most warming to AGW with high confidence, because of uncertainty related to natural variability. Let’s take that as fact. Still, in order to do things like attribution analyses, we need to estimate the contribution of AGW to recent warming. My question: isn’t the central estimate for that contribution still about 100%? Proving greater uncertainty broadens the range of possible values, but unless I am really missing something (possible), it broadens them in both directions. Natural forcings could be positive or negative over the recent past; establishing greater uncertainty does not tell us which. We still do not have a candidate for a “natural” warming influence, with solar forcing flat, and volcanic activity flat. So isn’t the most reasonable way to proceed to continue to calculate the effects of AGW as similar to all of the recent warming, given that it could be more (net negative natural forcing) as well as less (net positive natural forcing)? Until we have a strong natural forcing or forcings in mind, we don’t know which. Is this your thinking, or do you think there is a convincing case to be made that the uncertainty is mostly monopolar in the direction of positive natural forcings?And this follow-up:
Robert | January 9, 2012 at 9:55 am | @Dr. Curry Just to place some numbers around my thought from above (http://judithcurry.com/2012/01/06/the-new-climate-dice/#comment-157346) let’s suppose we consider the current warming, if 100% human-caused, compatible with a climate sensitivity of 1.5C — 4.5C/doubling. Now we suppose that there is a powerful natural variability that is responsible for 50% of the warming trend. We do not know what this might be; we have not been able to detect it, but since we don’t know everything about natural variability, we accept it as possible. Now the observed response of the climate system to human activity is compatible with a climate sensitivity of 0.75C — 2.25C/doubling. But if principled uncertainty, rather than observed natural warming, is the reason for supposing this to be so, then it would seem the opposite proposition is just as reasonable — that absent human influence, a cooling of 50% of the observed warming trend might be unmasked. Such that the actual net forcing producing the current warming is 50% of what we observe humans to be contributing, compatible with a climate sensitivity of 3C — 9C per doubling. If indeed both are possible, then instead of being constrained in a band from 1.5C — 4.5C, the modern climate response would be compatible with a climate sensitivity of 0.75C — 9C. Unless the uncertainty operates in only one direction, for some reason, it would seem that the assumption that current warming is 100% human-caused remains a reasonable working hypothesis, except insofar that if you are right about the greater uncertainty, it implies a much larger “fat tail” of climate sensitivity.Think she'll respond? I have my doubts.