What does Judith mean by natural?

Gavin Schmidt recently addressed Judith Curry’s 50:50 argument in a Realclimate post. In Judith’s response (which doesn’t really say much) she finishes with,

I do regard the emerging realization of the importance of natural variability to be an existential threat to the mainstream theory of climate variations on decadal to century time scales. The mainstream theory views climate change as externally forced, e.g. the CO2 control knob theory. My take is that external forcing explains general variations on very long time scales, and equilibrium differences in planetary climates of relevance to comparative planetology. But it does not explain the dominant variations of climate on decadal to century timescales, which are the time scales of relevance to policy makers and governments that are paying all this money for climate research.

So, unless I’m misunderstanding Judith she really is suggesting that natural variability could significantly influence warming on timescales as long as a century. Although Judith doesn’t specify, I’ll assume that the implication is that it could provide as much as half of the warming over the course of the instrumental temperature record – since 1880, so longer than a century but let’s just work from there.

So, I thought I’d try to put some numbers to this to see how plausible it is that natural variability could play a significant role in the warming since 1880. Let’s set down some basic numbers. We’ve warmed by 0.9^oC since 1880. In the absence of any other changes, this would increase the outgoing flux by 3 Wm^-2 (). However, we still have an energy imbalance of about 0.5 Wm^-2, so that suggests – assuming that the energy imbalance in 1880 was small – that we’ve seen an increase in radiative forcing of 3.5 Wm^-2 (well forcings plus feedbacks). The change in anthropogenic forcing over the same time interval is between 1 and 3 Wm^-2, with the uncertainty largely a consequence of uncertainties in the aerosol forcings.

So, when Judith suggests that a large part of the warming on century timescales could be natural, what does she really mean? There are two possibilities that I can think of and I’ll try to explain them below.

1. Half of the change in radiative forcing is natural and half anthropogenic – i.e., about 1.75 Wm^-2 each.
   • This would suggest that the aerosol forcing has to be on the high side.
   • If we assume that half the warming is natural and half anthropogenic, then it suggests that the feedback response to natural warming is significantly higher than that due to anthropogenic warming (since the anthropogenic forcing is unlikely to be less than 1 Wm^-2).
   • If half the warming is natural and half anthropogenic, then in fact the TCR would be between 1 and 2^oC and the ECS would be between 1.25 and 2.5^oC (since the anthropogenic forcing is between 1 and 1.75 Wm^-2 and the planetary imbalance would be 0.25 Wm^-2).
   • In some sense, this possibility doesn’t really make much difference as it would simply imply that a large part of the anthropogenic warming has been masked by a potentially large negative aerosol forcing.
2. The alternative is that Judith means that half the warming is natural and half anthropogenic and, consequently, that half the feedback response is natural and half anthropogenic [i.e., where is the anthropogenic forcing]. Let’s consider 3 different possibilities, low, high, and medium anthropogenic forcing.
   • The anthropogenic forcing is around 1 Wm^-2 (very large aerosol effect), and the feedback to natural and anthropogenic warming is around 1.25 Wm^-2 in each case.
     • This would suggest that the feedback is quite high since the natural and anthropogenic warming contributions are around 0.5^oC each.
     • In this scenario the TCR would be around 1.9^oC and the ECS would be around 2.5^oC. Again, it would seem to be a scenario in which aerosol forcing is masking a lot of anthropogenic warming and the long-term warming would be in line with IPCC scenarios.
   • The anthropogenic forcing is around 3 Wm^-2 (very small aerosol effect), and the feedback to natural and anthropogenic warming is around 0.25 Wm^-2 in each case.
     • The feedback is very small. This would appear to make it difficult to explain long-term natural warming since this would seem to require that the feedback exceed the Planck response.
     • The TCR would be around 0.6^oC and the ECS would be around 0.7^oC which would suggest that the equilibrium response to the doubling of CO₂ would lead to less warming than CO₂ alone (overall feedbacks would have to be negative).
   • The anthropogenic forcing is around 2 Wm^-2 (average aerosol effect), and the feedback response to natural and anthropogenic warming is around 0.75 Wm^-2 in each case.
     • The TCR would be just less than 1^oC and the ECS just about 1^oC, so suggests that a doubling of CO₂ would, in equilibrium, be associated with no feedbacks. This seems odd, since the initial assumption results in the feedback response being positive.
     • The feedback response to the natural warming is quite a bit smaller than the Planck response due to the natural warming. This would seem to make long-term natural warming quite difficult. Presumably natural warming can only be long-term if the feedback response exceeds the Planck response.

So, I’ve thrown this together a little fast and when I was thinking of this there were other factors that I’d thought of, but have now forgotten. From what I can see, there isn’t an easy way to construct a plausible scenario in which natural variability can have played a significant role in the warming over the last century or so. Again, this isn’t me saying that it can never do so, just that given the information we have for the last 100 years or so, and the state we’re in today, there doesn’t seem to be an easy way to construct a plausible scenario. If others can think of one, or can add other reasons why it’s implausible, feel free to do so.