The theory of global warming is solid, staid and uncontroversial. All things considered, if we double the concentrations of CO2 in the atmosphere, temperatures should increase by 1.1C over what they otherwise would have been.
The theory of atmospheric sensitivity is a different story. The political controversy that has raged since 1988 centers on the idea that our atmosphere is sensitive to changes and that changes produced by humanity–in particular our emissions of greenhouse gases–will cause more warming than just the 1.1C from the emissions themselves.
We don’t know what sensitivity is. In fact, there is more than one type of sensitivity and more than one definition. That doesn’t help matters.
Real Climate, the ‘voice’ of the climate establishment (that’s not meant to be a criticism, btw) has two posts up now on sensitivity. In their first post they acknowledge the difficulty of definitions:
“In practice, people often mean different things when they talk about sensitivity. For instance, the sensitivity only including the fast feedbacks (e.g. ignoring land ice and vegetation), or the sensitivity of a particular class of climate model (e.g. the ‘Charney sensitivity’), or the sensitivity of the whole system except the carbon cycle (the Earth System Sensitivity), or the transient sensitivity tied to a specific date or period of time (i.e. the Transient Climate Response (TCR) to 1% increasing CO2 after 70 years). As you might expect, these are all different and care needs to be taken to define terms before comparing things (there is a good discussion of the various definitions and their scope in the Palaeosens paper).”
The post talks about some recently published papers and ends with what appears to me to be a basic concession–that current thinking places sensitivity within a lower range than previously thought.
The IPCC has held that the likely range of sensitivity values are between 1.5C and 4.5C, writing in their Fourth Annual Report (known as FAR):
“The likely range for equilibrium climate sensitivity was estimated in the TAR (Technical Summary, Section F.3; Cubasch et al., 2001) to be 1.5°C to 4.5°C. The range was the same as in an early report of the National Research Council (Charney, 1979), and the two previous IPCC assessment reports (Mitchell et al., 1990; Kattenberg et al., 1996). …”
we conclude that the global mean equilibrium warming for doubling CO2, or ‘equilibrium climate sensitivity’, is likely to lie in the range 2°C to 4.5°C, with a most likely value of about 3°C. Equilibrium climate sensitivity is very likely larger than 1.5°C.
For fundamental physical reasons as well as data limitations, values substantially higher than 4.5°C still cannot be excluded, but agreement with observations and proxy data is generally worse for those high values than for values in the 2°C to 4.5°C range.”
Real Climate concludes its first post by writing “In the meantime, the ‘meta-uncertainty’ across the methods remains stubbornly high with support for both relatively low numbers around 2ºC and higher ones around 4ºC, so that is likely to remain the consensus range.”
Their second post details recent work that attempts to work around our ignorance of cloud feedbacks and comes up with a high figure (4C) for sensitivity that doesn’t include cloud effects. (I don’t find it convincing–but I’m not a scientist and my lack of conviction on this is based on other data, not that which they looked at–caveat lector.)
Now, the very probable reason that Real Climate has published two posts on climate sensitivity at this time is that conflicting work has recently seen the light of day that points at lower levels of sensitivity. (This is the way the political fight around climate change works–the activists have a ‘rapid response team’ that springs into action when questions arise or conflicting data is published. There in all honesty appears to be a coordinated campaign to drive an agenda, something that they in turn accuse skeptics of doing.)
Matt Ridley, author of The Rational Optimist, highlighted recent work by Nic Lewis, who used observational data to postulate that a doubling of CO2 will lead to a warming of 1.6°-1.7°C (2.9°-3.1°F). Ridley’s piece was also published in the Wall Street Journal. The articles (and a vehement, if knee-jerk, response by Joe Romm, was discussed at Bishop Hill’s blog here, here, here and elsewhere.
My own contribution to the debate, published in my other weblog here, is based on decidedly lower math. I noted that during the recent (hotly debated) plateau in temperatures, humankind has managed to emit one third of all the greenhouse gases they have ever spit into the atmosphere–without any tangible effects on temperatures. Now, climate science allows for uneven steps in temperature change, and I am perfectly comfortable with that. The current warming period is certainly characterized as a sawtooth form imposed on a rising trend. It is conceivable that this is just another pause that will be followed by another period of temperature rises.
But this pause in the temperature rises has lasted longer than previous pauses. Since 1998 there has been little if any net rise in temperatures. Should this pause continue for just a few more years it will mathematically invalidate many of the climate models’ predictions.
And it almost beggars belief that the sheer quantity of emissions since 1998 can have so little effect–if sensitivity is high. On the other hand, if sensitivity is as low as Nic Lewis postulates (as have others before him), it would make more sense that a massive outgassing of CO2 in a short timeframe could still have a small effect.
What we’re left with is the realization that this period will be the proving ground for the various theories of sensitivity. Global emissions are hardly likely to go down–indeed, they will probably continue to increase, as developing countries continue to burn incredible quantities of coal in their race to provide modern lifestyles to their citizens. By the end of the decade humanity will have emitted one half of their historical total of greenhouse gases since 1998.
The results will be interesting. If the current temperature plateau holds, the climate activists will have to maintain that the lag between emission and response is so great that previous temperature rises were quite possible linked to other phenomena than human CO2–or else revise their sensitivity figures.
If, on the other hand, temperatures begin once again to rise quickly, many skeptics will have to acknowledge many uncomfortable conclusions of the climate scientists they have been fighting so bitterly.
In either case, this decade will provide something the debate has sorely needed for 25 years–answers.