The Determination of Climate Sensitivity

[WeatherRusty]

The basic radiative physics which describes CO2 as a greenhouse gas is well accepted by nearly all the scientific community. To within a well confined margin for error, the Planck Temperature Response for a doubling of CO2 in Earth currently constituted atmosphere is ~1.2K. The uncertainty, therefore, with regard to the direct impact of CO2 on global temperature is very small.

If we doubled CO2 we could expect at radiative equilibrium, a temperature rise of 1.2C. No one would claim that to be a major problem. It would entail some adaptation, there would be winner and losers, but the consequences to the ecology and human condition would remain manageable.

1.2C is not the end of the story however. If the climate system were such that nothing else changed in response to a 1.2C warming we would know for relative certain what to expect from a global warming due to an increase in atmospheric CO2. The climate system is sensitive to a change in temperature however. The entire system will respond to a change in average temperature, such that local, regional and global climates will be affected.

Atmospheric water vapor will increase. Cloudiness, will likely increase. Ice will melt. Sea levels will rise. Vegetation and animals will migrate to keep up with shifting climate zones. Grass land becomes desert. Desert shifts to grass land. Grass land shifts to scrub brush and then forests. The global water cycle is affected. Precipitation patterns are altered. Jet streams migrate. etc.

So just how sensitive is the climate to a gradually changing temperature perturbation which becomes in isolation 1.2C at equilibrium? Many estimates, and varied approaches have been applied to arrive at a general range of 2C-4.5C. This is the so called Charney Sensitivity understood since the 1970s and adopted by the IPPC AR4 for it's 2007 report.

Can we do better than that wide range of probability to the 95% confidence level? The Charney Range represents a whole world of difference in outcome from one end to the other.

Here are two of the latest studies which attempt to hone in on a tighter estimated range.

Sorry, Abstract Only

Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum

Complete Paper

Climate Sensitivity Estimated From Earth's Climate History

[PhillipS]

The basic radiative physics which describes CO2 as a greenhouse gas is well accepted by nearly all the scientific community. To within a well confined margin for error, the Planck Temperature Response for a doubling of CO2 in Earth currently constituted atmosphere is ~1.2K. The uncertainty, therefore, with regard to the direct impact of CO2 on global temperature is very small.

If we doubled CO2 we could expect at radiative equilibrium, a temperature rise of 1.2C. No one would claim that to be a major problem. It would entail some adaptation, there would be winner and losers, but the consequences to the ecology and human condition would remain manageable.

1.2C is not the end of the story however. If the climate system were such that nothing else changed in response to a 1.2C warming we would know for relative certain what to expect from a global warming due to an increase in atmospheric CO2. The climate system is sensitive to a change in temperature however. The entire system will respond to a change in average temperature, such that local, regional and global climates will be affected.

Atmospheric water vapor will increase. Cloudiness, will likely increase. Ice will melt. Sea levels will rise. Vegetation and animals will migrate to keep up with shifting climate zones. Grass land becomes desert. Desert shifts to grass land. Grass land shifts to scrub brush and then forests. The global water cycle is affected. Precipitation patterns are altered. Jet streams migrate. etc.

So just how sensitive is the climate to a gradually changing temperature perturbation which becomes in isolation 1.2C at equilibrium? Many estimates, and varied approaches have been applied to arrive at a general range of 2C-4.5C. This is the so called Charney Sensitivity understood since the 1970s and adopted by the IPPC AR4 for it's 2007 report.

Can we do better than that wide range of probability to the 95% confidence level? The Charney Range represents a whole world of difference in outcome from one end to the other.

Here are two of the latest studies which attempt to hone in on a tighter estimated range.

Sorry, Abstract Only

Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum

Complete Paper

Climate Sensitivity Estimated From Earth's Climate History

Thank you, Rusty, for sharing those papers. There is a lot of material in the Hansen paper and I"ll have to read it several times before I feel I understand it all.

One 'back of the envelope' reality check that people can do for assessing estimates of climate sensitivity is to simply look at the global temperature record and the CO2 record and see how much rise we've already observed.

The latest BEST project findings are that the Earth has warmed about 1.5 C since 1750. I'll assume, arbitrarily, that half of that rise (0.75 C) is anthropogenic and half natural variability. (Feel free to split the rise however you feel appropriate) Now, remember, the AGW temperature rise to date is only the fast response feedback processes, the Earth has not reached equilibrium in any sense, and there is additional warming already committed to. But looking at the response we've measured for the fast feedback processes will at least bound the possible sensitivity estimate.

The current CO2 reading at Mauna Loa is 394 ppm and, given the pre-industrial level of 280 ppm, that means we've raised the CO2 level by ((394/280) - 1) = 0.4, or about 40%.

A 0.75 C rise due to a 40% increase in CO2 works out to a 1.9 C/doubling of CO2 for the fast feedback processes. The full climate sensitivity will be higher than that.

And if I change my assumption of the split between AGW and natural variability from 50/50 to, say, 75/25 then the fast feedback sensitivity is ((1.5 * 0.75) / 0.4) or 2.8 C/ doubling. Again, the full climate sensitivity will be higher.

For me this exercise indicates that the Hansen estimate of 3.0 +- 0.5 C/doubling is reasonable.whereas the Schmittner estimate of 2.3 -1.7 +2.6 C/doubling may be a bit low.

[ben4vols]

For me this exercise indicates that the Hansen estimate of 3.0 +- 0.5 C/doubling is reasonable.whereas the Schmittner estimate of 2.3 -1.7 +2.6 C/doubling may be a bit low.

What sensitivity would you get using models that are closest to current observations?

[PhillipS]

What sensitivity would you get using models that are closest to current observations?

You don't determine sensitivity from models. It can be estimated from radiative physics (Charney sensitivity), or from studies of how the climate responded in the past to changes in forcings ( the approach in the Schmittner and Hansen papers Rusty linked to).

Models incorporate the sensitivity, or range of sensitivities, chosen by the team building the model. For example, in the model Hansen used back in 1988 the sensitivity was 4.2 C/doubling - a value that with hindsight we believe was too high. But we've learned a lot over the quarter century since then.

If there is a particular model you feel has good results I'm pretty sure you can get its sensitivity values from its documentation. Most of that is on-line now. Let us know what you find out - that would be interesting.

[donsutherland1]

Some additional content from the Climate Sensitivity paper:

The resulting PDF, considering both land and ocean reconstructions, is multimodal and displays a broad maximum with a double peak between 2 and 2.6 K, smaller local maxima around 2.8 and 1.3 K, and vanishing probabilities below 1 K and above 3.2 K. The distribution has its mean and median at 2.2 and 2.3 K, respectively, and its 66 and 90% cumulative probability intervals are 1.7 to 2.6 K and 1.4 to 2.8 K, respectively. Using only ocean data, the PDF changes little, shifting toward slightly lower values (mean 2.1 K, median 2.2 K, 66% 1.5 to 2.5 K, and 90% 1.3 to 2.7 K), whereas using only land data leads to a much larger shift toward higher values (mean and median 3.4 K, 66% 2.8 to 4.1 K, and 90% 2.2 to 4.6 K)...

We propose three possible reasons that our study yields lower estimates of ECS_2xC than previous work that also used LGM data. First, the new reconstructions of LGM surface temperatures show less cooling than previous studies...

The second reason is limited spatial data coverage...

A third reason may be the neglect of dust radiative forcing in some previous LGM studies despite ample evidence from the paleoenvironmental record that dust levels were much higher.

Note: "PDF" is an acronym for posterity probability density function or illustration of the range of outcomes..