Arguments
A detailed look at climate sensitivity
Posted on 8 September 2010 by dana1981
Some global warming 'skeptics' argue that the Earth's climate sensitivity is so low that a doubling of atmospheric CO2 will result in a surface temperature change on the order of 1°C or less, and that therefore global warming is nothing to worry about. However, values this low are inconsistent with numerous studies using a wide variety of methods, including (i) paleoclimate data, (ii) recent empirical data, and (iii) generally accepted climate models.
Climate sensitivity describes how sensitive the global climate is to a change in the amount of energy reaching the Earth's surface and lower atmosphere (a.k.a. a radiative forcing). For example, we know that if the amount of carbon dioxide (CO2) in the Earth's atmosphere doubles from the pre-industrial level of 280 parts per million by volume (ppmv) to 560 ppmv, this will cause an energy imbalance by trapping more outgoing thermal radiation in the atmosphere, enough to directly warm the surface approximately 1.2°C. However, this doesn't account for feedbacks, for example ice melting and making the planet less reflective, and the warmer atmosphere holding more water vapor (another greenhouse gas).
Climate sensitivity is the amount the planet will warm when accounting for the various feedbacks affecting the global climate. The relevant formula is:
dT = ?*dF
Where 'dT' is the change in the Earth's average surface temperature, '?' is the climate sensitivity, usually with units in Kelvin or degrees Celsius per Watts per square meter (°C/[W m-2]), and 'dF' is the radiative forcing, which is discussed in further detail in the Advanced rebuttal to the 'CO2 effect is weak' argument.
Climate sensitivity is not specific to CO2
A common misconception is that the climate sensitivity and temperature change in response to increasing CO2 differs from the sensitivity to other radiative forcings, such as a change in solar irradiance. This, however, is not the case. The surface temperature change is proportional to the sensitivity and radiative forcing (in W m-2), regardless of the source of the energy imbalance.
In other words, if you argue that the Earth has a low climate sensitivity to CO2, you are also arguing for a low climate sensitivity to other influences such as solar irradiance, orbital changes, and volcanic emissions. Thus when arguing for low climate sensitivity, it becomes difficult to explain past climate changes. For example, between glacial and interglacial periods, the planet's average temperature changes on the order of 6°C (more like 8-10°C in the Antarctic). If the climate sensitivity is low, for example due to increasing low-lying cloud cover reflecting more sunlight as a response to global warming, then how can these large past climate changes be explained?
Figure 1: Antarctic temperature changes over the past 450,000 years as measured from ice cores
What is the possible range of climate sensitivity?
The IPCC Fourth Assessment Report summarized climate sensitivity as "likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values."
Individual studies have put climate sensitivity from a doubling of CO2 at anywhere between 0.5°C and 10°C; however, as a consequence of increasingly better data, it appears that the extreme higher and lower values are very unlikely. In fact, as climate science has developed and advanced over time , estimates have converged around 3°C. A summary of recent climate sensitivity studies can be found here.
A study led by Stefan Rahmstorf concluded "many vastly improved models have been developed by a number of climate research centers around the world. Current state-of-the-art climate models span a range of 2.6–4.1°C, most clustering around 3°C" (Rahmstorf 2008). Several studies have put the lower bound of climate sensitivity at about 1.5°C,on the other hand, several others have found that a sensitivity higher than 4.5°C can't be ruled out.
A 2008 study led by James Hansen found that climate sensitivity to "fast feedback processes" is 3°C, but when accounting for longer-term feedbacks (such as ice sheet
disintegration, vegetation migration, and greenhouse gas release from soils, tundra or ocean), if atmospheric CO2 remains at the doubled level, the sensitivity increases to 6°C based on paleoclimatic (historical climate) data.
What are the limits on the climate sensitivity value?
Paleoclimate
The main limit on the sensitivity value is that it has to be consistent with paleoclimatic data. A sensitivity which is too low will be inconsistent with past climate changes - basically if there is some large negative feedback which makes the sensitivity too low, it would have prevented the planet from transitioning from ice ages to interglacial periods, for example. Similarly a high climate sensitivity would have caused more and larger past climate changes.
One recent study examining the Palaeocene–Eocene Thermal Maximum (about 55 million years ago), during which the planet warmed 5-9°C, found that "At accepted values for the climate sensitivity to a doubling of the atmospheric CO2 concentration, this rise in CO2 can explain only between 1 and 3.5°C of the warming inferred from proxy records" (Zeebe 2009). This suggests that climate sensitivity may be higher than we currently believe, but it likely isn't lower.
Recent responses to large volcanic eruptions
Climate scientists have also attempted to estimate climate sensitivity based on the response to recent large volcanic eruptions, such as Mount Pinatubo in 1991. Wigley et al. (2005) found:
"Comparisons of observed and modeled coolings after the eruptions of Agung, El Chichón, and Pinatubo give implied climate sensitivities that are consistent with the Intergovernmental Panel on Climate Change (IPCC) range of 1.5–4.5°C. The cooling associated with Pinatubo appears to require a sensitivity above the IPCC lower bound of 1.5°C, and none of the observed eruption responses rules out a sensitivity above 4.5°C."
Similarly, Forster et al. (2006) concluded as follows.
"A climate feedback parameter of 2.3 +/- 1.4 W m-2 K-1 is found. This corresponds to a 1.0–4.1 K range for the equilibrium warming due to a doubling of carbon dioxide"
Other Empirical Observations
Gregory et al. (2002) used observed interior-ocean temperature changes, surface temperature changes measured since 1860, and estimates of anthropogenic and natural radiative forcing of the climate system to estimate its climate sensitivity. They found:
"we obtain a 90% confidence interval, whose lower bound (the 5th percentile) is 1.6 K. The median is 6.1 K, above the canonical range of 1.5–4.5 K; the mode is 2.1 K."
Examining Past Temperature Projections
In 1988, NASA climate scientist Dr James Hansen produced a groundbreaking study in which he produced a global climate model that calculated future warming based on three different CO2 emissions scenarios labeled A, B, and C (Hansen 1988). Now, after more than 20 years, we are able to review Hansen’s projections.
Hansen's model assumed a rather high climate sensitivity of 4.2°C for a doubling of CO2. His Scenario B has been the closest to reality, with the actual total radiative forcing being about 10% higher than in this emissions scenario. The warming trend predicted in this scenario from 1988 to 2010 was about 0.26°C per decade whereas the measured temperature increase over that period was approximately 0.18°C per decade, or about 40% lower than Scenario B.
Therefore, what Hansen's models and the real-world observations tell us is that climate sensitivity is about 40% below 4.2°C, or once again, right around 3°C for a doubling of atmospheric CO2.
Probabilistic Estimate Analysis
Annan and Hargreaves (2009) investigated various probabilistic estimates of climate sensitivity, many of which suggested a "worryingly high probability" (greater than 5%) that the sensitivity is in excess of than 6°C for a doubling of CO2. Using a Bayesian statistical approach, this study concluded that
"the long fat tail that is characteristic of all recent estimates of climate sensitivity simply disappears, with an upper 95% probability limit...easily shown to lie close to 4°C, and certainly well below 6°C."
Figure 2: Probability distribution of climate sensitivity to a doubling of atmospheric CO2
Summary of these results
Knutti and Hegerl (2008) presents a comprehensive, concise overview of our scientific understanding of climate sensitivity. In their paper, they present a figure which neatly encapsulates how various methods of estimating climate sensitivity examining different time periods have yielded consistent results, as the studies described above show. As you can see, the various methodologies are generally consistent with the range of 2-4.5°C, with few methods leaving the possibility of lower values, but several unable to rule out higher values.
Figure 3: Distributions and ranges for climate sensitivity from different lines of evidence. The circle indicates the most likely value. The thin colored bars indicate very likely value (more than 90% probability). The thicker colored bars indicate likely values (more than 66% probability). Dashed lines indicate no robust constraint on an upper bound. The IPCC likely range (2 to 4.5°C) and most likely value (3°C) are indicated by the vertical grey bar and black line, respectively.
What does all this mean?
According to a recent MIT study, we're currently on pace to reach this doubled atmospheric CO2 level by the mid-to-late 21st century.
Figure 4: Projected decadal mean concentrations of CO2. Red solid lines are median, 5%, and 95% for the MIT study, the dashed blue line is the same from the 2003 MIT projection.
So unless we change course, we're looking at a rapid warming over the 21st century. Most climate scientists agree that a 2°C warming is the 'danger limit'. Figure 5 shows temperature rise for a given CO2 level. The dark grey area indicates the climate sensitivity likely range of 2 to 4.5°C.
Figure 5: Relation between atmospheric CO2 concentration and key impacts associated with equilibrium global temperature increase. The most likely warming is indicated for climate sensitivity 3°C (black solid). The likely range (dark grey) is for the climate sensitivity range 2 to 4.5°C. Selected key impacts (some delayed) for several sectors and different temperatures are indicated in the top part of the figure.
If we manage to stabilize CO2 levels at 450 ppmv (the atmospheric CO2 concentration as of 2010 is about 390 ppmv), according to the best estimate, we have a probability of less than 50% of meeting the 2°C target. The key impacts associated with 2°C warming can be seen at the top of Figure 5. The tight constraint on the lower limit of climate sensitivity indicates we're looking down the barrel of significant warming in future decades.
As the scientists at RealClimate put it,"Global warming of 2°C would leave the Earth warmer than it has been in millions of years, a disruption of climate conditions that have been stable for longer than the history of human agriculture. Given the drought that already afflicts Australia, the crumbling of the sea ice in the Arctic, and the increasing storm damage after only 0.8°C of warming so far, calling 2°C a danger limit seems to us pretty cavalier."
This post is the Advanced version (written by dana1981) of the skeptic argument "Climate sensitivity is low". Note: a Basic version is on its way and should be published shortly.
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"Global temperature increases over the past decade have been on the low end of climate model projections."
That's like saying, 'there were ten days at the end of Spring that were on the low end of average temperatures... therefor it seems unlikely that temperatures will increase this Summer'. You're looking at a short term fluctuation, no greater than half a dozen such since 1880, and ignoring the long term trend.
That being the case, as presented your comment is nonsense. It is not possible for a method to be (or for the uncertainties to contain) "No, poorly understood, large uncertainties, very few studies or poor agreement, (un)known limitations, low confidence". For your comment to make sense, you need to split the analysis into the respective categories, and then apply the appropriate descriptor. If you do not do that, then you would need to apply all three cluster descriptions, for (for all paleo methods) they have both red, yellow, and green classifications.
More importantly, that analysis does not recognize the concordance across a range of paleo studies. Studies of both the Last Glacial Maximum (20 thousand years ago) and of Pliocene (5.3 to 2.6 million years ago, and when CO2 levels where last at their current levels) both study periods when the Earth did not have a "Similar climate base state" to its current condition, the LGM being about 6 degrees C colder, and the Pliocene was about 2-3 degrees warmer. Never-the-less, studies of both periods have come up with climate sensitivities in the range of 2 to 4 degrees C per doubling of CO2. The supposition that climate sensitivity for the current climate base state will some how be much lower than that range, when both warmer and colder conditions have a higher climate sensitivity is magical thinking.
In short, paleo-climate studies show the climate sensitivity to be very robust with respect to climate base state. That is not evident from looking at just studies of the LGM, or just studies of the Pliocene. Hence looking at the seperate categorization provided by Knutti and Hegerl with regard to climate base state is misleading if you do not recognize the robustness of the results.
While I have focused on the LGM and Pliocene as the best understood paleo-eras, the robustness of climate sensitivity has extended across a range of conditions from snowball Earth through to Saurian Sauna. There is undoubtedly some variation of climate sensitivity across that range, but that range is very likely to be smaller than the uncertainty in determination of climate sensitivity, ie, the range of climate sensitivities is likely to fall in the 2 to 4 degree C per doubling of CO2 range.
Finally, even if we suppose ourselves to be in a goldilocks zone for climate sensitivity, that is bad news. Suppose the modern climate sensitivity is in fact 2 degrees C per doubling, but that this is due to the fact that we are in a goldilocks zone. At that climate sensitivity, business as usual will still lift global temperatures by about 2-3 degrees C by the end of this century. But that is enough to lift us into a Pliocene base state with its higher (on this supposition) climate sensitivity but with our having much higher than Pliocene CO2 concentrations. In other words, we would face accelerating global warming even if we had managed to stop further emissions. In this scenario, the equilibrium temperature would still be that determined using a Pliocene (or even Cretacious) base state because our current activities would lift us to that level.
(From Park and Royer 2011, fig 9d. Alternative fits under different assumptions in figures 9 a-c and figure 10 should also be examined)
There are, of course, to periods of significant mismatch. That may be because of problems in the record of erosion (see figure 10 and related discussion). More probably, IMO, it is because particular geographical configurations changed the climate base state. Or it could even be because the climate sensitivity was significantly different in those periods (which is a distinct possibility from the geographical change of the climate base state).
Finally, the PDFs are indeed PDFs. Given a set threshold for explained variance in the CO2 concentration, the PDF maps the probability that a particular climate sensitivity (or climate sensitivity pairing, where glacial is distinct from non-glacial) will explain that degree of variance. However, like all statistical measures, a simplistic interpretation can be risky (and I am not the one too explain the risks of over interpretation in this case). However, it is not over interpretation to say that given the evidence in this study, "...the empirical PDFs for glacial climate sensitivity predict T2x(g)>2.0 °C with 99 percent probability, T2x(g)>3.4 °C with 95 percent probability, and T2x(g)>4.4 °C with 90 percent probability", and that "[t]he most probable values are T2x(g) 6° to 8 °C."
The wind shows a fairly drastic difference from glacial to the other three regimes indicating that measurements such as sensitivity of climate to a forcing change from the glacial to interglacial can't be applied to interglacial to future.
Tom in #72 replies that even if we are in a "Goldilocks" climate state, BAU will lift temperatures by 2-3C. My answer is there are not sufficient changes in the climate regime (compared to LGM) to do that. There are large decreases in wind (and consequent dust and other aerosols) that will not be duplicated to any significant extent in the forthcoming change from current to doubled CO2. The precip increases will work against temperature increases.
In fig 3b (post 72) the left two squares are red because the starting conditions (LGM) are too different from today's starting conditions and the sensitivity estimate cannot be applied. I am trying to show further that the sensitivity estimate is also an overestimate due to the greater magnitude of climate changes from LGM to present than present to doubled CO2. People may argue that the increased difference is accounted for simply by the increased GAT change, but that change was only 4-5C.
Please describe using the graphic how you conclude 1C is reasonable. The lowest high confidence intervals stop at 1.5C and most stop at 2C. There are long high tails. This graph has been posted many times to SKS since you started posting.
Can this graph be added to the climate graphics page?
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