Arguments
How sensitive is our climate?
What the science says...
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Net positive feedback is confirmed by many different lines of evidence. |
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Climate Myth...
Climate sensitivity is low
"His [Dr Spencer's] latest research demonstrates that – in the short term, at any rate – the temperature feedbacks that the IPCC imagines will greatly amplify any initial warming caused by CO2 are net-negative, attenuating the warming they are supposed to enhance. His best estimate is that the warming in response to a doubling of CO2 concentration, which may happen this century unless the usual suspects get away with shutting down the economies of the West, will be a harmless 1 Fahrenheit degree, not the 6 F predicted by the IPCC." (Christopher Monckton)
Climate sensitivity is the estimate of how much the earth's climate will warm in response to the increased greenhouse effect if we double the amount of carbon dioxide in the atmosphere. This includes feedbacks which can either amplify or dampen that warming. This is very important because if it is low, as some climate 'skeptics' argue, then the planet will warm slowly and we will have more time to react and adapt. If sensitivity is high, then we could be in for a very bad time indeed.
There are two ways of working out what climate sensitivity is. The first method is by modelling:
Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).
The second method calculates climate sensitivity directly from physical evidence, by looking at climate changes in the distant past:
Various paleoclimate-based equilibrium climate sensitivity estimates from a range of geologic eras. Adapted from PALEOSENS (2012) Figure 3a by John Cook.
These calculations use data from sources like ice cores to work out how much additional heat the doubling of greenhouse gases will produce. These estimates are very consistent, finding between 2 and 4.5°C global surface warming in response to doubled carbon dioxide.
It’s all a matter of degree
All the models and evidence confirm a minimum warming close to 2°C for a doubling of atmospheric CO2 with a most likely value of 3°C and the potential to warm 4.5°C or even more. Even such a small rise would signal many damaging and highly disruptive changes to the environment. In this light, the arguments against reducing greenhouse gas emissions because of climate sensitivity are a form of gambling. A minority claim the climate is less sensitive than we think, the implication being we don’t need to do anything much about it. Others suggest that because we can't tell for sure, we should wait and see.
In truth, nobody knows for sure quite how much the temperature will rise, but rise it will. Inaction or complacency heightens risk, gambling with the entire ecology of the planet, and the welfare of everyone on it.
Basic rebuttal written by GPWayne
Update July 2015:
Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial
Last updated on 5 July 2015 by skeptickev. View Archives
Hey, so I was linking your excellent version of Knutti and Hegerl graphic used in this post and noticed that it uses a potentially confusing notation both here and in the original paper. The 90% confidence interval is labelled "very likely" and the 66% confidence interval is labelled as "likely." That's sensible from a science perspective, but a bit confusing in that values from the 66% region are more likely to be the drawn values than those in the 66-90% interval. Not sure there's any way to better label the figure, but I thougt I'd just put that out there to see if there's a less confusing way of doing so....
Paul,
The 90% interval is from 5% to 95% so it includes the 66% confidence interval. Therefore it is more likely to occur since if the 66% occurs the 90% must also occur.
Michael, I'm well aware of that. My point is that if one didn't go digging through to the original article AND understand IPCC terminology AND frequentist statistics, the graph would seem to be confusing.
If it were labelled for example "66% confidence interval" and "90 % confidence interval" one wouldn't have to go chasing footnotes to understand it....
Paul from VA @310.
You are right that it is confusing, but because it is actually more confusing than you say, as it also refers to "the most likely value."
The caption for Figure 4 (in the Advanced version of this post) says "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)." The original Knutti & Hegerl paper sort of copes with this by talking of "most like values" and "likely ... and very likely value ... ranges" (my emphasis) but I would consider this poor description for a Review Article where the audience is very likely less attuned to the underlying science and so more reliant on the actual descriptions presented. And at SkS the audience is even less steeped in the science (although it is an advanced level SkS post).
The problem is also encountered in the other Knutti & Hegerl figure used in the advanced level post (SkS figure 6) where the terms "most likely warming" and "likely range" cope reasonably well. Yet if this is an advanced level post I would have thought the concept of a confidence interval would be preferable as suggested @ 310.
Paul and MA,
This discussion seems to me to come down to how useful the IPCC terms are in a scientific paper. These terms have been discussed a lot beore and some people do not like them. On the other hand, people also did not like using numbers before the IPCC adopted the current terms. It seems to me extremely likely that the scientists reading the review paper are familiar with these terms, the paper is not intended for a lay audience. Most of the users here at SkS are also familiar with these terms. They are not perfect, but they are what we currently have. I imagine that if we switched to new terms someone else would complain. It is difficult to please everyone.
Perhaps you could write a new post that explains things better? Good explainations are always welcome.
Jutland @313, the 7.8 deg C value is for the Earth System Climate Sensitivity, ie, the change in temperature for an initial doubling of CO2 after all feedbacks, including slow responding feedbacks from ice sheets, etc, have stabilized. The value is similar to other reasonable estimates, but the Earth System Response will take several thousands of years to stabilize. The value is therefore largely irrelevant to temperatures over the next century or so. Further, provided we do stop emitting CO2 at some point in the next century, equilibriation of CO2 concentration between the surface and deep oceans will reduce CO2 concentrations to about 50% of their peak increase over preindustrial values, so that the Earth System Response would be to a much lower overall CO2 concentration.
Far more relevant to the immediate future (ie, next 100-200 years) are the Transient Climate Response and the Charney Climate Sensitivity, which the Apollo-Gaia project shows as 3 C (close to IPCC central estimates). The only policy relevant impact of the Earth System Response is that it shows that a stable solution to the problem of global warming will require zero net anthropogenic emissions. Merely reducing emissions to 20% of current values is not a stable long term response.
Jutland@313
The biggest mistake in that booklet is their application of Earth System Sensitivity, which by their own definition works in millenial timescale, to the problem of AGW mitigation, which works on a century (or couple of centuries) timescale. The ESS by thier own definition, is a speculative measure, based on inaccurate deep-paleo data. You cannot expect ESS to play out fully within the mitigation timeframe (until say 2100) IPCC is concerned about. Beyond the timeframe of few centuries, the CO2 level may drop signifficantly due to ocean invasion, so most of the ESS feedbacks may not (and likely will not) play out. The same applies to Hadley & Hansen sensitivities: their positive feedbacks are not rellevant within the timeframe considered. By the same token, the rock weathering negative feedback does not play out within interglacial cycles of 100ky, therefore we don't talk about it while considering Milankovic forcings. While taking about this century, Charney sensitivity is the only one that we can be certain to play out.
Even more erroneour (actually ridiculous for me) is their calculation of Earth System Sensitivity in this booklet.
Check out the figure 8 on age 13. They claim ESS being far more accurate than other sensitivities, because it's derived from "high precision mathematics". That's just pathetically ridiculous. They don't mention how imprecise their input data is: just few points of highly uncertain values from 100 or 40 milion years ago. I'm sorry but if you are trying to estimate ESS from so highly uncertain old data (even ignoring the paleo-expert assertions that Earth sensitivity was different at that time due to continental configurations, etc.), your "high precision mathematics" won't help you to find the precise parameter you're looking for.
"There are some of us who remain so humbled by the task of measuring and understanding the extraordinarily complex climate system that we are skeptical of our ability to know what it is doing and why." Dr John R Christy
Well-quoted, Earthling. I, too, am "skeptical" of John Christy's ability to know what the climate is doing and why.
@316, you mean I've got more reading to do?
Stub for Klapper to move conversation from Guardian to SkS where he believes there are more informed commenters than me.
Just to pick up the conversation with Klapper.
I don't know why you're asking me questions like this that are best answered by people who are experts in the field. All I can do is try to read the relevant research and give my non-professional opinion.
What I'm asking you is, on all these questions you're asking, which you seem to think are evidence of a failed theory of AGW, who are the experts you're asking? You say they're not answering these questions, but are you actually asking anyone who actually would best know the answer?
Rob Honeycutt @321, there has been a recent paper by Smith et al (Feb, 2015) on "Earth's energy imbalance since 1960 in observations and CMIP5 models". For your discussion with Klapper, the key graphs are figs 3 a and b.
To interpret that, No is the net downward energy flux at the Top of the Atmosphere (ie, TOA energy imbalance) determined from observations, being the net difference between satellite observed outgoing long wave radiation and incomeing short wave radiation benchmarked against ocean heat content data from July 2005 to June 2010. Ht MOSORA is the ocean heat content from a Met Office reanalysis. That makes it semi-emperical, being emperical over those zones of the ocean of which we have observations, but using a computer model constrained to the emperical values over those zones where we have observations to fill in those zones in which we do not have observations. Ho and Ht CMIP5 are the multimodel mean equivalents.
Several things are worth noting in Fig 3a. First, No (ie the TOA energy imbalance) from observations and models match closely except for the period of 1972-82. They certainly match well over the last decade, although the observed No is slightly less than the modelled No in that period (of which more later). Second, TOA energy imbalance and OHC should match closely, and do for the models. There are, however, wide disparities between them in observations. That indicates there are more problems with the observations than there are with the model/observation comparison. (For what it is worth, the problems with observations probably relate to the limited region of the ocean in which OHC is directly observed, coupled with problems in the reanalysis.)
Fig 3b is much simpler, and simply shows a direct mean TOA energy imbalance comparison between models and observations over various periods. As you can see, the observations are statistically indistinguishable from the models for all periods. More importantly, "the missing 0.5W/m2 between models and reality" is seen to be a fiction. The actual difference over the most recent decade is 0.11 W/m^2. The 0.5 figure is based on old figures from CMIP 4 and far less accurate observations, and even then is exagerated by rounding. That Klapper is still using it shows he is clinging to old data simply because it is convenient for his message.
The paper also has some interesting information about the cause of the discrepancy between models and observations, encapsulated in Fig 4:
As you can see, the discrepancy between model and observed short wave radiation (ASR) is greater and more persistent than the discrepancy in longwave radiation (OLR) after 2000. Ergo the primary cause of the 0.11 W/m^2 discrepancy between models and observations is the reduced observed shortwave radiation compared to the models. At least part of the explanation of that is that the models cease to use historical data from about 2000 onwards, and hence do not include the short wave forcing from a series of recent volcanoes. If that forcing were included, the discrepancy between models and observations would be smaller, possibly non-existent.
(Note to Rob - I've spelt out in detail a number of points I know you know quite well for the benefit of Klapper and other potential readers.)
Perhaps Tom Curtis might use this recent study to add to the "It's the Sun" post, a counter to the myth that the Earth's temperature still is catching up to the increased input from the Sun that happened before around 1960? The counter to the myth is that if the myth is true, energy imbalance should be decreasing since then, as the increased outgoing radiation due to the Earth's higher temperature increasing compensated for the now-stable input from the Sun. Pretty please?
@Rob Honeycutt #321:
"... who are the experts you're asking?..."
You, and Tom Curtis and if not direct me to the peer-reviewed research that you know of. I admit I have in the past used Skeptical Science as a sounding board for ideas I have, since after a few back and forths on the numbers I can normally see if there is a concrete reason to reject the reason or not.
@Tom Curtis #322:
"...First, ...the TOA energy imbalance...from observations and models match closely except for the period of 1972-82"
Where would you get observations from 1972 for the TOA energy imbalance? For that matter exactly how accurate are the current observations for the TOA imbalance? There's an post over at the Guardian on the water vapour/climate change story by "MaxStavros" which claims the satellite numbers in raw form show an imbalance of 6.5W/m2 at the TOA. Since we know that is impossible the number has been adjusted down to something more believable. I can understand the instruments on the satellite are precise but not accurate, but that means the "observations" are not that reliable. I'm guessing the most reliable number is ocean heat, but that is true only since the ARGO era, from 2004 or 2005. From the NODC data, the warming rate of the oceans, corrected to global area, is about 0.5 W/m2. This is close to other estimates. The following example is ocean plus melting, plus land, but since most of the heat goes into the oceans we would expect the ocean only and total should be close (and they are).
Here's a quote from Jame Hansen et al 2012 at the NASA website: "We used other measurements to estimate the energy going into the deeper ocean, into the continents, and into melting of ice worldwide in the period 2005-2010. We found a total Earth energy imbalance of +0.58±0.15 W/m2 divided as shown in Fig. 1"
http://www.giss.nasa.gov/research/briefs/hansen_16/
Here's the problem with an energy imbalance of +0.58W/m2: the models show a much larger TOA energy imbalance. The GISS model shows +1.2W/m2, and the CMIP5 ensemble mean is +1.0 W/m2 for the 2000 to 2015 period.
[JH] Link activated.