Sense and Sensitivity

September 24, 2013


IPCC AR5, at least the first parts of it, will be coming out this friday.

Don’t ask me how it is that IPCC still wants to put out their most important media product in a friday news dump. I’m too tired to flog that horse.

Anyway, leading up to that, rather than pretend I’ve got the leaked documents, I thought it would be productive to post discussions of what are sure to be some of the major talking points – they’ve already emerged – climate sensitivity and the purported “pause” in global warming.

Zeke Hausfather, a careful researcher and writer, has been doing some thinking about the issues.

Zeke Hausfather at the Yale Forum on Climate Change and the Media:

Climate ‘skeptics’ down-play the sensitivity of Earth’s climate to increased CO2 emissions and concentrations, and so might some policy makers. In the end, it’s the emissions and concentrations that most matter rather than uncertainties about climate sensitivity.

Climate sensitivity is suddenly a hot topic.

Some commenters skeptical of the severity of projected climate change have recently seized on two sourcesto argue that the climate may be less sensitive than many scientists say and the impacts of climate change therefore less serious: A yet-to-be-published study from Norwegian researchers, and remarks by James Annan, a climate scientist with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).

While the points skeptics are making significantly overstate their case, a look at recent developments in estimates of climate sensitivity may help provide a better estimate of future warming. These estimates are critical, as climate sensitivity will be one of the main factors determining how much warming the world experiences during the 21st century.

Climate sensitivity is an important and often poorly understood concept. Put simply, it is usually defined as the amount of global surface warming that will occur when atmospheric CO2 concentrations double. These estimates have proven remarkably stable over time, generally falling in the range of 1.5 to 4.5 degrees C per doubling of CO2.* Using its established terminology, IPCC in its Fourth Assessment Report slightly narrowed this range, arguing that climate sensitivity was “likely” between 2 C to 4.5 C, and that it was “very likely” more than 1.5 C.

The wide range of estimates of climate sensitivity is attributable to uncertainties about the magnitude of climate feedbacks (e.g., water vapor, clouds, and albedo). Those estimates also reflect uncertainties involving changes in temperature and forcing in the distant past. But based on the radiative properties, there is broad agreement that, all things being equal, a doubling of CO2 will yield a temperature increase of a bit more than 1 C if feedbacks are ignored. However, it is known from estimates of past climate changes and from atmospheric physics-based models that Earth’s climate is more sensitive than that. A prime example: Small perturbations in orbital forcings resulting in vast ice ages could not have occurred without strong feedbacks.

Water Vapor: Major GHG and Major Feedback

Water vapor is responsible for the major feedback, increasing sensitivity from 1 C to somewhere between 2 and 4.5 C. Water vapor is itself a powerful greenhouse gas, and the amount of water vapor in the atmosphere is in part determined by the temperature of the air. As the world warms, the absolute amount of water vapor in the atmosphere will increase and therefore so too will the greenhouse effect.

That increased atmospheric water vapor will also affect cloud cover, though impacts of changes in cloud cover on climate sensitivity are much more uncertain. What is clear is that a warming world will also be a world with less ice and snow cover. With less ice and snow reflecting the Sun’s rays, melting will decrease Earth’s albedo, with a predictable impact: more warming.

There are several different ways to estimate climate sensitivity:

  • Examining Earth’s temperature response during the last millennium, glacial periods in the past, or periods even further back in geological time, such as the Paleocene Eocene Thermal Maximum;
  • Looking at recent temperature measurements and data from satellites;
  • Examining the response of Earth’s climate to major volcanic eruptions; and
  • Using global climate models to test the response of a doubling of CO2 concentrations.

These methods produce generally comparable results, as shown in the figure below.

Figure from Knutti and Hegerl 2008.

The grey area shows IPCC’s estimated sensitivity ranges of 2 C to 4.5 C. Different approaches tend to obtain slightly different mean estimates. Those based on instrumental temperature records (e.g., thermometer measurements over the past 150 years or so) have a mean sensitivity of around 2.5 C, while climate models average closer to 3.5 C.

The ‘Sting’ of the Long Tail of Sensitivity

Much of the recent discussion of climate sensitivity in online forums and in peer-reviewed literature focuses on two areas: cutting off the so-called “long tail” of low probability\high climate sensitivities (e.g., above 6 C or so), and reconciling the recent slowdown in observed surface warming with predictions from global climate models.

Being able to rule out low-probability/high-sensitivity outcomes is important for a number of reasons. For one, the non-linear relationship between warming and economic harm means that the most extreme damages would occur in very high-sensitivity cases (as Harvard economist Marty Weitzman puts it, “the sting is in the long tail” of climate sensitivity). Being able to better rule out low probability/high climate sensitivities can change assessments of the potential economic damages resulting from climate change. Much of the recent work arguing against very high-sensitivity estimates has been done by James Annan and Jules Hargreaves.

The relatively slow rate of warming over the past decade has lowered some estimates of climate sensitivity based on surface temperature records. While temperatures have remained within the envelope of estimates from climate models, they have at times approached the 5 percent to 95 percent confidence intervals, as shown in the figure below.

Figure from Ed Hawkins at the University of Reading (UK).

However, reasonably comprehensive global temperature records exist only since around 1850, and sensitivity estimates derived from surface temperature records can be overly sensitive to decadal variability. To illustrate that latter point, in the Norwegian study referred to earlier, an estimate of sensitivity using temperature data up to the year 2000 resulted in a relatively high sensitivity of 3.9 C per doubling. Adding in just a single decade of data, from 2000 to 2010, significantly reduces the estimate of sensitivity to 1.9 C.

There’s an important lesson there: The fact that the results are so sensitive to relatively short periods of time should provide a cautionary tale against taking single numbers at face value. If the current decade turns out to be hotter than the first decade of this century, some sensitivity estimates based on surface temperature records may end up being much higher.

So what about climate sensitivity? We are left going back to the IPCC synthesis, that it is “likely” between 2 C and 4.5 C per doubling of CO2 concentrations, and “very likely” more than 1.5 C. While different researchers have different best estimates (James Annan, for example, says his best estimate is 2.5 C), uncertainties still mean that estimates cannot be narrowed down to a far narrower and more precise range.

Ultimately, from the perspective of policy makers and the general public, the impacts of climate change and the required mitigation and adaptation efforts are largely the same in a world of 2 or 4 C per doubling of CO2concentrations where carbon dioxide emissions are rising quickly.

Just how warm the world will be in 2100 depends more on how much carbon is emitted into the atmosphere, and what might be done about it, than on what the precise climate sensitivity ends up being. A world with a relatively low climate sensitivity — say in the range of 2 C — but with high emissions and with atmospheric concentrations three to four times those of pre-industrial levels is still probably a far different planet than the one we humans have become accustomed to. And it’s likely not one we would find nearly so hospitable.

Zeke Hausfather, a data scientist with extensive experience with clean technology interests in Silicon Valley, is currently a Senior Researcher with Berkeley Earth. He is a regular contributor to The Yale Forum (E-mail:

19 Responses to “Sense and Sensitivity”

  1. omnologos Says:

    largely the same in a world of 2 or 4 C per doubling of CO2concentrations

    Is this the magical thinking about the 2C value, coming back from the dead?

    • Is this the magical thinking about the 2C value, coming back from the dead?


      I hear your opinion but where is the beef? Flapping your gums is cheap and free.

      • omnologos Says:

        no Jeffery…where is the beef in saying that 2C or 4C, it doesn’t really matter?

        As if 2C were a magical threshold.

        • omnologos… Have you tried reading where the 2C figure comes from?

        • [And a small clue, omnologos. The 2C threshold is different from 2C for climate sensitivity.]

          • omnologos Says:

            yes yes Rob it’s just that, as if by magic, 2C (temp increase) is written in the treaties and 2C (sensitivity) is the new lower threshold above which things are bad bad bad.

            it’s like the 97% figure that keeps popping up on different topics. soundbites wouldn’t sound too different from those.

          • I’m wondering if you are confused between the 2*C goal not threshold to limit earth’s temperature change by our switch over to clean energy.

            We burn all the fossil fuels possible, then we reach the game over threshold of having a decent earth to live on. We start heading over 1000 ppm co2 and over, we reach Joe Romm’s “Hell and High Water” scenario.

  2. There is sensitivity, and then there is sensitivity over time. The climate system has some small, some big and some huge lags.

    Look at it this way, the Milankovitch Cycles essentially provide a zero net change in forcing and yet that has been enough to throw us into and out of ice ages. The Milankovitch cycles are very gentle, what we have done is far from gentle.

    The climate system does seem to have some stepped responses. Comparing today against 1998, is like trying to determine the rise of a stair case by measuring the flat of a tread.

  3. Indeed there is lots of discussion about sensitivity, but I would like to suggest that ignores two fundamentally important factors.

    The first is that sensitivity is based on stabilisation of the temperature following the CO2 increase. That may not necessarily take place immediately. Indeed, I have seen nothing which indicates how long it may take (there may be plenty – I just haven’t seen it). If it takes place immediately, then that is much more of a problem than if it takes centuries to reach equilibrium.

    The second factor is that this is based on a doubling of CO2. We appear to be on track to significantly exceed that, making the estimates rather academic.

  4. redskylite Says:

    The effect of GHG on planetary atmospheres has been known and well proven for two whole centuries and just because the “hockey stick” rise in temperature on Earth does not conveniently and uniformly occur with the measured increase in atmospheric concentrations of CO2 (like it would in a simple model) because of many natural external reasons (known and unknown), doesn’t stop the known effect of GHG occurring. We are concentrating a lot on CO2 but also there is a lot going on with methane, which I am sure will also be covered in the AR5 report.
    There must be a lot of energy used melting polar sea ice, once the ice is gone then and the energy is no longer needed in the melting process and will be free to warm the surface – warming will surely take off. It is going to be a stepped process, tipping points will occur. I am alarmed to see Russia exploring for new oil reserves in the Arctic – there is an abundance of barrels there, hopefully between the IPCC, world politicians and scientists, economics and common sense we can still limit the damage before it is far to late. There is always Mars I guess. Not sure if I can plough through the whole AR5 series, but I’m sure the reputable press and reputable climate sites will amplify the critical parts.

  5. Peter Mizla Says:

    One thing is for certain– the skeptics will get far more media coverage then the actual caveats of the IPCC report in its totality. Governments basically want to wait far longer in doing anything about emissions at at all- probably the 2030s= perhaps mid century- and any report that hints of lower climate sensitivity as a ‘possibility’ but still unlikely- and a so called ‘pause’ in warming- another cherry picked piece of hokum, will get lots of media play. Governments do not want to switch from fossil fuels. Therefore CO2 will keep rising for the foreseeable future at an increasing rate. This of course could change if in the years ahead we begin to see the climate really begin to ‘tip over’ and catastrophes begin to take a toll on the global economic system. For now however I frankly see NO reductions in C02 coming- and no restrictions or taxes of any kind taking place. We are on a pace to see 800ppm by 2090.

    • ubrew12 Says:

      I agree with your judgement, and in part its because we are all treating this like its just a bad dream, but thankfully a dream that we still control. We’re laser-focused on the remaining uncertainty in Climate Sensitivity, while our OWN inability to emotionally process what is happening, and effect Change, is the Greater Uncertainty.

      It’s like that dream that you’re falling from a great height, but can observe without emotion because ‘you know’ at the last moment you’ll sprout wings or turn out to have springs for legs, or something. Intellectually, we know we’re falling, but emotionally, we don’t seem to know what that means yet. But surely something wonderful will happen at the last second to save us, because its still ‘our dream’, isn’t it? To some degree, its not just the deniers who are in denial.

      We made it through the Cold War, didn’t we? No Nuclear Winter. It was just a bad dream, but we ‘knew’ it would have a happy ending. We can’t even emotionally process the recent revelation that we VERY nearly vaporized North Carolina, just as Camelot was being constructed (“The document says the bombs should have detonated — parachutes were deployed and triggers were armed, but one low-voltage switch failed to activate as it should have”). And what is scary about this recent revelation is how we very nearly didn’t even intellectually process it, 50 years later, in the U.S. Someone from another country had to go dig it out of our collective amnesia.

      Go back to sleep. It was just a bad dream…

  6. Scott Mandia Says:

    As climate expert Dr. John Harte wrote in an email regarding a reduced lower-end sensitivity:

    Suppose you had been told for many years that drinking a small amount of a pesticide would result in a probability of consequent death that was somewhere in the interval 15% – 75%. If tomorrow you were told the actual range is 10% – 75% would you be any more likely to take a swig?

  7. ubrew12 Says:

    While its important to hammer down the eventual response to CO2 doubling, I think it’s also important to emphasize the Extra Uncertainty caused by today’s wholly unnatural Rate of Change. You can appreciate it here, in this RealClimate 11,000 yr Marcott plot:

    It’s like the difference between gradually bringing a pot of water to a rolling boil, and overheating the same pot until the rushing steam pumps the water out of the pot and all over the stove-top. In both cases, you’re just bringing water to a boil, but the consequences are very different.

    The recent understanding that much Antarctic glacial ice is actually being melted from the ocean BELOW it may be an example of ‘explosive boiling’ in action. Much of this planet may just not have enough time to ‘get out of the way’ of these changes, which means some kind of collision, which is never good (insert ‘Deer in Headlights’ image here).


    The peer-reviewed study published in the Quarterly Journal of the Royal Meteorological Society argues that conventional conclusions on climate sensitivity – the extent to which global temperatures respond to greenhouse gas emissions – underestimate the role of some amplifying feedbacks that may intensify climate impacts in ways that many models tend to overlook.

    Traditional estimates of climate sensitivity such as that adopted by the IPCC focus on “fast feedbacks” like water vapour, natural aerosols, clouds, and snow cover, but do not sufficiently account for slower feedbacks including “surface albedo feedbacks from changes in continental ice sheets and vegetation”, and climate greenhouse gas feedbacks “from changes in natural (land and ocean) carbon sinks.”


    Long term warming favors the higher sensitivities.

  9. […] If you have  not read them, take in Stefan Rahmstorf’s piece here, as well as another good one by Zeke Hausfather. […]

  10. […] even as I’m working with video of Zeke Hausfather of Berkeley Earth predicting more adjustments to the satellite record – news breaks, of course, of new […]

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