Climate Sensitivity

Three Weeks Before U.N. Climate Meeting: It's Worse Than You Think


Clouds in the models

Here's a consistent trend—just before the annual United Nations Climate Conference convenes, climate researchers take the opportunity to publish their more alarming findings. On November 26, the 18th Confererence of the Parties (COP-18) to the U.N. Framework Convention on Climate Change will meet in lovely Doha, Qatar. Of course, this is not skullduggery, just the prudent pursuit of publicity.

As example of this phenomenon, the journal Science is publishing today an article, "A Less Cloudy Future: The Role of Subtropical Subsidence in Climate Sensitivity," (sub required) which aims to refine predictions of climate sensitivity. Climate sensitivity is generally defined as how much a doubling of carbon dioxide in the atmosphere would boost average global surface temperature. The new study looks at how well 16 prominent climate models simulate the observed relative humidity of the earth's subtropical regions. They focus on this because current climate models do not handle the effects of clouds very well and the goal is to associate seasonal changes in cloudiness over the subtropics with changes in humidity. As the press release for study explains:

Climate model projections showing a greater rise in global temperature are likely to prove more accurate than those showing a lesser rise, according to a new analysis by scientists at the National Center for Atmospheric Research (NCAR)….

NCAR scientists John Fasullo and Kevin Trenberth, who co-authored the study, reached their conclusions by analyzing how well sophisticated climate models reproduce observed relative humidity in the tropics and subtropics.

The climate models that most accurately captured these complex moisture processes and associated clouds, which have a major influence on global climate, were also the ones that showed the greatest amounts of warming as society emits more greenhouse gas into the atmosphere.

"There is a striking relationship between how well climate models simulate relative humidity in key areas and how much warming they show in response to increasing carbon dioxide," Fasullo says. "Given how fundamental these processes are to clouds and the overall global climate, our findings indicate that warming is likely to be on the high side of current projections."

How much warming are the researchers talking about? The consensus estimate of climate sensitivity has hovered around 5 degrees Fahrenheit for the past 30 years. The new study reports:

Estimates based on observations show that the relative humidity in the dry zones averages between about 15 and 25 percent, whereas many of the models depicted humidities of 30 percent or higher for the same period. The models that better capture the actual dryness were among those with the highest ECS [equilibrium climate sensitivity], projecting a global temperature rise for doubled carbon dioxide of more than 7 degrees F [emphasis added]. The three models with the lowest ECS were also the least accurate in depicting relative humidity in these zones.

Keep in mind that it is generally agreed that the earth's average temperature has increased by about 1.5 degrees Fahrenheit over the past century as atmospheric carbon dioxide increased from the pre-industrial 280 parts per million (ppm) to 395 ppm now.

If carbon dioxide emissions continue at their current levels or increase, atmospheric carbon dioxide will double before the end of this century. If this study is right, it's going to get a lot toastier around planet earth than many climate models are projecting. But some researchers disagree.

In September, University of Alabama in Huntsville climatologist John Christy testified [PDF] before the House Energy and Power Subcommittee where he compared how well the leading climate models matched actual trends in global average temperature. See chart below:

climate models vs. actual temperatures

Note the the actual data cited by Christy are the circles and squares at the bottom and include two different satellite data series and data series from the National Climatic Data Center and NASA. In Christy's analysis nearly all of the climate models are projecting much higher global average temperatures than the thermometers and satellites are finding. Yes, indeed the science is settled.

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  1. …climate researchers take the opportunity to publish their more alarming findings.

    This year’s most alarming finding? Climate researchers have rendered themselves irrelevant.

  2. Meanwhile the evidence that the IPCC is overstating the sensitivity of T to CO2 by an order of magnitude continues to mount:

    Nic Lewis on Statistical errors in the Forest 2006 climate sensitivity study

    The code, written in IDL, that Forest has made available is both important and revealing. Important, because all or much of it has been used in many studies cited, or expected to be cited, by the IPCC. That includes, most probably, all the studies based on simulations by the MIT 2D model, both before and after AR4. Moreover, it also includes a number of detection and attribution studies, the IPCC’s “gold standard” in terms of inferring climate change and establishing consistency of AO-GCM simulations of greenhouse gas induced warming with observations. Much of the core code was originally written by Myles Allen, whose heavily-cited 1999 Allen and Tett optimal fingerprinting paper, here, provided the statistical theory on which Forest 2006 and its predecessor and successor studies were based.

    The IDL code is revealing because it incorporates some fundamental statistical errors in the derivation of the likelihood functions from the model-simulation ? observation discrepancies.

    1. The IPCC’s alteration of Forster & Gregory’s model-independent climate sensitivity results

      The IPCC Fourth Assessment Report of 2007 (AR4) contained various errors, including the well publicised overestimate of the speed at which Himalayan glaciers would melt. However, the IPCC’s defenders point out that such errors were inadvertent and inconsequential: they did not undermine the scientific basis of AR4. Here I demonstrate an error in the core scientific report (WGI) that came about through the IPCC’s alteration of a peer-reviewed result. This error is highly consequential, since it involves the only instrumental evidence that is climate-model independent cited by the IPCC as to the probability distribution of climate sensitivity, and it substantially increases the apparent risk of high warming from increases in CO2 concentration.

  3. “If this study is right, it’s going to get a lot toastier around planet earth than many climate models are projecting.”


    I find solace in the myriad incorrect predictions since Global Warming became the official Environmental Scare, replacing Biodiversity Loss (that was a real dud), which followed the now-defunct threat of Global Cooling (which actually scared people for a little while).

    Now maybe this study is the correct one. But there’s this whole probability thing…

    “If carbon dioxide emissions continue at their current levels or increase,”

    If they don’t, it will be due to a huge technological shift, like widespread local thorium reactor use, or because of a massive die-off of homo sapiens.

    The first seems unlikely, because burning stuff has been humanity’s chief source of BTUs since we discovered the campfire. Even new technology won’t make this source of energy disappear. The second doesn’t sound very good, except to serious eco-nazis.

  4. so, peak oil will make carbon-dioxide mediated climate change irrelevant?

  5. Wouldn’t increased cloud cover mean more sunlight reflected into space?

    If so, that makes me wonder if global warming is a self-limiting phenomenon. Higher temperatures lead to more water vapor in the air, which leads to more cloud cover, which leads to more sunlight reflected into space, which leads to lower temperatures.

    1. There will be an equilibrium, but at a higher overall temperature.

    2. Well, that’s the whole thing. Both the positive and negative feedbacks are legion, and they don’t know how to calculate them, which is why their model predictions are all wrong.

    3. I don’t see where there is a prediction of increased cloud cover.

      Clouds both reflect sunlight to space and heat (infrared radiation) back to Earth’s surface. Their net effect depends on their thickness, extent, and altitude. The net effect of clouds is likely small and positive. But the uncertainty still is large, relative to the net effect.

      Cloud feedbacks are one of the few that are not fully understood.

      That is why Fasullo and Trenberth chose to measure humidity during the dry season of about the least cloudy region.

  6. What Christy neglected to mention:
    1) The circle points represent satellite measurements of lower atmosphere temperatures. These, of course, will be lower than surface temperatures. You cannot equate surface temperature to these satellite measurements. So, strike all the circles.
    2) All of the measured temperatures fall within the uncertainty range of the model predictions.
    3) While it is true that the mean model prediction 0.23C/decade is above the measured increase of 0.17C/decade for the period shown, the last decade experienced cooling conditions that should have caused cooling. These include a strong La Nina storing heat in the oceans, reduced solar output, and increased aerosol emissions (think China coal fired power plants).
    Christy has been caught misinforming Congress time and again.

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