Climate Sensitivity

Climate Worst-Case Scenarios May Not Go Far Enough, Cloud Data Shows 0620 - Clouds provide a substantial amplifying feedback.  Climate sensitivity is about 5°C for doubled CO2, not 3°.

     In the the 3-panel figure, not only do high ECS models match observatons much better than low ECS models do at 35-60°S latitude (see this commentary in blue), high ECS models also match observations better at 40-60°N latitude.

U.N. Climate Panel Confronts Implausibly Hot Forecasts of Future Warming 0721

Why Low-End Climate Sensitivity Can Now Be Ruled Out 0820 - take 3, by some authors of the study

Why Clouds Are the Key to New Troubling Projections on Warming 0220

     Newer global climate models (GCMs) do a much better job of handling clouds, for which droplet size is very important, as is large scale (10s to 100s of kilometers): micro and macro in the same model.
    These newer GCMs find that clouds diminish as Earth's surface warms.  See reasons in figures in the   Albedo - Clouds   page: How Climate Change Breaks Up Clouds.
And in the cloud graphs with trend lines, not as far above in the   Albedo - Clouds   section.
    Taking into account the feedbacks involving cloud changes, these newer GCMs find climate sensitivity (∆°C for ∆ ppm CO2) of not ~ 3°,
which has the central estimate for decades, but
4 to 5.6°C.
    Climate modelers and the IPCC sometimes refer to these new and improved models as "hot models".

     Real-world data from satellites suggests that the modelers’ predictions may already be coming true.  See the 3-panel graph at the top of this Climate Sensitivity page.

    "Past models have overestimated how much ice in these clouds will turn to liquid water in a warmer world — and so overestimated both the thickness of future clouds and their ability to keep us cool.  Eliminating that bias, says Tan, could increase climate sensitivity by as much as 1.3°C."
    For example, French scientists at the National Center for Scientific Research concluded that the new models predicted that rapid economic growth driven by fossil fuels would deliver temperature rises averaging 6 to 7°C (10.8 to 12.6°F) by the end of the century. They warned that keeping warming below 2°C was all but impossible.

2 Million Year Record Indicates 5°C Warming from 400 ppm CO2, 9° from 560 ppm 0916 - Figure below is from Hansen, also shown a little farther below.

Making Sense of Palaeoclimate Sensitivity- Hansen 1112 - PDF - also above (Distant Past) and on Water page.  It has climate sensitivity estimates from 38 pale-climate studies.  Hansen "discards" 11 of the 38.

Below, Hansen draws Figure a from 800 K years, like Figure c (earlier, also by Hansen).  Figure b is drawn from the last 20 M years, according to van der Wal.  Figure d melds all 3.  Slow feedbacks include albedo (reflectivity) changes due to changes in vegetation and extent of ice sheets, as well as plate tectonics, weathering, and some aspects of the carbon cycle.  Fast feedbacks include cloud coverage, snow extent, sea ice, upper ocean heating, carbon emissions from permafrost and methane hydrates, and most aspects of dust and aerosol changes.

     Dr. Fry's analysis and projections suggest that climate sensitivity is not one number, but increases as more ice (in sheets) is available for albedo changes (6-8°C / doubled CO2) in the heart of an ice age.  Conversely, it decreases in a warmer world with less ice left to melt, until it reaches about 2° when no icde is left to melt.

Section Map: Heat