Climate Sensitivity

Global Warming Has Accelerated - Hansen 0125.pdf

     Hansen et al. make the case that the IPCC's 3°C central estimate of climate sensitivity (warming per doubling of CO2 level) is an underestimate, that it should be 4.5°C or higher.  The mistake comes from an underestimate of particulate (sulfte) pollution, which has been failed to be measured by satellites.

     In particular, climate sensitivities of 4.5 and 6.0°C better match the patterns in the observational data, including considerable warming in 2023 and 2024, especially when tied to the reduction in sulfates from ships, which is pronounced in open ocean areas that had so little other sulfate pollution, thanks to the rules promulgated by international shipping organization starting in 2020.

CO2 Puts Heavier Stamp on Temperature than Previously Thought 0624.rtf - from Dutch and English researchers.  From the Abstract:  “.... From 15.0-0.3 Myr ago, our reconstructed pCO2 values steadily decline from 650 ± 150 to 280 ± 75 ppmv, mirroring global temperature decline. Using our new range of pCO2 values, we calculate average Earth system sensitivity and equilibrium climate sensitivity, resulting in 13.9°C and 7.2°C per doubling of pCO2, respectively.  These values are significantly higher than IPCC global warming estimations, consistent [with] or higher than some recent state-of-the-art climate models, and consistent with other proxy-based estimates.”

     In the figure below, Observations fill the right-most of the 3 panels.  The High ECS models, on the left, shows a much better fit than the Low ECS models in the enter panel.  This is most true in winter (November-March near 60°N latitude in northern winter and April-october near 60°S in southern winter).  The high ECS model also matches observations better from 10 to 30°S over May to December.

Climate Worst-Case Scenarios May Not Go Far Enough, Cloud Data Shows 0620.rtf - 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 observations 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.

     Shown below are climate sensitivities (eventual temperature response to doubled CO2 levels) in Global Climate Models.  CMIP6 GCMs are more recent and more detailed than CMIP5 models.  They especially do a better job of modeling clouds in detail.

     Climate sensitivities of -1 to +6 are shown on the bottom axis.  For 3 decades, the IPCC saw +3 as the central estimate for climate sensitivity, with a modest 1 to 1.5 error band.  More recent models, below, mostly show higher sensitivities than +3.

     Abbreviations shown below, just above the -1 to +6 warming key, are TR = Tropics, EX = Extra-Tropical, both for Forcing by greenhouse gases (other than water vapor), which leads to heating.  LR and RH are for Longwave Radiation and Relative Humidity.

     Albedo is the % of incoming solar radiation that Earth reflects, mostly from sea ice and snow, plus a little bit from land ice.  Clouds refers to the albedo (reflectivity) of clouds, directly and indirectly, from changes mostly in cloud area, altitude, and opaqueness.

     Notice that more recent models, which handle clouds better, yields more heating per doubled CO2.

Why Some Climate Models Are Running Red Hot - Clouds, Water, Ice 0522.rtf

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

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

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

     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 2 x ppm CO2) of not ~ 3°, which has the central estimate for decades, but
4.0 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.rtf 

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