Data

Future CO2 GT, PPM, ∆°C, SLR, by Gene Fry (Dec. 2018 edition)
      This is the spreadsheet model underlying the results shown below.  An annotated version of the 4 graphs is shown below them.

     Note that stopping human CO2 emissions (indeed GHG emissions) is only enough to hold eventual warming to 5°C.  But failing to stop them now is worse, much more than the difference between 1.5 and 2.0°C.  At 5°C warming, civiization starts to fall apart very noticeably.  Food supplies would be cut in half and fresh water becomes scarce almost everywhere.  Unless we end carbon emissions soon AND remove most GHGs we have put in the air, Earth’s carrying capacity will shrink a lot.  Then human population will probably fall well below 1 billion.  It would be concentrated in Antarctica, Siberia, the Canadian Arctic, Alaska, and Greenland.  The population decline will be very messy (wars, famine, etc.)

     The only way to keep temperatures to tolerable levels is stop putting GHGs in the air soon AND remove most of the CO2 we have put in the air.  Even that will likely not be enough to hit 2° on the way back down until well after 2400.  Solar radiation management may provide a way to get the rest of the way back down to

     About 39% of the atmospheric warming observed since 1975 is due to albedo changes.  That includes 22% from cuts in our sulfur emissions,  11% from snow melting earlier in the spring, 6% from reduced Arctic sea ice area, and less than 1% from reductions in area covered by land ice.  Feedbacks from more water vapor in the air and from cloud cover changes due to these 4 albedo changes are included in these %’s.  The water vapor and cloud feedbacks account for 41% of the 39%,(i.e., 16% of observed warming since 1975) .  Water vapor feedbacks are also included in the direct warming from greenhouse gases such as CO2, which account for the remaining 61% of the warming since 1975.

     In the future, the fraction of warming from albedo changes and their feedbacks will grow, as the fraction from additional greenhouse gases shrinks

     Mean northern snow cover is 25 million sq km.  This is 2.5 times ice cover, and at lower latitudes.  This yields higher mean sun angles, for greater effect.

     Chan et al. (2016) estimated that altered northern hemisphere snow cover timing from 1982 to 2013 cut snow’s radiative forcing (cooling) effect by 0.12 W per square meter.  Dividing by the warming during that period yields 0.21 W / sq m / °C in lost cooling effect.

    Future snow coverage will decline as the land surface warms.  Future snow cover shrinkage is modeled by using analog cities: cities at about the same longitude and distance from oceans, progressively farther south.

For example, Winnipeg snow cover days analogs are Fargo, Sioux Falls, Omaha, & Kansas City.  As warming proceeds, Winnipeg snow days fall, to Omaha’s & even to zero, if winters there are warm enough (global ∆ = 8°C).  Shown below are the data for some of the analog cities.  +3°C, +5°C and +7°C are local temperatures.  They correspond roughly to 2, 3, and 4°C GLOBAL warming at the land surface.