Tuesday, August 28, 2018

California's Fourth Climate Change Assessment

California's Fourth Climate Change Assessment was released yesterday and contains some sobering findings for the future of snow and skiing in the Sierra Nevada.  The assessment can be accessed at www.climateassessment.ca.gov, with the regional report for the Sierra Nevada available at http://www.climateassessment.ca.gov/regions/docs/20180827-SierraNevada.pdf.

California is perhaps one of the more climate-vulnerable regions of the contiguous U.S. and is already feeling the effects of global warming.  On the other hand, it has incredible expertise to draw from, with the regional report from the Sierra Nevada seeing contributions from manny oustanding scientists, led by Mike Dettinger of the USGS.

Some key points from report on the Sierra Nevada:

  • Climate change is already underway
  • Recent/ongoing trends foreshadow changes to come
  • By the end of the 21st century, temperatures are projected to warm 6-9˚F
  • Future precipitation is expected to be within ±15% of current totals, but extremes (deluge and drought) will increase
  • Snowpacks will "very likely be eradicated below about 6,000 feet elevation and will be reduced by more than 60% across nearly all the range."  

Using ten different climate models, they specifically examine projections of historical and future climate, the latter under two scenarios, RCP4.5 in which greenhouse gas emissions peak around 2040 and then decline, and RCP8.5 in which greenhouse gas emissions rise throughout the 21st century.  Below are projections of temperature and precipitation for four regions within the Sierra Nevada. 

Source: Dettinger et al. (2018)
Under RCP8.5, locations at or below 6000 feet in the Sierra Nevada experience at least an 80% reduction in April snow-water equivalent.  The smallest declines occur in the southern Sierra due to it's higher altitude and colder climate.  Soil moisture declines at all locations. 

Source: Dettinger et al. (2018)
The graph below presents the probability that the April snow water equivalent will fall in the historical lowest 10% of seasons.  Translation: this is the likelihood of a truly shitty snow season.  In 2000, this comes near 10% as one would expect.  In the Northern Sierra under RCP8.5%, the odds increase to 90% by the end of the century.  In other words, nine out of every ten years would be truly shitty.  The odds of a truly shitty season are lower, but still higher than current, in the northern Sierra Nevada under RCP4.5 in which greenhouse gas emissions peak in 2040 and then decline.  Increases in the odds of truly shitty seasons are much smaller in the southern Sierra Nevada which thanks to altitude sees less vulnerability of the April snowpack.  It's a bit unfortunate that they don't present data for later months when declines in the southern Sierra may be more dramatic. 

Source: Dettinger et al. (2018)
Overall, these results are consistent with the expectations that snow at lower, warmer elevations is more vulnerable to warming.  It also highlights that fact that although we are already committed to some future climate change, reductions in greenhouse gas emissions can still reduce the magnitude of that warming and its associated impacts. 


  1. I tried to find the third report as well (couldn't find it by region but did find the overall report). Seems like this is an increase in expected warming from the previous report a few years ago (previous report had statewide going up 4.6-8.6 degrees. What is the main driver of this? Increases in projected co2 rates?

    1. The 6-9 noted above is strictly for the Sierra Nevada. The statewide projection is 5.6-8.8, closer to your numbers. I don't have time to go back to the 3rd assessment report. It is possible that the new report is using more recent climate projections and scenarios, which would explain some small differences.

  2. More direr predictions. But no thought going into cause, green house gases, earth getting closer to the sun, temperature of core earth increasing? Thoughts?
    Is there a chance humans could change the change even if possible? Is just every human born another heat source and having population increase is equal to temperature increase?

    1. Extensive work being done in this area of course with an extensive body of evidence. Arguing that there has been no consideration of other possible drivers is something with no basis. Here is a summary: https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter10_FINAL.pdf.