Western Snow Trends and Global Warming: Part II

I've been on the road attending the American Meteorological Society Conference on Mesoscale Meteorology, so I missed yesterdays exciting weather and haven't been thinking much about climate change.  Nevertheless, I owe a post on this subject.

In our previous post, we discussed a recent study by Pierce and Cayan (2013) in which they examined the uneven response of several snow measures to climate change.  These measures included:

• SWE: The amount of water in the snowpack on 1 April

• SWE/P: The fraction of cold-season (1 October - 31 March) precipitation P that remains in the snowpack on 1 April.  

• Snowfall (SFE): The total cold-season snowfall measured as the amount of water in the snow (abbreviated as snowfall water equivalent, SFE).

• SFE/P: Fraction of total water in the cold-season precipitation that falls as snow

Their analysis suggested that long-term trends in these snow measures will emerge at different points in the future as the climate warms over the western U.S.  They suggested that increases in temperature and decreases in the fraction of precipitation that falls as snow would emerge first (as they have), with declines the fraction of cold-season precipitation that remains in the snowpack on 1 April and the amount of water in the snowpack on 1 April emerging later.  Amongst the snow variables examined, trends in snowfall will be the hardest to detect and emerge the latest.

Their results are based on downscaled climate model projections.  Because projections of climate require a vast amount of computer power, they typically are done at low resolution.  These models divide the atmosphere into cells that are perhaps 50 or 100 miles on a side, which is far to coarse to resolve the effects of the mountains of the western US.  Downscaling means that you take that coarse climate model projections and you use either statistical or regional modeling systems to generate a higher resolution projection.  In the Pierce and Cayan (2013), this downscaling effort involved the use of a hydrologic modeling system known as VIC, which allowed them to examine changes in snowpack.

Here we examine some of the regional contrasts in snow measure trends, with regions defined based on the color coding below.  Note that the Wasatch region spans a wide range of snow climates including the Tetons, Wind Rivers, Bear River Range, Uinta Mountains, Wasatch Mountains, and the plateaus and ranges of southern Utah.

Source: Pierce and Cayan (2013)

The figure below summarizes how the snow climate in each of 8 western US regions changes during the 21st century according to the average of the downscaled climate models (note that this averages out the year-to-year variations, revealing the long term trend.  It is based on a "moderate growth" emissions scenario in which equivalent CO2 concentrations reach about 800 ppm (we are at 400 ppm today) and breaks the total cool-season (Oct-Mar) precipitation down into three categories:

• Rain: Precipitation that fall as rain
• Melts: Precipitation that falls as snow, but melts and is not retained in the snowpack on 1 April
• SWE: Precipitation that falls as snow and is retained in the snowpack on 1 April.

Over the Washington Cascades, which feature a temperature-sensitive snow climate, there is an increase in the fraction of precipitation that falls as rain (grey region), an increase in the fraction of precipitation that falls as snow but melts before 1 April (pink region), and a decrease in the amount of snow retained in the snowpack on 1 April (blue region).   On a region-wide basis, the decline in the latter is about 35%.  Note that this is an average for the entire region, so there will be variations with elevation.  

Multi-model average of the fate of precipitation within eight regions over the western United States
during the 21st century.  Rain indicates the amount of cool-season precipitation that falls as
rain, melts indicates the amount of cool-season precipitation that falls as snow but melts before
1 April, and SWE indicates the amount of cool-season precipitation that falls as snow but
remains in the snowpack on 1 April.  Source: Pierce and Cayan (2013).  

On the other hand, in the colder Wasatch region, these changes are not as dramatic.   There is some increase in the fraction of precipitation that falls as rain and the fraction of snow that melts before 1 April, but the total decline in snowpack is about 22%, smaller than found in the other regions except the Colorado Rockies.

So, barring a shift in storm track not anticipated by these models, the snow climate of the Wasatch region and the Colorado Rockies have some "insurance" against global warming.  This is especially true at upper elevations.  It also means that trends in snow measures like 1 April snowpack SWE will take longer to emerge than found in warmer snow climates to the west.  These results are, of course, based on downscaled climate model projections and they are very dependent on the rate of warming and the quality of precipitation projections produced by those models.  Ultimately, what happens will depend on how high greenhouse gas concentrations go, how sensitive the climate system is to those concentrations, and whether or not there is a more dramatic shift in the storm track than anticipated by the climate models over the western U.S.