Climate change and the local weather


The unusual weather this winter - wet in October and December, then almost totally dry until mid April - raises a serious question: was this persistent pattern a forerunner of what we should expect from global warming? Or was it just a fluke?

The scientific answer is complicated and incomplete, but progress is being made. Two important peer-reviewed scientific studies were recently published in technical journals on this matter, using computer-based modeling. One of them (Swain et al, Nature Climate Change, 2018) predicts that precipitation in California will become much more "volatile". Many winter seasons will feature precipitation that is far above normal. These wet years will be interspersed by seasons with precipitation that is far below normal. More frequent seasons of drought will occur. The swings from one situation to the other will become more extreme. The effects of such an increase in volatility in the local area are obvious. The extreme drought winters will likely lead to more wildfires in the following summers. Unfortunately, the increasingly frequent heavy precipitation events ("atmospheric rivers") that occur in some years will not efficiently replenish the groundwater because much of the deluge of excess water will just run off the surface rather than slowly seep into the ground.

Increasing extremes of wet vs. dry is one aspect of future weather. The other aspect is how much total precipitation will actually occur in the future, on the average. In general, warmer temperatures (due to anthropogenic warming, i.e., increased heat-trapping fossil fuel-produced carbon dioxide) will lead to more evaporation of water from the ocean. "What goes up must come down", so the total precipitation over the earth will definitely increase with any increase in sea surface temperatures. But land mass sizes, shapes, and locations, topography, seasons, and of course latitudes vary a lot over the earth, so some areas will see gains in precipitation and other areas will see losses. How does Northern California fare in this human-induced redistribution of the water wealth?

The other study (Dong and Leung, Geophysical Research Letters, 2021) finds that winter precipitation will likely increase in Northern California in the December through February periods, and generally decrease the rest of the year. Since most of the precipitation around here "normally" occurs in the December-February period anyway, that means an overall increase in annual precipitation.

Here is a map of the predictions for average changes in precipitation over the coming decades:

The map depicts expected changes between the historical averages in 1962-2005 and the computer modeled averages for 2056-2099. It shows, in general, that the "wet get wetter and the dry get dryer". Over this period, some areas (deepest brown) are predicted to suffer a loss of 15" of annual precipitation while others (darkest blue) are predicted to gain about 15". Most of those areas of increased rain are over the ocean where it does no good to ease the droughts over land.

We are in the red rectangle to the upper left. The other four black rectangles share with us a "Mediterranean climate", meaning cool wet winters and very warm and dry summers. All of the rectangles reside on the West Coasts of their respective continents, in the mid-latitudes. But strangely enough, only our home rectangle is predicted to see an increase in precipitation; the other four will likely suffer a decrease. Locally, that amounts to a gain of about 7" for a typical year in the 2056-2099 period, over the historical 1962-2005 period.

The study suggests that the projected local precipitation increase is due to CO2-induced sea-surface temperature warming in the subtropical Pacific, which strengthens a persistent low pressure zone over the Aleutians and forces a well-defined jet stream just to its south to project farther east, thereby reaching California before splitting and spreading out. This enhanced and eastward-extended jet stream will carry more strong storms directly into Northern California.

For this area, at least, the predictions of these two studies mean that "fluke" weather will become the norm. Air temperatures will increase, and droughts may become more extreme, and flooding may become more common. But at least we likely won't see less total precipitation on average. However, higher temperatures in winter will mean that the rain/snow line will retreat to higher elevations, meaning more rain and less snow in our towns. Only the highest elevations (above, say, about 8000', where it remains too cold for rain) will see an increase of snow. The result will be more liquid runoff during the winter, but frozen snowpack that otherwise lasts through late spring will be reduced in most places. The higher temperatures in summer will mean more evaporation from the soil and from the woody "fuels", thereby increasing the wildfire danger, despite the occasional years of much greater winter precipitation.