Despite the rainfall that el Niño brings, California hasn’t yet recovered from its multi-year drought. Some have tried to deny any relationship between the drought and man-made climate change, but they usually miss the point entirely: that whether or not global warming increased the likelihood of the drought, it has indeed increased its severity.
New research by Woodhouse et al. drives home the point, not about the California drought but about the upper Colorado River basin. Water from the Colorado River is a big part of the supply for many western states (including California), and its availability has been on the decline. In some places, Lake Mead for example, the water level is troublingly low and continues to decline. This research, however, focused on the upper Colorado River basin by studying streamflow at Lees Ferry, and looking for possible relationships to precipitation, temperature, and soil moisture.
The best predictors they found were cold season precipitation (October through April) when the snowpack builds up, spring-summer temperature (March through July), and November soil moisture storage. All three are factors in the river’s streamflow, but it’s no surprise that the amount of precipitation is most important; that’s what determines how much water is in the snowpack, the main source for the upper Colorado. It’s not too hard to see its impact by plotting annual time series for both variables (flow during the water year, and Oct-Apr precipitation) on the same graph, using “normalized” data so they’re on roughly the same scale:
The relationship with temperature is also visually apparent, in a similar plot of normalized values:
The real test, however, is whether or not temperature is a significant predictor even after allowing for other factors. If we model the river flow as a function of both precipitation and soil moisture, then study what’s left over (the residuals), we can still see a relationship with temperature:
It turns out that yes, temperature is a factor in the Colorado river flow; higher temperature means reduced flow. This isn’t just due to the fact that both flow and temperature are trending (flow decreasing while temperature increases long-term); statistical significance remains (extremely strong) even if we allow for a simple time trend in our model.
What’s troubling is the size of the trend; river flow decreases about 10% for each degree Celsius temperature increase.
There is now a host of research establishing the relationship between temperature and drought severity, showing the effect of increased evaporation and changes to the timing and rapidity of snowmelt. This can have severe consequences beyond the problems associated with drought such as California has had to endure, things like lengthening of the wildfire season. Yet many will continue to deny any relationship between global warming and drought frequency or severity. Unfortunately for us, nature pays no mind to their eyes-wide-shut brand of skepticism, and when she shows us the consequences of our actions, we cannot help but see them.
This blog is made possible by readers like you; join others by donating at Peaseblossom’s Closet.