U.S. Floods, Droughts and Global Warming: Another Wardrobe Failure

Global Science Report is a feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

It is the current rage in the mainstream media and the government to tie almost everything into human-caused global warming—from a sluggish economy to Ebola,  and everything in between (and then some).

In fact, virtually none of these claims are supported by a consensus of evidentiary science. Here is (yet) another example, debunking the popular notion floods are being worsened by dreaded climate change caused by pernicious economic activity.

Clinically speaking, a “flood” is actually an extreme excursion in streamflow. So, if changes in streamflow are related to long-term changes in climate, and we accept that the majority of those latter changes are caused by said economic activity (we don’t), then our activities should increase streamflow and therefore the frequency of floods (or their opposite, droughts).

Two scientists from the U.S. Geological Survey (USGS), Gregory McCabe and David Wolock, recently examined historical (1951-2009) streamflow records from 516 rivers and streams that they considered to be only minimally impacted by human development. They first sorted the data into regional patterns, and then compared the temporal behavior of these patterns to  common historical climate indices—such as well-known patterns of atmospheric circulation, sea surface temperatures, or even large-scale warming.

It turns out that there weren’t any relationships between streamflow and the larger atmospheric phenomena.  Or at least, so very few that they are hardly worth mentioning.

Here is how McCabe and Wolock describe what they (didn’t) find:

Comparing time series of climate indices…with the time series of mean [stream] flow for the 14 clusters [patterns] indicates weak correlations that are statistically significant for only a few clusters. These results indicate that most of the temporal variability in streamflow in the conterminous U.S. is unpredictable in terms of relations to well-known climate indices. [emphasis added]

In other words, trends and/or variability in larger-scale features of the climate (including rising temperature from global warming) are not very strongly (if at all) related to regional and temporal characteristics of streamflows across the U.S.

And before anyone starts to argue that we have left out the direct (i.e., local) effect of global warming—that warmer air holds more moisture and thus it can rain more frequently and harder—McCabe and Wolock report very few long-term trends that would be indicative of steadily rising moisture levels. Instead, the find the historical records dominated by periods of multidecadal variability. In their own words:

Analyses of the annual mean streamflow time series for the 14 streamflow clusters indicated periods of extended wet and dry periods, but did not indicate any strong monotonic trends. Thus, the mean cluster streamflow time series indicate nearly random variability with some periods of persistence.

The bottom line is that McCabe and Wolock do not identify any behavior in historical U.S. streamflow records that is suggestive of an influence from human-caused global warming.

So next time you hear that there are increasing droughts or floods in the U.S. and that they are, through some convoluted explanation, “consistent with” global warming, remember two things: 1) “consistent with” is not the same as “caused by” and, 2) the consensus science linking global warming to changing streamflow characteristics across the U.S. is lacking.


McCabe, G. J., and D. M. Wolock, 2014. Spatial and Temporal Patterns in Conterminous United States Streamflow Characteristics. Geophysical Research Letters, doi:10.1002/2014GL061980