According to reports from NOAA, the U.S. just saw its wettest 12 month period since record keeping began 124 years ago. The fact that this stretch of extremely wet weather was preceded by a time of extraordinary drought during the 2010s is also notable. Because it is exactly this kind of swing from one extreme to the next that you would expect in a world being forced to warm by fossil fuel burning.

(Annual precipitation has increased by about 7 percent across the contiguous U.S. during the past Century. This jibes with our understanding of atmospheric physics in which the rate of evaporation and precipitation increase as the amount of atmospheric moisture climbs by 6-8 percent for each 1 degree C of global warming. It’s worth noting that though precipitation is increasing, it doesn’t mean that soils, in general, hold more moisture. This is due to the fact that rising temperatures also increase the rates at which soils dry. And because precipitation and drying are not spread evenly, you tend to get regions and times of preference for more intense storms or more intense drought. Image source: NOAA. Hat tip to Weather Underground.)

In this part 2 of our hydrology and climate change discussion, I’ll take a look at some of the drivers for the extreme swing from U.S. drought to deluge. The first being that overall global surface warming in the range of 1.1 C is having the effect of amping up global evaporation and precipitation rates by 6-8 percent. In the U.S. this larger climate change influence helped to spur the multi-year droughts across the U.S. west as well as severe drought years for the Central and Eastern U.S.

On the flip side of the hydrological spectrum, warmer land surfaces and oceans have helped to fuel storms through increased evaporation of water moisture — pumping more water vapor into storms and enabling convection. For the past 12 months this has manifest in the form of the powerful and moisture-rich Hurricane Florence. It has also generally loaded the dice for powerful storms and flooding rains as a persistent trough swung over the Central and Eastern U.S. during spring of 2019.

(Examining climate change’s influence on the wettest 12 months in the last 124 years.)

The recent 12 month record wet period thus fits into a regime of extremes. What these larger trends mean is that in the future the U.S. is likely to continue to experience bipolar precipitation patterns — with hard swings between deeper dry and more intense wet periods coming to dominate as the Earth heats up.

The primary mitigation for this continuing trend is tamping down human based carbon emissions. And a clean energy transition away from fossil fuel burning is central to that more optimistic prospect.

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(UPDATED)