Here are the conclusions of a freshly minted study, titled “Characterizing Recent Trends in U.S. Heavy Precipitation” from a group of scientists led by Dr. Martin Hoerling from the NOAA’s System Research Laboratory in Boulder, Colorado:
Analysis of the seasonality in heavy daily precipitation trends supports physical arguments that their changes during 1979-2013 have been intimately linked to internal decadal ocean variability, and less to human-induced climate change…Analysis of model ensemble spread reveals that appreciable 35-yr trends in heavy daily precipitation can occur in the absence of forcing, thereby limiting detection of the weak anthropogenic influence at regional scales [emphasis added].
Basically, after reviewing observations of heavy rains across the country and comparing them to climate model explanations/expectations, Hoerling and colleagues determined that natural variability acting through variations in sea surface temperature patterns, not global warming, is the main driver of the observed changes in heavy precipitation.
They summed up their efforts and findings this way (emphasis also added):
In conclusion, the paper sought to answer the question whether the recent observed trends in heavy daily precipitation constitute a strongly constrained outcome, either of external radiative forcing alone [i.e., greenhouse gas increase], or from a combination of radiative and internal ocean boundary forcing. We emphasized that the overall spatial pattern and seasonality of US trends has been more consistent with internally driven ocean-related forcing than with external radiative forcing. Yet, the magnitude of these forced changes since 1979 was at most equal to the magnitude of observed trends (e.g. over the Far West), and in areas such as the Far Northeast where especially large upward trends have occurred, the forced signals were several factors smaller. From the perspective of external forcing alone [i.e., changes in atmospheric carbon dioxide], the observed trends appear not to have been strongly constrained, and apparently much less so than the efficacy of an external driving mechanism surmised in the National Climate Assessment.
Hoerling’s team tried to say it nicely, but, basically they’re saying that the federal government’s assessment of the impacts of climate change greatly overstates the case for linking dreaded carbon dioxide emissions to extreme precipitation events across the United States (Note: We weren’t as nice when saying that, in fact, the National Assessment Reportoverstates the case for linking carbon dioxide emissions to darn near everything.)
This is not to say that Hoerling and colleagues don’t think that an increasing atmospheric concentration of carbon dioxide isn’t supposed to lead to an enhancement of heavy precipitation over the course of the 21st century. (If they didn’t say that, they’d probably be exiled to the federal climatologist rubber room). Rather, they think that folks (including the president and the authors of the National Climate Assessment) are far too premature in linking observed changes to date with our reliance on coal, oil, and natural gas as primary fuels for our energy production.
Whether or not at some later date a definitive and sizeable (actionable) anthropogenic signal is identifiable in the patterns and trends in heavy precipitation occurrence across the United States is a question whose answer will have to wait—most likely until much closer to the end of the century or beyond.
Hoerling, M., J. Eischeid, J. Perlwitz, X. Quan, K. Wolter, and L. Cheng, 2016. Characterizing Recent Trends in U.S. Heavy Precipitation. Journal of Climate. doi:10.1175/JCLI-D-15-0441.1, in press.