precipitation

Heavy Rains Increasing, but Not Disproportionately So

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.”

A new paper has been published in the journal Geophysical Research Letters that examines trends in heavy rainfall amounts across the U.S. The paper is authored by Newcastle University’s Renaud Barbero and colleagues, and, to summarize, finds that the heaviest rainfall events of the year have been increasing in magnitude since 1951 when averaged across nearly 500 stations distributed across the U.S. (note: results from individual stations may differ from the general finding).

Someone with a critical eye might ask the real question, which is “how much?” That such a number does not jump out of this paper—a cynic would say—probably means it is very small. Read on and you will find the answer.

That rainfall on the rainiest day of the year is increasing is, of itself, hardly surprising considering that the total annual rainfall amount averaged across the U.S. has also been increasing during this same period (again, results from individual locations/regions may (and do) depart from this generality).

Changes in heavy rainfall like this are often luridly described as a “disproportionate increase” in extreme events, or that extreme precipitation increases are “worse than expected.”

Spin Cycle: Attributing Louisiana Floods to Global Warming

The Spin Cycle is a reoccurring feature based upon just how much the latest weather or climate story, policy pronouncement, or simply poobah blather spins the truth. Statements are given a rating between 1-5 spin cycles, with fewer cycles meaning less spin. For a more in-depth description, visit the inaugural edition.

In mid-August a slow moving unnamed tropical system dumped copious amounts of precipitation in the Baton Rouge region of Louisiana. Reports were of some locations receiving over 30 inches of rain during the event. Louisiana’s governor John Bel Edwards called the resultant floods “historic” and “unprecedented.”

Some elements in the media were quick to link in human-caused climate change (just as they are to seemingly every extreme weather event). The New York Times, for example, ran a piece titled “Flooding in the South Looks a Lot Like Climate Change.”

We were equally quick to point out that there was no need to invoke global warming in that the central Gulf Coast is prime country for big rain events and that similar, and even larger, rainfall totals have been racked up there during times when there were far fewer greenhouse gases in the atmosphere—like in 1979 when 45 inches of precipitation fell over Alvin, TX from the slow passage of tropical storm Claudette, or in 1940 when 37.5 in. fell on Miller Island, LA from another stalled unnamed tropical system.

But we suspected that this wouldn’t be the end of it, and we were right.

Six Decades of Temperature and Precipitation in Kentucky

Air temperature and precipitation, in the words of Chattopadhyay and Edwards (2016), are “two of the most important variables in the fields of climate sciences and hydrology.” Understanding how and why they change has long been the subject of research, and reliable detection and characterization of trends in these variables is necessary, especially at the scale of a political decision-making entity such as a state.

Two Millennia of Snowfall Accumulation in Antarctica

Providing the rationale for their work, Roberts et al. (2015) write that “the short and sparse instrumental record in the high latitudes of the Southern Hemisphere means investigating long-term precipitation variability in this region is difficult without access to appropriate proxy records.” It was therefore the objective of this team of nine researchers to extend the duration of the Law Dome, East Antarctica, snowfall accumulation record back in time an additional 750 years so that it would cover over two millennia.

The resultant 2035 year-long proxy (22 BC to 2012 AD) is presented in the figure below. As reported by the authors, the average long-term snow accumulation rate was calculated as 0.686 m yr-1 (27 inches) ice equivalent, which rate they say “is in agreement with previous estimates, and further supports the notion that there is no long-term trend in snow accumulation rates, or that any trend is constant and linear over the [2035-year] period of measurement.”

If this number seems low for such an icy continent, the fact is that most high-latitude locations in both hemispheres would qualify as deserts based upon annual precipitation. In many places, it is literally “too cold to snow” as the frigid air can hold only tiny amounts of moisture.

There were several decadal-scale oscillations in the record, described by the authors as “common,” with “74 events (33 positive and 41 negative) of at least a 10-year duration in the record.”  The three longest periods of above average integrated snowfall occurred over the intervals 380-442, 727-783, and 1970-2009, while the three longest periods of below average integrated snowfall occurred during 663-704, 933-975, and 1429-1468.

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