This is the wrong question, really, but if pushed, one would have to answer, “Well, not directly.” The problem with the question is that it conflates the two very different concepts of weather and climate. A better-posed question would be, “Is Harvard’s wet June, punctuated by heavy downpours, consistent with climate change?” And there, unfortunately, the answer is clearly “Yes.”
Mark Twain supposedly observed, “Climate is what we expect, weather is what we get.” Climate is generally regarded as the long-term statistical average of day-to-day weather as measured over periods of up to 30 years. While Harvard’s weather may well change on an hourly basis, climate is relatively stable and reflects our general expectations of seasonal weather patterns. Thus, no single, short-term weather event can be said to be linked to or to be caused by climate change.
This graph shows the percentage of the land area of the contiguous 48 states where a much greater than normal portion of total annual precipitation has come from extreme single-day precipitation events. The vertical blue bars represent individual years; the red line across the years is a nine-year weighted average. The yelow bars have been added by the author to highlight trends identified by EPA. CLICK TO ENLARGE
Source: US EPA, Heavy Precipitation: Climate Change Indicators in the U.S. (2013)
Consequences of a warming planet
But basic high-school physics strongly suggests that, on average, more precipitation awaits us on a warming planet, and we expect to receive more and more of it in so-called “extreme precipitation events”—heavy and sustained rainfalls and downpours.
That basic physics? As global warming proceeds, surface waters increase in temperature, and the vapor pressure of that water increases exponentially. Vapor pressure is a measure of liquid water’s natural tendency to evaporate. The higher its temperature and vapor pressure, the greater is the amount of water evaporated. An increase in temperature of two degrees Celsius, or 3.6 degrees Fahrenheit, increases vapor pressure and thus the concentration of water in saturated air by 13.5 percent, and the considerable thermal energy required for vaporization rises into the skies along with the increased mass of water vapor that carries it. Warm, humid, and buoyant air is what produces clouds and fuels thunderstorms.
What goes up…
What goes up must come down, although not necessarily in the same place! As the inherent buoyancy of warm, humid air or the advance of cold fronts pushes parcels of water-laden air higher into the atmosphere, the water-saturated air expands and its temperature drops to the point where water condenses and falls as rain. At the same time, the stored thermal energy that went into evaporating that water—its so-called “latent heat”—is released and converted, in part, to kinetic energy of motion of air aloft. This generates the violent updrafts and downdrafts that accompany thunderstorms.
So the higher the earth’s surface temperature, the greater the quantities of water and energy carried aloft to be released in subsequent rain events. On average, then, the planet can expect a rainier future. But certain places at certain times will experience higher-than-average levels of precipitation, while other places at other times will experience drying, even drought—all dependent on shifting patterns of atmospheric circulations and ocean currents.
Data from the National Oceanic and Atmospheric Administration (NOAA) and Environmental Protection Agency (EPA) show that across the Northeast precipitation has increased during the past century by 10 to 20 percent—more than for the contiguous 48 states as a whole—and further increases in this range are projected for the coming century.
…comes down
Of course, we don’t really experience “average precipitation.” We’re much more interested in how that rainfall is packaged, especially when it arrives in heavy or “extreme” events. Climate scientists and meteorologists consider an event to be “extreme” if its total rainfall lands it in the highest 10 percent of the distribution. Harvard has just experienced some intense rain; going forward, we expect that a higher fraction of our annual precipitation will be delivered to us in concentrated fashion. Climate change is projected to increase not only the annual amount of precipitation received but also the intensity of rainfall events.
On the one hand, the anticipated increase in extreme events can be explained mathematically, as a consequence of what happens to the “tail” of a distribution when the distribution as a whole shifts to the right. Consider the distribution of probabilities that rainfalls will be of a certain size. Most rainfalls will bring near-average amounts of precipitation and will constitute the broad center region of the distribution; downpours will constitute relatively infrequent, low-probability events out at the “tail,” or extreme end, of the distribution.
As annual average precipitation and the amount of rain in a typical rainfall increase, the number of rainfalls that qualify as extreme will also increase. Moreover, both the number and size of these extreme rainfalls will increase in a way that will be out of proportion to, and greater than, the increase in the averages, due to the shape of the distribution curve. Remember being graded “on the curve” in school, where the curve describing the distribution of grades had a big, broad hump in the middle, mostly C’s, and just a few A’s and F’s at the tails? Now imagine what happens to the proportion of A-plus grades that will be awarded if the teacher decides to push the curve to the right and bump the average grade up to a B.
Sluggers on steroids
But the popular metaphor of “the slugger on steroids” provides a more compelling and intuitive explanation. We’ve always had downpours, just as baseball sluggers have always hit home runs. Always have and always will, so no single home run hit in the future by a slugger “juiced” on steroids can be attributed to drug use, just as June’s heavy rainfalls cannot unambiguously be attributed to climate change. But the trends or patterns are there, and the effect of doping will show up in the slugger’s statistics—both in higher batting percentage and a greater number of home runs hit.
Those home runs are analogous to extreme precipitation events, which are expected to occur with increasing frequency in a globally warmed “climate on steroids.”
What say the data? NOAA has compiled historical data for “extreme one-day precipitation events” going back to 1910, as shown in the accompanying graphic. The EPA interprets these data as follows: “The prevalence of extreme single-day precipitation events remained fairly steady between 1910 and the 1980s, but has risen substantially since then.” Still other data on NOAA’s “Climate Extremes” website show that the frequency of extreme rain events and their rate of increase over time have been larger for the Northeast than for the country as a whole.
Steve Matson, a chemical engineer by training, is a 30-year resident of Harvard and has recently taught clean energy technology and policy at Tufts University.
