UNIVERSITY PARK, Pa. – Near the end of this torrid, drought-ridden summer, it no doubt will surprise many Pennsylvanians to learn that the state receives more rain than it used to.
But that’s a fact, according to climate experts at Penn State.
“The amount of precipitation that we get here in Pennsylvania has gone up 10 percent in the past century, since 1895 to be exact,” said state climatologist Paul Knight. “It’s a well-known fact among meteorologists – we are getting more rain than we used to.
“Most people don’t realize it because the amount of water we use has increased greatly. So the increase in precipitation has barely kept up with the increase in water use.”
And even though the current drought is part of a four-year span of below-average precipitation, there is nothing to suggest that Pennsylvania is turning into a mountainous expanse of arid landscape, according to Bryan Swistock, extension water resources specialist in the College of Agricultural Sciences.
“I don’t think that anybody is prepared to say that the current drought is really a long-term weather pattern that will end in Pennsylvania getting, say, 25 inches of rain a year, rather than the 40-some inches we get now.
“There is no reason to think that will happen,” Swistock adds. “We are just in a dry cycle. Even the models of global warming don’t have a great deal of strength when they try to predict precipitation trends. The models for global warming say some areas will get wetter and others will get dryer. The ones I have seen don’t suggest any significant changes for Pennsylvania.”
There are examples of up-to-a-decade-long dry periods in recent history, Swistock points out, such as the parched 1960s and the “dust bowl” era in the 1930s.
Inexact science. Global warming does seem to be showing up in Pennsylvania, Knight points out, but the effects are subtle. Temperatures are averaging about a degree higher now than they were just generations ago.
But he notes that scientists are not sure if there is some connection between increased precipitation and slightly higher temperatures overall.
“The trend for more precipitation does roughly mirror the increase in temperature,” he said. “I would not rule out that it may be related to global warming since it has happened in the same time frame as the rise in precipitation.”
Winter changes. In Pennsylvania, like the rest of the country, temperature increases have mostly occurred at night and in the winter.
“I’m saying this in the face of what we all agree has been a hot summer. But really our summer days have not changed much,” said Knight. “In general, the nights are warmer, however. The folks who are modeling global warming say that this is what they expect – that temperature increases should happen at night. More greenhouse gases and more water vapor in the air will hold in the warmth.”
Hard to argue. It is difficult to dispute the theory of global warming now, said Knight, who points to the trend that almost every year during the 1990s broke the previous year’s record as the warmest ever.
But while it has been established that the weather hasn’t changed much when judged over decades or even centuries, what has changed is how much it varies within those longer periods. More simply put, Knight explains, we seem to have more droughts, more floods, more damaging storms, more rapid swings in winds, temperatures, precipitations and short trends.
“There is clear evidence of greater climate variability both here in Pennsylvania and across the country,” he said. “It appears that humans are playing a role in varying the climate. But how much, we are not sure yet. It’s a complicated problem and I don’t agree with those who say humans are completely to blame. I do believe we are playing a role.”
Land use impact. Knight contends that land-use patterns may be affecting the climate in Pennsylvania more than the release of greenhouse gases.
“Increases in pavement and development are affecting the environment by influencing our weather,” he explains. In highly urbanized areas, such as the eastern and western portions of the state, so much impermeable (paved) surface has been created that a huge volume of water runs off land and is lost, never penetrating into the ground, Knight said.
And he cites studies that conclude that the tiny particles in power plant emissions, which are carried into the atmosphere and cause clouds, do not behave like larger natural particles such as dust.
“Because they are smaller they tend to hang around longer, but they don’t cause rain and snow as much as the larger particles that are put into the atmosphere by natural causes,” he said. “The larger particles from natural processes produce different drop sizes. The smaller particles from combustion form smaller droplets that don’t grow.”
Strange cycle. Knight did observe one Pennsylvania precipitation trend in the last decade that he can’t begin to explain, but there was no denying it.
“Starting in 1991, we had sort of a quirky growing season cycle, that alternated annually between dry and wet,” he explained. “For example, in ’91 it was dry, in ’92 it was wet. For the whole decade, the odd years were dry and the even years were wet – it was fascinating.
“The cycle ended in 2001,” he added. “I don’t know why it was there, but it was a pretty clear trend through the ’90s.”
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