Global warming study treads on thin ice

Written by Ralph Schwartz | The Bellingham Herald | July 31, 2012

By Ralph Schwartz

A new study released today by Environment Washington links global warming to the increasing frequency of extreme rainstorms, but a local weather expert warns that this may not be right.

The 43-page study, “When It Rains, It Pours” (available here) analyzes data taken across the United States showing that the frequency of the most severe storms has increased by 30 percent, from once a year to once every nine months or so. It also concludes that the amount of rain coming out of the biggest storms has increased by 10 percent.

The report takes this data and makes two claims, one bolder than the other:

1) These trends (more frequent severe storms, higher rainfall amounts in the biggest storms) are not random.

2) These trends are caused by global warming.

The first claim is pure statistical analysis. The report shows it is reasonable to believe that something other than the random ups and downs of the weather is causing the increased rainfall.

The second claim relies on the argument that warmer temperatures bring more evaporation and enable the atmosphere to hold more water vapor. This produces bigger clouds capable of more severe storms, Environment Washington spokeswoman Samantha Cramer said.

The report also cites previous studies that linked global warming to enhanced rainfall, including those by the Intergovernmental Panel on Climate Change, which won a Nobel Peace Prize in 2007 for its global warming research.

The panel may have concluded (with what they call “medium confidence”) that severe storms will occur more frequently across the globe as the atmosphere warms. That doesn’t mean the Environment Washington study backs that up, according to Cliff Mass, professor of atmospheric sciences at the University of Washington in Seattle.

Mass is among the large majority of scientists who agree that global warming is happening. But the atmosphere is too complicated to draw such easy connections between a global phenomenon such as global warming and weather patterns over the past six decades, he said.

“This is something you’ve got to be really careful about,” Mass said about this connection.

It’s “a little bit of a stretch,” he said.

Mass hadn’t read the latest report but was conversant in it; he had read closely an earlier version of the study, which had many of the same results. He even duplicated the study’s results in his own research, which looked at rainfall trends along the west coast.

But if this trend is real and not random, and if it’s not caused by global warming, then what could be causing it?

“There’s all kinds of things happening. All kinds of natural variability,” Mass said. He gave as an example snowfall amounts in the Cascade Mountains. A colleague at the University of Washington was claiming a decade ago that smaller snowpacks in the Cascades since the 1950s were a sign of global warming. Mass countered that there was another, natural climate phenomenon at work. In fact, the snowpack in the Cascades has not changed over the past 30 years.

In other words, your conclusions can depend significantly on the time period you choose to look at. There’s no compelling reason to believe Environment Washington’s time period, 1948 to 2011, is dominated by the global warming effect.

Not that this should provide fodder for global-warming deniers. Mass said it’s “pretty clear” global warming is real.

“And the main effects are ahead of us,” he said.