News•April 26, 2013

Wild Weather Swings May Be a Sign of Climate Change

For a political candidate, being labeled a “flip-flopper” can be a career killer. Just ask Secretary of State John Kerry, who lost his 2004 presidential race in part because of his reputation for voting against bills before he voted for them. Increasingly, though, the label also applies to North American weather, which has been lurching from one extreme to the next in a pattern that is consistent with global warming.

Flooding on Grand Avenue in Chicago after heavy rains struck the city in mid-April.
Credit: NWS via John Trillik.

Climate studies have warned us to expect more frequent and intense extreme events, such as heavy rain and snow storms, along with heat waves. While weather variability is nothing new, the wild swings in weather — termed “weather whiplash” and that have recently occurred across the Midwest and South Central states during the past few years, from record flood to record drought and back to record flood — may be an example of what’s in store as global warming continues to alter the atmosphere.

As exhibit A, consider what is going on right now in the nation’s heartland.

Chicago has already had its wettest April on record, with nearly 9 inches of precipitation, and its wettest start to the year, with a little more than 17 inches of precipitation so far. That compares to the 26.91 inches of precipitation that fell during all of 2012 (weather records in Chicago date back to 1871).

Chicago had a bout of especially heavy rainfall on April 17-18, which set a record for the heaviest two-day rainstorm there, and which led to major urban and river flooding.

The wet and cool conditions in Chicago this spring are a sharp reversal of fortune compared to last year, when the city experienced its warmest March on record, and severe drought conditions were taking hold starting in April.

“This year, of course, we cannot buy any spring warmth,” said a report from the National Weather Service in Chicago.

The heavy rains in the Midwest have helped to raise Great Lakes water levels from record lows set during last year’s drought to more average levels. According to WGN-TV meteorologist Tom Skilling, the U.S. Army Corps of Engineers has found that Lake Michigan's level had risen by 9 inches during April alone.

“Each inch increase on Lake Michigan is the equivalent of 390-billion gallons of water, so we're talking about a whole lot of water there,” Skilling wrote on his Facebook page. “We've more than halved the deficit in Lake Michigan's water level between this year and last. Lake Michigan — only a month ago 17 inches lower this year than last — is now just 7 inches lower than a year ago. And the other Great Lakes have posted water-level increases since April 1 as well,” Skilling wrote.

In August 2012, drought had left runoff in the Upper Mississippi River Basin in the bottom 10 percent. Now, water levels are in the top 95 percent.
Click image to enlarge. Credit: Climate Central.

Other areas have also been experiencing sudden shifts in the weather, particularly the Ohio and Mississippi River Valleys. Record rains in the Ohio Valley in 2011 caused the Mississippi River to reach record to near-record crests downstream from the intersection with the Ohio River. At New Madrid, Mo., the crest of 48.35 feet on May 6, 2011 was the highest recorded, forcing the Army Corps of Engineers to blow up levees along the river to lower water levels. That flooded farmland, but saved more heavily populated areas.

Yet just 15 months after the record flooding, on Aug. 30, 2012, the river dropped to the lowest level on record at New Madrid. That forced the Army Corps to reverse tactics, this time dredging the river to try to ensure the river remained navigable for barge and ship traffic.

Now the river is back in flood, with record highs noted in some locations along the Mississippi River’s tributaries.

“The recent flooding is due to an inordinately wet April,” said NWS climate services program manager Victor Murphy. “Portions of northern and central Illinois and eastern Iowa have received from 200 percent to 400 percent of normal precipitation over the past 30 days,” Murphy said.

While spring flooding is typical in the Midwest as snow cover melts and heavy spring rainfall occurs, the recent extreme swings between record dry conditions and record flooding are anything but normal.

Trend in 1-day precipitation extremes through 2012 for the lower 48 states.
Click image to enlarge. Credit: NCDC.

The swings may be connected in part to climate change, since the atmosphere is now carrying more moisture than it used to due to warming air and ocean temperatures. Climate studies have projected that precipitation extremes will become more frequent and severe. Some studies have already found an increase in heavy precipitation events across the Northern Hemisphere and in North America, including in the Midwest.

The proximate cause for the floods and the drought have had to do with sharp and extended variations in the jet stream, which in the case of flooding means storms were steered into the Midwest, or in the case of drought means they were steered away from the region. However, the long-term increase in water vapor due to manmade global warming acts to worsen both extremes by providing more water for storms to work with, therefore dumping heavier rains, or making temperatures hotter than they otherwise would be during a drought, thereby drying soils out even more.

Some recent studies have also linked increased variability in the jet stream itself to rapid Arctic climate change, and this is an area of ongoing scientific research.

According to a draft federal climate assessment released in 2012, heavy precipitation events in the U.S. have increased during the past half-century. Managing increased variability, with the climate lurching from dry to extremely wet and back again, is already putting a strain on businesses and municipalities that have to manage the consequences, from shipping companies trying to move their products along a shrunken or swollen Mississippi River to cities trying to prevent flooding of entire neighborhoods.

Another area that has suddenly reversed its fortunes is the state of Georgia, which got rid of its two-year drought in just two months. As recently as February, 81 percent of the state was in drought. But heavy rainfall that began late in February and continued through April has slashed the portion of the state in drought conditions down to a measly 2 percent.

“This is due largely to Georgia having its wettest February on record, with a statewide average precipitation of 9.92 inches, or more than double the 100-year average of 4.5,” Murphy said in an email conversation.

So yes, when it comes to the atmosphere, the flip-flopper label certainly applies, at least lately. And we better get used to it.