In the winter, there was the record snowfall across the Northeast. Record rainfall and floods in Ohio Valley followed in April and May. The Southwest has been plagued by drought for months, while tornadoes have devastated the Midwest and South. Record heat is scorching most of the country this week — just days after snow fell in Hawaii. Meteorologists predict a harrowing hurricane season.
Is some underlying climactic condition driving these extreme weather events? There are two potential candidates: The first is La Niña, trade winds that blow through every five years, cooling the water of the equatorial Pacific Ocean and shifting the jet stream northwest. The second is global warming, the average worldwide temperature increase of 1 to 2 degrees Fahrenheit over the past century that has resulted from the dramatic rise of carbon dioxide levels in the atmosphere.
La Niña and global warming are both partly responsible for some of the episodes of wild weather, experts say. However, natural atmospheric variability has also come into play this year; to some extent, the pile-on of wild weather is random chance.
Drought here, floods there
According to Martin Hoerling, a research meteorologist and chairman of the climate variability research program at the National Oceanic and Atmospheric Administration (NOAA), "Some aspects of the recent U.S. climate anomalies, for example, the Texas, Arizona and New Mexico drought, are likely linked to a common factor: the cumulative effect of the prolonged La Niña event that has lasted from summer 2010 until this spring." [Read: La Niña Ends, So What's Next?]
"The impact of La Niña is also to generate wet Ohio Valley conditions in late winter," Hoerling told Life's Little Mysteries, a sister site to LiveScience. This year, however, the rains were heavier than in any prior year, La-Niña-year or otherwise. "The extreme nature of these rains is beyond what can be reconciled with La Niña alone."
Most NOAA researchers believe the extreme flooding is partially a result of climate change induced by global warming. "The flooding and the heavy rainfall are consistent with what we expect with global warming," said David Easterling, chief of the Global Climate Applications Division at NOAA's National Climatic Data Center. "Looking at some of the modern trends, we've seen increases in the amount of water vapor in the atmosphere, drawing a direct link between what's happening in the Midwest and global warming."
On top of global warming and La Niña, natural weather variability is undeniably playing a role, just as it does every year. "Natural atmospheric variability almost certainly contributed significantly to these extreme events," Hoerling wrote in an email. There have always been droughts and floods, and while global warming and La Niña may have helped them along this year, they may have hit hard regardless.
The nearly nationwide heat wave of the past week was caused by what meteorologists call a "high pressure ridge," a long strip of air that has high atmospheric pressure, making it warm and dry. While such ridges come about naturally, the record high temperatures this time around are linked to global warming, experts say.
The average temperature of the Earth has shifted higher in recent years, and that means the bell curve that represents the annual temperature variation has shifted over as well. Along with the higher average temperature, you therefore also get higher extremes. "An [average] warming of climate would indeed increase the probability of extreme hot periods," Hoerling wrote.
The link between global warming, La Niña and this year's violent tornadoes is unclear. Some scientists speculate that La Niña did have an impact by dying down earlier in the year than usual. "If La Niña had maintained its strength, perhaps we wouldn't have seen so many tornadoes," Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory in California, told OurAmazingPlanet, a sister site to LiveScience.
The jet stream, an "atmospheric fence" where cool, dry air meets warm, moist air, is usually pushed north by the La Niña trade winds in the spring, Patzert explained. This year, La Niña's early departure left the jet stream draped across the middle of the country, mixing cool, dry northern air with warm, moist southern air in tornado alley — a recipe for disaster.
Did global warming make the tornado season even worse? NOAA scientists have been trying to figure that out; so far, they say, the evidence is inconclusive.
Thermodynamic instability, the condition that causes thunderstorms by forming vertical clouds, combined with wind shear, which stabilizes the updraft and causes it to rotate, are the "perfect storm" that leads to a funnel. Trends associated with global warming do indeed amplify the first condition: Hotter air holds more moisture, so global warming increases the likelihood of vertical clouds forming. But you can't get tornadoes without wind shear, and "the expectation is that there will be less of that wind shear with global warming," Easterling said.
So with one tornado-conducive factor being increased and the other being decreased, it's really hard to say how global warming affects tornado activity. The scientists are still looking into it. "Inconclusive data does not lead you to conclude that there's no evidence of a connection," Hoerling said.
The scientists point out that there has always been extreme weather, and this year's events aren't entirely caused by La Niña and climate change. "I don't know whether climate has varied more extremely or not during recent decades," Hoerling wrote in an email.
Furthermore, "there is much more to extremes that just temperature and precipitation. It's for example not clear how strong cyclones and their associated wind speeds will change [due to climate change]. There has been no evidence for a detected change in hurricanes or other violent wind storms. Yet, there is an indication that heavy daily rain events have increased in some regions during some seasons.
"In sum, there are many measures of extremes, and it's unclear if any aggregate change in extremes has indeed occurred," Hoerling said.