Understanding extreme weather in an era of climate change

NOAA

The US has clearly seen some pretty extreme weather events over the last year. These events have caused both billions of dollars in property damage and endless arguments over how much can be attributed to climate change. Even as scientists work on the problem of attribution, the public has often made up its mind on what's to blame.

To try to bring some sanity to the discussion, the meeting of the American Association for the Advancement of Science hosted a session on US weather extremes. Although there were a variety of talks, three presentations nicely captured the challenges: one on the state of the US climate, another on a recent climate event, and a third on trying to convey all of this to the public.

Turning up the heat

The first speaker was Donald Wuebbles of the University of Illinois. He started out by saying that you can view the climate as a bell curve, with extreme heat and cold events occurring where it starts to flatten out to the left and right. In that view, changing the climate could do any of three things. The curve itself could shift, with hot events becoming more common and cold events becoming less frequent. You could also potentially flatten the curve, with the typical climate remaining roughly the same but the instances of extreme events increased. Or, he said, you could do both.

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2012 was hottest on record in the US, warm everywhere else

Of the 118 years on record, 2012 came out on top in 19 states.

NOAA

When 19 of the contiguous 48 states have the warmest years on record, it's pretty much guaranteed to be a record year for the country as a whole. But saying that 2012 had record heat doesn't truly capture how much of an aberration the year was for the US, according to the annual report from NOAA. Another 10 states had their second-warmest years on record; seven more had their third. The end result was a year that was a full degree Fahrenheit above the previous record holder, and 1.8°C (3.3°F) above the average from last century.

Most of the warmth came during the early part of the year, which was accompanied by significant drought conditions across a wide section of the country. NOAA's "climate extremes index," which incorporates temperature, precipitation, and other trends shows the entire South (from Texas to the East Coast) experienced its most extreme weather in the spring; everything but the West Coast was much above average during that time. The index shows that 2012 was either the most extreme on record or came in second to 1998, depending on whether you included hurricane activity.

For some regions of the country, snow took the year off. As NOAA puts it, "The 2011/12 winter season was nearly non-existent for much of the eastern half of the nation." The result was the third-lowest snow cover on record in the 48 contiguous states, with only a relatively heavy winter in the Pacific Northwest keeping matters from being worse. The low snowpack contributed to the drought conditions that prevailed over most of the summer.

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Why European forecasters saw Sandy’s path first

Satellite image of Hurricane Sandy.

NASA Earth Observatory

Look, it’s not easy to predict the weather. And while it’s frustrating to have weekend plans spoiled by unexpected showers, the stakes are much higher for potentially catastrophic storms like hurricanes. The warning provided ahead of “Superstorm” Sandy’s arrival on the East Coast saved lives, without question, and that’s a success for weather science. But the story carries a reminder that there’s room for improvement in the US, where many argue forecasting tools have been neglected amid continual budget crunches.

Seven days before Sandy made landfall in New Jersey, the atmospheric crystal ball was partly cloudy. The US National Weather Service forecast model showed a chance that Sandy might come ashore, but indicated that it was more likely the storm would spin off into the Atlantic. The European Centre for Medium-Range Weather Forecasts (ECMWF) model, however, definitely pointed the storm ashore. It would be about three days before the US model totally converged on the Europeans' forecast.

The fact that the National Weather Service was able to issue detailed warnings well ahead of Sandy is both a testament to what these amazing models are able to do, and the hard work of the professionals behind the scenes.

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Fresher ocean water can boost hurricanes

Hurricane Omar

NASA Earth Observatory

There are two things coastal residents nervously look for in hurricanes (more generally, “tropical cyclones”). First, where will the storm make landfall? Second, how strong will it be? Tropical cyclones are fueled by warm water, so sea surface temperatures have a lot to do with how a storm plays out.

Normally, a tropical cyclone traveling over warm water will cause the water to cool as the strong winds stir up the water, which brings cooler water to the surface. This helps limit the growth of the cyclone, because it effectively diminishes its own fuel source. However, that doesn’t always happen, and the storm can continue to grow as a result.

In 2008, Hurricane Omar spun its way through the Caribbean. It was not an especially damaging storm, but it did give researchers some food for thought. If you look at a map of Omar’s effect on sea surface temperatures, you’ll see that as it neared the Virgin Islands, it stopped causing the warm sea surface to cool. Later, the normal cooling behavior resumed. So what happened?

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For the US, July was the warmest month we’ve ever seen

The state-by-state rank of July temperatures out of 118 years of records. Only one state set a record at 118, but a huge band of states ranked above 110.

NOAA

NOAA has released the numbers on July's temperatures within the US, and they continue the trend of what's been an exceptionally scorching year. With temperatures a full 3.3°F above the twentieth century average, it was the hottest month ever recorded within the US. While only one state (Virginia) set a record high monthly temperature, 32 different states across a broad sweep of the country had months that were in the top 10; combined, that made for an exceptional month.

But July is only the latest in a string of exceptionally hot months. The first seven months of this year were the hottest on record, and the 12-month period that includes it also set a record (narrowly beating out the 12 months that ended this past June). But simply describing records doesn't truly convey what an outlier this year is for the US; fortunately, NOAA has provided a graph that does so. As it clearly shows, every month since February is a serious outlier from historic conditions.

Enlarge / How exceptional has 2012 been? The dark red trace shows that almost every month has been an outlier.

NOAA

Rainfall continued to be a problem, with up to 22 percent of the country reaching exceptional or extreme drought readings. The problem is exacerbated by the location of that 22 percent: the states that were the furthest below normal rainfall stretch east from Nebraska and Kansas to Illinois, which covers the US's agricultural heartland.

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Greenland hit by dramatic ice thaw

Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory

Data from satellites monitoring the Greenland ice sheet show that 97 percent of the ice sheet surface has begun to thaw.

On 8 July, Modis satellites that study the surface temperature of the ice showed that about 40 percent of the ice sheet had melted at or near the surface—slightly below the average of 50 percent at this time of year.

Just a few days later, however, that proportion had risen dramatically to 97 percent, covering almost the entirety of the massive island, from the low-lying, sparsely covered edges to the three-kilometer-thick (1.86mi) interior.

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Ideology clouds how we perceive the temperatures

Areas currently experiencing drought conditions in the US. Their residents are likely to actually perceive that there's been a shortage of rain.

NOAA

Earlier this year, we covered some polling data in which people were asked what factor shaped their acceptance of climate change. Buried in the data were two apparently contradictory findings: there is a large partisan divide in acceptance of climate change, but most respondents said they base their acceptance on their personal experience of the weather. Assuming that hot weather shows no partisan bias, this doesn't make much sense—political beliefs shouldn't influence what we think about the weather.

And yet they do. That's the conclusion of a new paper that dives into extensive polling data to find out how people perceive different trends in the climate. The results show that not all weather events are created equal. When it comes to things like flood and droughts, most people seem to have accurately registered the recent trends in their area. But when the subject shifts to temperatures, the actual trends become irrelevant, and ideology and political beliefs shape how people perceive things. As the authors put it, "the contentious nature of the climate change debate has influenced the way in which Americans perceive their local weather."

That authors of the study used data from about 8,000 poll responses, obtained between 2008 and 2011. The surveys included questions about how people perceived the weather in recent years. For temperatures, they were asked whether they were higher, the same, or lower than in past decades. Similar questions were asked about the frequency of floods and droughts. The survey also asked for self-assessments of political leanings, and included several questions that got at core ideological beliefs (such as egalitarian or individualist tendencies).

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Colder winters may be new normal due to melting Arctic ice

Recent summer melts have left lots of the ocean exposed to sunlight.

NASA

In early 2011, the US and Europe froze, even as Greenland and Alaska experienced unusual periods of warmth. This year, the US and Europe were baking as the winter drew to an end, even as cold air hovered over Central Europe and Asia. In the Northern Hemisphere, extreme winter weather tends to be associated with the negative phase of the Arctic Oscillation, a wind pattern that dominates the polar region. And a consensus is building that changes in the Arctic may have permanently placed the Oscillation in the negative mode, leading to stable changes in the winters of the Northern Hemisphere. Cornell professor Charles H. Greene has just published a review of this idea, and we talked with him about what the warming Arctic might mean for the US and Europe.

Greene's paper describes a key determinant of the Northern Hemisphere's winter weather: the Arctic Oscillation. When that's in its positive phase, a strong set of winds called the Polar Vortex forms. These winds help trap Arctic air masses at the pole, keeping the cold out of the mid-latitudes. This also allows the jet stream to take a more direct route around the globe, moderating the weather.

But, over the last few years, the Oscillation has been strongly negative; in fact, in 2010, we saw a record for the most strongly negative period we'd ever recorded. During this phase, the winds of the Polar Vortex weaken, allowing the cold Arctic air to intrude or mix into the air at lower latitudes. As a result of this, Greene told Ars two things happen to the jet stream: it gets substantially weaker, and it tends to meander widely from north to south as it traverses the globe. This can lead to the severe chills the US and Europe have experienced over the past several winters, but the meandering jet stream can also draw warmer southern air north, as happened in the US this spring.

Shuttle-monitoring radar can watch raindrops in a cloud

Doppler weather radar at the National Severe Storms Laboratory.

NOAA

Turn on the Weather Channel, and you might see storm-tracking images from your friendly local Doppler radar system. Although your average storm front is quite large, this technology can also track smaller objects—much smaller, in fact. A Doppler radar system originally designed to track small debris particles falling off the Space Shuttle has now been used to detect individual rain drops within clouds. The hope is that this work will give us a better understanding of the dynamics that help drive weather.

First, some radar basics. The term is actually an acronym for “radio detection and ranging,” and it was originally developed before and during World War II to detect incoming ships and aircraft. A radar system transmits radio wave or microwave signals, some of which are reflected back to the receiver when they hit a large change in density—typically, a solid or liquid object. Electrically conductive materials like metal and water show up particularly well.

All ground-based radar systems used to track moving objects are actually Doppler radars, which rely on the Doppler shift in the radio waves to determine the relative velocity of whatever they're tracking. This is the same Doppler effect that causes the wail of an ambulance siren to shift in frequency as it zips toward or away from you. The same principle applies to electromagnetic radiation: the frequency of reflected waves increases (with respect to the original transmitted waves) when an object is moving toward the receiver, and decreases when it is moving away. Doppler radar signal processors pick up on these changes to track the relative velocity of objects.

It wasn’t your imagination: US experienced warmest March ever

As record temperatures swept through the Midwest and trees bloomed early across the Northeast, lots of talk focused on what an unusually warm start spring was having. The folks at the National Oceanic and Atmospheric Administration have now crunched the numbers, and found that it wasn't just unusually warm—March was bizarrely hot. With 15,000 record high temperatures set in the US, it was far and away the warmest March in the nation's history, and only a single month—January of 2006—was as far off from the monthly average.

Only one of the 48 contiguous states (Washington) was below normal, and a huge slice down the center of the country was bathed in bright red in NOAA's map, indicative of record high temperatures. The heatwave was partly responsible for moving the first quarter of the year into the top slot of the US record books. The high temperatures also kicked off an unusually early spring cluster of tornadoes in the Midwest.

Neither NASA nor NOAA have managed to do the global monthly averages yet, so it's not clear if our experience was shared by much of the rest of the planet (the US occupies a relatively small fraction of its surface). So far this year, the global means have been pretty mundane. They're above last century's average, but not by a lot, and every month has been above that average since early 1994.

NOAA indicates that it was a specific weather pattern that pushed heat into the central US. One of the key drivers of global temperature, the tropical Pacific's surface temperatures, remain in a cooler, La Niña state, so it's unlikely the rest of the world shared in our warmth.

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