'March summer' linked to global warming
An early "March summer" in Canada and the United States with record high temperatures may be a symptom of global warming, researchers said. Record-breaking summer-like conditions have been reported across North America following an unusually mild winter, meteorologists said.
In Canada, the temperature in Saint John, New Brunswick, hit 77 degrees Fahrenheit on March 21, smashing the previous record high for March of 64 degrees, NewScientist.com reported. "We've never seen these kinds of temperatures before," Dave Phillips, a senior climatologist at Environment Canada, said. "It's quite remarkable." "The duration, areal size, and intensity of the 'summer in March' heat wave are simply off-scale," Jeff Masters of the Weather Underground Web site in San Francisco said. "The event ranks as one of North America's most extraordinary weather events in recorded history."
A large loop in the jet stream over the continent, funneling warm air northward from the Gulf of Mexico, remained "stuck" in place for more than a week, a phenomenon known as a blocking pattern, Masters said. And Phillips points out that air flowing northward in the spring would normally be cooled as it passes over cold, snowy ground, but the mild winter has left very little snow on the ground and the air is hardly cooling at all.
There is evidence global warming can both reduce snow amounts on the ground and influence atmospheric conditions aloft such as the jet stream, researchers said. "Global warming boosts the probability of really extreme events, like the recent U.S. heat wave, far more than it boosts more moderate events," climate scientists Stefan Rahmstorf and Dim Coumou of RealClimate.org wrote in a blog post.
Fonte:
UPI.com
Has global warming brought an early summer to the US?
North America has been experiencing unusual weather of late. After a mild winter over much of the continent, last week it experienced record-breaking summer-like conditions. In Canada, for instance, the thermometer in St John's, New Brunswick, hit 25.4 °C on 21 March, smashing the previous record high for March of 17.5 °C. "We've never seen these kinds of temperatures before. It's quite remarkable," Dave Phillips, a senior climatologist at Environment Canada, a government agency, told local media.
"The duration, areal size, and intensity of the 'summer in March' heat wave are simply off-scale," says Jeff Masters of the Weather Underground. "The event ranks as one of North America's most extraordinary weather events in recorded history." New Scientist takes a closer look.
The "summer in March" has now come to an end but what caused it?
Meteorologists have been pointing to two main factors. First, as pointed out by Masters, there was a big loop in the jet stream over the continent, funnelling warm air northwards from the Gulf of Mexico. This loop in the jet stream remained "stuck" in place for over a week, a phenomenon known as a blocking pattern.
And the second factor?
Phillips points out that air flowing northwards in the spring would normally be cooled as it passes over cold, snowy ground. But this year there is very little snow because of the mild winter and the air was hardly cooled at all.
So is there a link with global warming?
There may be. "Global warming boosts the probability of really extreme events, like the recent US heat wave, far more than it boosts more moderate events," point out climate scientists Stefan Rahmstorf and Dim Coumou in a
blogpost on RealClimate.org.
How could global warming have produced an off-the-charts event like the "summer in March"?
Nobody can say for sure, but we can speculate. First, sea-surface temperatures in the Gulf of Mexico have been higher than normal in the past couple of months, due to global warming, which means the air that flowed north would have been warmer to start with. That fits with global warming trends.
There is also some evidence that global warming is reducing the snowpack, at least in the western US. Global warming may also have influenced the jet stream pattern. According to research published last week by Jennifer Francis of Rutgers University (Geophysical Research Letters, vol 39, L06801), the rapid warming in the Arctic is affecting atmospheric circulation further south, making weather patterns more persistent – more blocking, in other words – which makes some kinds of extreme weather, such as heatwaves, more likely.
So several different factors, each made more likely by global warming, combined to produce this very extreme event?
Possibly. "I think [that is] right but it is hard to prove," said Kevin Trenberth of the US National Center for Atmospheric Research, when New Scientist asked him about this. Masters, meanwhile, described the idea as "entirely plausible".
Does this mean more extremely warm springs are likely?
Not necessarily. The confluence of events that caused the "summer in March" might still be exceedingly rare, even if the odds of such an event occurring are much higher than before. Again, nobody really knows. Masters says more detailed climate models are needed to track synergistic effects and anticipate extreme events such as this. However, the "summer in March" is cause for concern.
In 2004, Paul Epstein of Harvard Medical School's Centre for Health and the Global Environment, and James McCarthy of Harvard University, claimed: "We are already observing signs of instability within the climate system [...] there is no assurance that the rate of greenhouse gas build up will not force the system to oscillate erratically and yield significant and punishing surprises."
As Masters notes, significant and punishing surprises are exactly what we have been seeing in the US and elsewhere recently. And if such an "off the chart" event can occur when the world has warmed by less than 1 °C, what sort of extreme events will occur by 2050, when the planet could be as much as 3 °C hotter?
Fonte:
NewScientist