USA –-As wildfires devour acreage across the West and a heat wave broils in the East, the question seems natural: Are we feeling the effects of global warming? Scientists still answer cautiously. No single weather event, they say, can be linked directly to global climate change. But some researchers have begun to draw a broader connection between sweltering temperatures, tinder-dry forests and a warming planet.
While direct links are still elusive, the statistics, they say, reveal climate change hovering in the background – pushing conditions more consistently toward extremes, and causing those extremes to increase in severity.
“One of the things I try to impress upon people is that the environment that all the storms are forming in nowadays, that all the weather occurs in, is different than it used to be because of climate change,” said Kevin Trenberth, a climate researcher at the National Center for Atmospheric Research in Boulder, Colo.
Since the 1950s, heat waves have increased in number and duration across the world, and also have grown hotter, Trenberth and his co-authors wrote in a recent review of other climate studies published through May 2012.
New record high temperatures in the contiguous United States are outpacing record lows, a trend that also is swinging sharply upward, he said. In previous decades, the ratio was roughly equal.
“By the time we got to the 2000s, it was running, in the U.S., about 2 to 1,” Trenberth said. “This year it has been running, on average, about 10 to 1. This is an indication that this is more than natural variability. This is clearly an indication of climate change. So many people, at the moment, can look out the window and see aspects of climate change in action.”
The first five months of the year also were the hottest on record for the contiguous states, he said.
In March, the same states broke more than 15,000 records, but it seemed to go largely unnoticed.
“In the East, many people felt they didn’t really have a winter,” Trenberth said. “And many people enjoyed that. Then we got to March, and we had June temperatures in March. Again many people enjoyed it; it was glorious.”
The warmth was still familiar, just at the wrong time of year, he said.
“But by the time we get to June, running with temperatures above average, then we’re no longer in familiar territory,” he said.
Dry conditions in the West, meanwhile, have left the Rockies with little snow and vegetation parched, raising the risk of wildfire. And a large number of wildfires are burning in the West, including in Colorado, Wyoming and Montana.
Trenberth still agrees that individual weather events can’t be blamed directly on climate change. But as records and unusual events accumulate, the fingerprint becomes visible.
“Heat waves, droughts, wildfires – that’s when it really shows up, because we’ve gone outside the bounds of previous existence,” he said. “There’s a non-trivial component of climate change that’s having an influence.”
Other scientists generally agree that climate change is occurring, and that it is mostly driven by human activity. But many, including ocean and climate researcher Bill Patzert at the Jet Propulsion Laboratory in Pasadena, are more cautious about assigning it a major role even for extreme events. “Global warming is a large stage on which everything is being played out, but it hasn’t been that big yet,” Patzert said.
Regional climate factors can swamp global effects, he said. While global warming has amounted to about 1.6 degrees Fahrenheit over the past 130 years, “in any given year, a big El Niño or La Niña can actually change the global temperature by half a degree Fahrenheit,” he said. “That’s a lot bigger.” And human choice plays a big role in wildfire. A long history of putting out small fires in the West, he said, has allowed fuel to build up, so that when fire does occur it tends to become explosive.
In Southern California, including Orange County, where the spring and summer so far have been mild but where hot Santa Ana winds tend to kick up in the fall, more homes have been built over the decades near wild land and in wind corridors.
In other words, human-generated carbon dioxide, or CO2, will definitely have its effects, but human choices will have stronger effects in the short term.