A cold front is sweeping across Europe after gripping swathes of North America last month, but the deep freeze does not mean the threat of global warming has abated, caution scientists.
"The major trend is unmistakably one of warming," Michel Jarraud, secretary general of the World Meteorological Organisation (WMO), told AFP. "If we look at the trajectory over the last 160 years, it overlays a large natural variability, and that's what causes confusion."
The cooler weather that was a hallmark of 2008 can be explained partly by La Nina, a reversal of the phenomenon by which warm waters build up on the surface of the Pacific, said Jarraud. "The problem is that people are confusing weather with climate," Susan Solomon, a top scientist on the UN's Intergovernmental Panel for Climate Change (IPCC), said in a recent interview.
"Weather is important locally, and from year to year. But what you really have to look at when you are interested in climate is the larger scale - the whole world - and the longer term," she said. Even if 2008 was on balance chillier than 2007, it still ranks as the 10th warmest year on record, she pointed out.
And over the long haul, average global surface temperatures have climbed significantly - between 0.75 and 1.0 degrees Celsius (1.35 and 1.8 degrees Fahrenheit) - since 1850, when accurate weather statistics were first recorded. The IPCC has said that by century's end, the increase will very likely be 2.4 to 4.0 C (4.3 to 7.8 C).
Exactly how much will depend on the extent to which humans can reduce atmospheric concentrations of carbon dioxide (CO2), the principal greenhouse gas. The coldest place was Dippoldiswalde-Reinberg near Dresden, eastern Germany, where the mercury plunged to - 27.7 C (- 4 F). Snow fell in France's Mediterranean port of Marseille.
Annual variations mask the incremental upward creep of the thermometer due to global warming, which averages out to only two hundredths of a degree Celsius per year, noted Jean Jouzel, one of France's leading climate scientists. One way to see the transition, he said, is to look at the impact of climate change on plant life.
"We see very clearly, for example, that the grape harvest in France has moved inexorably forward by several weeks over the last 50 years. But from one year to the next, it can start a few days earlier or later," he said. Scientists now agree that climate change is a reality, and that human activity is largely to blame, but they still differ on the pace at which it is unfolding.
And they are still divided over the perceived danger of runaway warming caused by self-reinforcing "positive feedbacks". The melting of the Arctic ice cap, for example, is both a symptom and a cause of global warming.
It is a sign of rising temperatures and also discreetly adds to the warming cycle. Ice reflects solar rays, so the loss of this floating cover exposes the sea to more of the Sun's heat. To make things even more difficult to decipher, global warming is not a linear process.
According to a study published last year in the scientific journal Nature, the overall warming trend could be slowed over the next decade by a shift in ocean currents, especially a weakening of the Gulf Stream that warms the north Atlantic.
Cold snaps are routinely seized upon by a dwindling rearguard of climate skeptics as "proof" that climate change is exaggerated or an outright fabrication, Jouzel said.
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