NASA scientists based at the Goddard Space Flight Centre in the United States used a computer simulation to study the effects of volcanic aerosols, polar ozone depletion, solar radiation, and greenhouse gases on global warming. According to the researchers, the climate model revealed that greenhouse gas forcing must be the primary factor driving warmer winters in North America, Europe and Asia over the last 30 years as it is the only method which is capable of the large sustained increase in the winds observed over this period.

By enhancing the temperature gradient across the jet core, greenhouse gases in the atmosphere cause the winds between the mid latitudes and the pole to strengthen. These stronger winds pick up and transport the warmer air from over moist oceans to the continents. Warm air from the Pacific Ocean increases the temperature of western North America, and Atlantic Ocean air is responsible for the same effect in Eurasia. When winds become weaker, the winters are correspondingly cooler, say the researchers.

“Surface temperatures in the Northern Hemisphere have warmed during winter months up to 9°F (5°C) over the last three decades, over 10 times more than the global annual average 0.7°F (0.4°C),” says Drew Shindell, one of the NASA researchers. “Warmer winters will also include more wet weather in Europe and western North America, with parts of western Europe the worst hit by storms coming off the Atlantic.”

The same effect is not felt in the Southern Hemisphere because it is colder than the north, and polar wind circulation over the Antarctic is already very strong, says the NASA team.

The researchers also examined the effects of volcanic activity between 1959 and 2000, identifying volcanically active and non-active years. The conclusion that they were able to reach was that because volcanic forcing is intermittent and decays rapidly, it appears unlikely to have contributed greatly to the long-term observed warming trend. Furthermore, large volcanic eruptions, such as that of Mount Pinatubo in the Philippines in 1991, inject aerosols into the atmosphere and have a global cooling effect during the years following an eruption.

The effects of solar radiation on stratospheric ozone were also considered not to be as significant as that of greenhouse gases. Gavin Schmidt, another researcher on the project, noted that long-term changes in solar irradiance have influenced the upper atmosphere. “However, it is unlikely that solar variability has been responsible for much of the observed trend in increasing the polar winds,” he pointed out.

Because the upper polar atmosphere becomes colder when ozone is depleted, the winds circling the pole are slightly enhanced, says Shindell. “However, greenhouse gases have the biggest impact on the strengthening of the polar winds, and in turn, the warming of the northern hemisphere during winter months,” he said.

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