NASA map showing differences in Earth's average surface temperature is based on satellite data. Warmer areas are shown in red and cooler areas in blue. Greenhouse gases in the atmosphere are the major reason the Northern Hemisphere has had warmer winter weather over the past three decades, according to new NASA computer models. Carbon dioxide, methane, and other gases increase the strength of polar winds that have a powerful effect on the northern climate. The polar winds blow from high in the stratosphere over the North Pole to the troposphere below it, where weather occurs.
Greenhouse gases make the polar winds blow stronger over the oceans, where they pick up warmer air.
Pacific Ocean air heats up western North America, while Atlantic air moves over eastern North America and Eurasia. Increased levels of greenhouse gases enable the strengthened polar winds to last longer into the spring in the Northern Hemisphere. This in turn speeds up spring's arrival at these latitudes. In the Southern Hemisphere, greenhouse gases do not have the same effect since Antarctic air is colder and polar wind circulation there is already very strong. Satellite data shows that surface temperatures in the Northern Hemisphere have warmed during the winter months by up to 9 degrees Fahrenheit over the past three decades.