The concept of global warming is compelling, but how can last year's cold winter fit into that pattern?
—Bill Price, Madison, Wis.
First, short-term cooling (a few years) does not contradict the concept of global warming — longer-term trends are what count; and second, one must consider the planet as a whole. Last year's harsh winter was a regional event affecting mainly the eastern U.S., whereas most of the Northern Hemisphere experienced a mild winter. Last winter, the average temperature range between the middle and high latitudes of the Northern Hemisphere was less than usual because of pronounced warming in the Arctic. This weakened the pattern of Northern Hemisphere jet streams, which draw their energy from zones of great temperature contrast, thereby allowing a greater-than-usual southward penetration of polar air.