Predictions early in 2005 that the year would be the warmest on record turned out to be off the mark.
A new study finds last year tied for the second-warmest year since reliable records have been kept starting in the late 1800s.
The global average temperature in 2005 was 0.54 degrees Fahrenheit (0.3 Celsius) warmer than the long-term average, tying a mark set in 2002.
But a puzzling general pattern, seen the past three decades, persisted: The most significant warming occurred in the Arctic, where the ice cap is shrinking at an alarming pace.
Seven times faster
Since November 1978, the Arctic atmosphere has warmed seven times faster than the average warming trend over the southern two-thirds of the globe, based on data from NOAA satellites.
The warmest five years since the 1890s, when reliable record-keeping began:
2. 2002 (tie)
Scientists agree the planet is warming. Ground in the Northern Hemisphere that's been frozen since the last Ice Age is melting and collapsing.
But they are still debating exactly how much and to what extent humans are contributing by burning fossil fuels that create greenhouse gases.
Lack of understanding
In a report last May, researchers said they know very little about how Earth absorbs and reflects sunlight, crucial factors that control climate. Other studies have indicated that increased output from the Sun is responsible for more of global warming than was previously realized.
"Obviously some part of the warming we've observed in the atmosphere over the past 27 years is due to enhanced greenhouse gases. Simple physics tells you that," Christy said. "But even if you acknowledge the effects of greenhouse gases, when you look at this pattern of warming, you have to say there must also be something else at work here."
Nobody's sure what that might be.
"The carbon dioxide from fossil fuels is distributed pretty evenly around the globe and not concentrated in the Arctic, so it doesn't look like we can blame greenhouse gases for the overwhelming bulk of the Northern Hemisphere warming over the past 27 years," Christy said. "The most likely suspect for that is a natural climate change or cycle that we didn't expect or just don't understand."
Opposite of expectations
Over the past 27 years, since the first temperature-sensing satellite was launched, the overall global temperature has risen 0.63 degrees Fahrenheit, while the hike in the Arctic has been 2.1 degrees.
"The computer models consistently predict that global warming due to increasing greenhouse gases should show up as strong warming in the tropics," Christy said.
Yet the tropical atmosphere has warmed by only about 0.3 degrees Fahrenheit in 27 years.
A study last year examined natural climate change going back more than 1,000 years. How do the recent changes stack up?
"It would be fairly rare to have this much warming all from natural causes, but it has happened [in the past]," Christy said. "What we've seen isn't outside the realm of natural climate change."
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