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Posted at 10:45 AM ET, 12/13/2010

With climate change, long-term trends are key

As the start of the New Year rapidly approaches, you're going to hear conflicting news about whether 2010 was the warmest year in the instrumental record. The first salvo has already been fired.

On Friday, NASA reported that the "meteorological year" spanning from December 2009 to November 2010 was the warmest in that agency's 131 years of record keeping. Never mind that the meteorological year is relevant only to meteorologists - the news still made headlines.

The two other groups that maintain official global surface temperature data - the U.S. National Oceanic and Atmospheric Administration (NOAA) and the UK Met Office Hadley Centre, which works in conjunction with the University of East Anglia Climatic Research Unit - will soon release data that may conflict with NASA somewhat, by ranking 2010 differently. NOAA, for example, currently ranks 1998 as the warmest year on record, rather than 2005, which is in NASA's top spot.

So, what should you, dear readers, make of the varying rankings? And does it really matter if 2010, compared to 2005 or 1998, was officially number one?

The bottom line is that all of the data as measured by land, sea, air, and even from space, shows 2010 has been an unusually warm year globally. This can be partially linked to El Nino conditions that were present up until last spring, but also to increasing concentrations of greenhouse gases in the atmosphere, which scientists say are very likely responsible for a large portion of the long-term warming trend.

This year's warmth is especially noteworthy because 2010 fell in a period of lower solar irradiance, which can help cool the climate. "The new record temperature in 2010 is particularly meaningful because it occurs when the recent minimum of solar irradiance is having its maximum cooling effect," wrote James Hansen, longtime director of NASA's Goddard Institute for Space Studies, along with three colleagues.

Some climate skeptics often point to the sun as the overriding driver of recent climate fluctuations, rather than greenhouse gases, but most published studies have refuted this argument.

The warm year is also interesting because a strong La Nina event got underway late last summer, and has helped to dampen global average temperatures since then.

Another key point to remember is that the temperature differences from one year to the next are not very meaningful, and even in this year's case can turn out to be very small. In NASA's release last week, scientists noted that the December 2009 to November 2010 period stands just a few hundredths of a degree warmer than the same period in 2005.

As Hansen and others explain in a research paper that is pending publication in a scientific journal, each climate monitoring center uses their own methods to track and rank global temperatures, which can lead to discrepancies between them. The NASA group uses techniques that infer temperatures across the Arctic, where available data indicates the most dramatic warming is occurring, but temperature-recording stations are sparse. (The paper in press describes NASA's methods in much greater detail).

The analysis in this graphic, which was prepared by research scientist Claudia Tebaldi, a colleague of mine at Climate Central, shows the differences between the three datasets over time.

It drives home the point that over the long-term, NASA, NOAA, and the UK's records are all consistent in showing a sharp uptick in global temperatures beginning during the latter half of the 20th century, and continuing on through the present day. Furthermore, NASA, NOAA, and the UK data all show the 2000s were the warmest decade since instrumental records began.

In fact, the NASA analysis released last week rebuts the argument that warming has slowed or ceased in the past decade. "Contrary to frequent assertions that global warming slowed in the past decade... global warming has proceeded in the current decade just as fast as in the prior two decades," Hansen and his colleagues wrote.

It's that long-term trend that is most important, given the numerous factors that can influence year-to-year weather and climate. So when you hear conflicting stories about whether this year was really the warmest, remember the overall context in which this first, second, or perhaps third-ranking warmth is occurring.

The views expressed here are the author's alone and do not represent any position of the Washington Post, its news staff or the Capital Weather Gang.

By  |  10:45 AM ET, 12/13/2010

Categories:  Climate Change, Climate Change, Climate Change

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