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Posted at 03:57 PM ET, 04/19/2012

Earth’s coolest March since 1999; global warming on the wane?


The March global temperature compared to the 20th century average dating back to 1880. Red bars indicate years with above average temperatures, blue bars indicate years with below average temperatures. (NOAA)
The heat in the U.S. during March was off the charts. How many times have you heard about the 15,000+ warm weather records that were set, many crushed? But the U.S. makes up just two percent of the planet and the rest of it wasn’t so hot. In fact, by recent standards, the Earth was cool - the coolest March since 1999.

Does the relatively cool March dispel global warming, or - at least - represent a small piece of evidence refuting it?

Related link: NOAA March 2012 State of the Climate Report

In short, no. March was still the 16th warmest on record since 1880, 0.83F above average. It marked the 325th straight month of above average global temperatures - coinciding with the inexorable increase in greenhouse gas levels.

But temperatures don’t rise in a straight line - or monotonically - in response to greenhouse gases. Even when the overall temperature trend points up, natural cycles draw peaks and valleys into the long-term profile (see top image). In 2012, the fading La Nina - associated with below average surface temperature in the tropical Pacific - and other factors likely exerted a small cooling effect on the global temperature.


Temperatures compared to average during March. Red areas are warmer than average, blue areas are cooler. (NASA)
Australia and Alaska, for example, were cooler than normal. Arctic sea ice reached one of its largest March extents of the last decade, almost recovering to average levels (the extent was 3.4 percent below average).

Related link: North America swelters in March heat (NASA)

But that doesn’t mean the warming pressure from elevated greenhouse gas concentrations stopped. (In fact, a NOAA analysis found it enhanced the U.S. heat wave by 5-10 percent.)

The global view of temperatures (above) show many more areas with warmer than average temperatures - with the continental U.S. and northwest Europe the most prominent and extreme examples.

The small downtick in the global temperature during March is a powerful reminder that 1) what happens in our backyard does not necessarily reflect the Earth as a whole and 2) natural variability plays an important role in month-to-month temperatures.

But March was still globally warm and a tick down compared to recent years means little in the long-term analysis of temperature trends - the standard for evaluating climate change.

(Note - if you examine the red bars over the last decade or so in the top chart - you may notice the appearance of a flat trend - or pause in the longer-term warming. Such a signature also appears in annual average temperatures. A forthcoming blog post will analyze what might be behind that.)

By  |  03:57 PM ET, 04/19/2012

 
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