Monday, August 13, 2012

Welcome to Warmer America

On August 6th, NOAA's National Climatic Data Center reported that July was the single warmest month in the 117-year temperature history of the continental United States. During July, over half of the continental US experienced temperatures in excess of 2°F above average, an unprecedented area and magnitude of warmth:

July 2012 Temperature anomalies; darkest red is 8°F above average. Source: NOAA

July's record temperature are just the latest in an exceptional year of heat for the continental US. A late June super-heat wave led to 208 locations tying or breaking their all-time highest temperature records-- all of this, despite the fact that June isn't even the warmest month of the year! From Alaska to Arkansas, a vast swath of the country set or tied their highest-ever June temperature during this heat wave:

U.S. Monthly Highest Max Temperature Records set in June 2012. Source: NCDC/NOAA

And this hot summer is a continuation of the status quo for the past several months. This past March was the warmest March in recorded history for the continental US. To date, the year 2012 is more than 4°F warmer than the 20th century average, smashing records of previous warm years:

Evolution of yearly temperature anomalies for the contiguous US (2012 in red). Click for larger. Source: NCDC/NOAA

All told, the past 12 months have been the warmest 12-month period in recorded history for the US. In fact, since May of 2011, every month has been warmer than its climatological average, a fourteen-month stretch of warmer-than-average temperatures.

What are the odds that 14 straight months would be warmer than average? One can assume there is a 50% chance that any given month will be warmer (or cooler) than average. The odds of having 14 such warmer-than-average months in a row, as we've had since May of last year, would thus be equivalent to the odds of flipping a coin heads fourteen times in a row: one in sixteen thousand.*

Of course, 14 straight months of anomalous warmth are more likely if the chance of warmth is better than 50/50. Since the 1980s, scientists have postulated that carbon dioxide-forced global warming would lead to increases in the frequency of heat waves and droughts, in effect "stacking the deck" in favor of warmer conditions. In 1981, climatologist Jim Hansen of the Goddard Institute of Space Studies, writing in Science, hypothesized "the creation of hot, dry conditions in much of the western two-thirds of the United States" as a result of additional atmospheric carbon dioxide.

Thirty-one years later, Hansen's predictions appear substantiated, as the US is currently in the grips of the worst drought since the 1930s Dust Bowl. 62% of the country is experiencing drought conditions, and the US Department of Agriculture is projecting that the 2012/13 corn yield will be the lowest in 17 years due to "extreme heat and dryness" across the Great Plains. As a consequence, global food prices spiked 6% in July, led by a 17% increase in the price of cereals and a 23% surge in the price of corn:

Figure 4: Spot price for December, 2012 Corn Futures (cents/bushel). Note the spike in prices during the late June super-heat wave. Source: CME Group

As with any climatic changes, there are winners and losers. Locally, farmers in North Carolina may gain from the high corn prices (above), with the USDA projecting a 30% increase in NC corn production over last summer despite the drought conditions. Globally, however, the World Bank notes that rising food prices tend to exacerbate political and social conflict, especially in developing countries.

Globally, the hot summer of 2012 in the US contrasts with cooler-than-average conditions in the UK and other locations. But the two do not fully offset: Like every year since 1977, global temperatures in 2012 will be above the 20th century average:

Measured global temperature anomalies, 1900-2011, with future projected temperatures. Click for larger. Sources: NOAA Climate Services, IPCC

By 2030, global temperatures are expected to be 0.9°F (0.5°C) warmer than today, rising to almost 2°F (1.1°C) warmer than today by 2050 (above). With such an increase in background warmth, hot months--and hot years--will undoubtedly become hotter.

Will the warmth continue in the US? As far as this year is concerned, it doesn't matter: the first half of 2012 has been so warm that, even if temperatures in the rest of 2012 are in line with 20th century averages, 2012 will still be the warmest year in US history. And with a developing El Niño historically favoring a warmer late fall in the central and eastern US, it is likely that the second half of 2012 will continue to be warmer than average:

Historical temperature anomalies in November and December during El Niño events. Red colors indicate warmer-than-average conditions. Source: NOAA Climate Prediction Center

In the meantime, go ahead and pencil in 2012 as the warmest year in US history. Just don't expect it to stay in the record books for very long.

* Note from above: This probability assumes that the chances of any given month being warmer or cooler than average is independent of all other months, which isn't necessarily true. In reality, the odds are probably slightly higher than one in 16,000, but difficult to quantify exactly.

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