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Globe Records its Seventh-Warmest July on Record, Arctic Melt Speeds Up

Abnormally warm conditions in much of the United States, Northern Europe, Western and Eastern Russia, and parts of the Arctic helped propel July 2011 to the seventh-warmest July on record, the National Oceanic and Atmospheric Administration (NOAA) reported yesterday. This makes July the 317th month in a row that global average surface temperatures were above normal. The year-to-date is now the 11th warmest period on average.

In the U.S., July was the fourth-warmest such month on record, with long-lasting heat waves affecting nearly all areas east of the Rocky Mountains. Along the West Coast, however, conditions were cooler-than-average. The heat toppled thousands of records (see our interactive data explorer for details on record warm overnight temperatures), and Oklahoma set a particularly notable record, showing the highest-ever statewide average monthly temperature for any month in any state in the US since instrument records began in 1895.

Some of the temperature records set during July 2011, according to data from NOAA. Credit: Russell Freedman.

As I wrote on July 20: "The current heat wave is consistent with climate science research, which shows that as the climate warms, heat waves in general are becoming more likely. Such heat waves can be especially deadly when they feature high humidity and unusually warm overnight low temperatures that prevent people from cooling off after a hot day. On July 19 alone, 317 record warm minimum temperatures were either set or tied in the U.S., according to the National Climatic Data Center. Of those records, 84 either set or tied all time warm minimum records." 

For added perspective on how heat extremes are likely to change as the climate continues to warm, check out an interview I did with Stanford University's Noah Diffenbaugh, who has published several recent studies on the topic.

NASA recently published some interesting visuals showing the factors that helped lead to the prolonged heat events. Although some of them are rather technical, and meant more for weather nerds like me, they offer a neat tour of the atmospheric anomalies that caused so many Americans to suffer through July.

Here's one of the images:

NASA data showing average winds and water vapor in the lower atmosphere, as sensed by a NASA satellite and computer models. Note the southerly flow of warmer, more humid air into much of the US. Credit: NASA.

NASA researchers discussed the heat wave information on their website, stating:

 

It is worth noting that even though the pressure anomalies were small, they could drive changes in the atmospheric circulation sufficient to break heat records at the surface. The configuration of the North Atlantic high pressure in July 2011 has been consistently driving surface winds from the warm and humid tropics into much of the continental United States. The result is apparent — as AIRS data show, much of the U.S.A. and Mexico were under distinctively higher humidity, and this humid air contributed greatly to keeping nighttime temperatures at anomalously high levels.

 

In the Arctic, the sea ice melt season is entering its final stretch before the Sun sinks lower on the horizon and cooler air arrives once again. According to data from the National Snow and Ice Data Center, the average Arctic sea ice extent during July was the smallest since satellite monitoring began in 1979, at 21.6 percent below average. The NSIDC and other monitoring groups have found that one of the routes through the famed Northwest Passage is largely free of ice, as has occurred during recent years since Arctic sea ice extent declined to a record low in 2007. The weather pattern in the Far North is currently favoring more rapid sea ice melt, and weather conditions during the rest of the summer will be critical in determining whether sea ice extent sets a new record low. Regardless of the final figures, it's the long-term trends that are important, as Climate Central's Mike Lemonick detailed last month. Researchers have observed more indications that Arctic sea ice cover is thinning in addition to downward trends in area.

As the NSIDC states in a recent news release:

Sea ice thickness is also an indicator of the health of the ice cover; thick ice is resistant to melt. Specialized buoys managed by the U.S. Army Cold Regions Research and Engineering Laboratory help supplement limited satellite measurements of sea ice thickness. The buoys provide accurate data at specific locations, and can tell us whether thickness changes are due to surface melt, melt at the bottom of the ice floe, or ice growth. These buoys are deployed on thick multiyear ice, which provide long-lasting, stable platforms.

Data from six of these buoys through July 20 show that this year, the ice surface is melting faster than the underside of the ice. As the sun starts to sink on the horizon, surface melt will slow. However, ocean waters warmed during the summer will continue to melt the ice from below, reducing ice thickness and extent into September.

Another measure of Arctic sea ice is the ice volume, which is a measure of ice thickness and sea ice area. Sea ice volume has been running well below the 1979-2010 average, as well as below the 2007 level, and projections show that the minimum volume in September, before the sea ice begins to freeze up again, "will very likely finish below 2007 and could even reach a record low volume."

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