July's heat stress

Climate Central

July was the fourth-warmest such month on record globally, and the 329th consecutive month with a global-average surface temperature above the 20th-century average, according to an analysis released Wednesday by the National Climatic Data Center (NCDC).

The combined-average July temperature over global land and ocean surfaces was 61.52 degrees Fahrenheit (16.4 degrees Celsius), which was 1.12 degrees Fahrenheit above the 20th-century average. This was the 36th straight July with a global temperature above the 20th-century average.

The last time the globe experienced a cooler-than-average July occurred in 1976, when Gerald Ford was the US president.

The globally averaged temperature over land areas was the third highest for July on record. For Northern Hemisphere land areas only, however, it was the warmest July on record, which is significant since this is where most of the planet's land masses are located.

According to NCDC, which is part of the National Oceanic and Atmospheric Administration (NOAA), July marked the fourth consecutive month that the Northern Hemisphere has set a monthly land temperature record. The Southern Hemisphere land temperature during July, on the other hand, was only the 33rd warmest for the month.

Higher-than-average monthly temperatures were especially pronounced across the US, which beat July 1936 for the title of the warmest month on record in the lower 48 states. The extreme drought worsened during the month, with the worst drought categories doubling from 10 per cent to 22 per cent between July and August. The dry weather and extreme heat “devastated crops and livestock from the Great Plains to the Midwest,” NOAA said.

Sea ice extent trends, showing the rapid melt of sea ice during the 2012 melt season so far. Credit: NSIDC.

Studies show that manmade global warming is shifting the odds in favour of more extreme heat events, and projections also show that drought conditions may become more common and severe in parts of the world as greenhouse gases continue to build up in the atmosphere.

One study published on August 6 by NASA climate scientist James Hansen found that the odds of extreme heat events such as the Texas heat wave of 2011 most likely would not have occurred without manmade global warming, although many climate scientists not involved with the study stopped short of tying extreme weather events to global warming so definitively.

While most regions of the globe experienced warmer-than-average monthly temperatures in July, Australia, northern and western Europe, eastern Russia, Alaska, and southern South America were all cooler than average. In Australia, July was the eighth-coldest July in the country's 63-years of record-keeping.

During July, sea surface temperatures continued to increase in the eastern and equatorial Pacific Ocean, a sign of developing El Niño conditions, according to NOAA. After two years of La Niña conditions, which featured cooler-than-average waters in the tropical Pacific, the developing El Niño is already boosting global temperatures. The January-to-July period is now tied with 2001 as the 10th-warmest such period on record, NOAA reported. El Niño years tend to be warmer than average worldwide.

July featured a precipitous drop in Arctic sea ice cover, which declined by 1.2 million square miles – equivalent to the states of Alaska, California, and Texas combined. Antarctic sea ice was 2.2 per cent above average during July.

A marked decline in Arctic sea ice cover, along with increasing sea ice cover around the South Pole, is consistent with projections from climate change computer model simulations.

The month also featured an unusual melt event in Greenland, when the portion of the ice sheet that was experiencing surface melting expanded to cover nearly the entire ice sheet for several days, an event that had not been seen for 150 years. A study released Wednesday found that more of Greenland's ice sheet has already melted so far this season compared to the past 30 years.

Typically, about half of the ice sheet experiences summer melting, with widespread and prolonged melting of higher elevation areas considered to be a rarity.

This article was originally published by Climate Central. Republished with permission.