How hot was 2012? Hottest on record in US, by a long shot

The year 2012 was the warmest on record for the continental United States, eclipsing 1998's record average temperature of 54.3 degrees by a full degree Fahrenheit.

While one degree's difference may not seem like much, the spread between the record coldest year, 1917, and the previous record warm year, 1998, is just 4.2 degrees F. With 2012's record-high reading, the gap has grown by 25 percent, according to preliminary data from the National Climatic Data Center (NCDC) in Asheville, N.C.

Last year marked the 15th consecutive year of above-normal average annual temperatures for the continental US.

Global warming "has had a role in this," says Jake Crouch, a climatologist at the NCDC, during a briefing Tuesday on the year's data. Annual average temperatures for the lower 48 states have been increasing over the past century, although he noted that it's difficult to know how much of the warming in 2012 could be pegged to human-induced climate change versus natural variability.

Regionally, the Northeast, Southwest, South, and North West Central US posted record high annual temperatures, with the North West Central US coming in at 3.9 degrees F. above the long-term average, followed by the Northeast at 3.4 degrees F. above the long-term average.

Beyond temperatures, the continental US posted the second worst year, after 1998, for severe weather, as measured by the NCDC's US Climate Extremes Index.

The most pervasive severe conditions in 2012 centered on the ongoing drought in the US. At one point in July, moderate to exceptional drought, as measured by a gauge known as the Palmer Drought Severity index, covered 61 percent of the US. Other measures still put the drought coverage at 61 percent of the country, covering most of the western two-thirds of the US.

Toward the end of last year, a dearth of water flowing into the Mississippi River threatened to shut down barge traffic along a key section between St. Louis and Cairo, Ill. But the US Army Corps of Engineers released water from the Carlyle Lake Reservoir in Illinois, which fed water into the river. The Army Corps also used explosives to pulverize rocks on the river bottom that had become threats to navigation as the water level fell. These two actions, Corps officials say, are expected to keep the Mississippi open to barge traffic through the end of January.

Still, long-term forecasts project river levels falling to historic lows, according to the American Waterways Operators, a trade group for tug, towboat, and barge operators in the US.

In the western US, hot, dry conditions contributed to wildfires that, nationwide, torched 9.2 million acres – the third-largest area affected in the past 13 years. Idaho, Montana, and Oregon were hardest hit, with wildfires covering more than 1 million acres in each state. Colorado experienced its most expensive fire, while New Mexico battled its largest fire on record.

East of the Mississippi, extreme weather also made itself felt in hurricane Isaac, tropical storm Debbie, and at the end of the Atlantic hurricane season, hurricane and post-tropical cyclone Sandy. The storms helped ease drought conditions in the Southeast.

Whereas temperatures warmed in part by climate change contributed to record heat and drought, Sandy's storm surge came atop seas that have risen at New York Harbor at a pace of an inch per decade during the past century. Part of that rise is due to factors such as local land subsidence. But part is also due to global warming, as rising temperatures warm seawater, which expands, and melt glaciers and ice caps, whose melt water and ice end up in the ocean.

By some estimates, Sandy, which outlasted tropical storm Tony as the final storm of the 2012 Atlantic hurricane season, caused at least $65.6 billion in damage and killed at least 235 people in seven countries – 131 in the US.

Indeed, 2012 tied four other years – including 2010 and 2011 – as the third most active hurricane season on record. The season saw 19 named storms, 10 of which became hurricanes. One, Michael, became the season's only major hurricane, sporting sustained winds of 115 miles an hour. But from birth to demise, the storm remained in the middle of the North Atlantic, far from land.

A swath of the US from Indiana to Maryland also took a beating during the summer from a storm whose powerful, expanding downdrafts along the storm's leading edge generated winds that plowed through the region, downing power lines, damaging homes, and killing 22 people.