The Heat Is Online

2012 smashes record for hottest year in US

Turning Up the Heat: Hottest Year on Record in the Lower 48 States

Climate Central, Dec. 26. 2012

2012 will go down in history as the hottest year on record in the continental U.S., pushing 1998 into second place. In line with the global warming trend spurred by steadily rising carbon emissions, seven of the top 10 warmest years in the 48 states have occurred in the past 15 years.

Like so much recent record-breaking weather, 2012 isn’t just going to top the previous record, 2012 is looking to smash it, by more than 1°F. In mid-December, Climate Central projected that 2012 average temperature for the continental U.S. at 55.34°F compared to the previous record set in 1998 of 54.32°F. For perspective, 1°F is one-quarter of the difference between the coldest and warmest years ever recorded in the U.S.

The country endured many unusually long-lasting and severe heat waves as temperatures spiked in March, and never fell below average. March was the warmest such month on record in the lower 48 states, exceeding the average monthly temperature by 7°F. After the March heat wave, the contiguous U.S. recorded its warmest Spring, largest seasonal departure from average, third-warmest summer, and warmest 12-month period, all new marks since records began in 1895.

The average springtime temperature in the lower 48 was so far above the 1901-2000 average — 5.2°F, to be exact — that the country set a record for the largest temperature departure for any season on record.

The dominance of mild conditions is starkly apparent when looking at the balance between daily record highs set or tied this year and daily record lows. During 2012, there were 33,753 daily record-high temperatures set or tied, compared to just 6,303 daily record-low temperature records. That means that for every five daily record highs, there was just one daily record low set or tied.
When taking into account record-warm overnight low temperatures as well as record-cold overnight low temperatures, the ratio was closer to 4-to-1. No matter how you slice it, warm temperatures absolutely dominated the U.S. weather in 2012.

As the climate has warmed during the past several decades, there has been a growing imbalance between record daily high temperatures in the contiguous U.S. and record daily lows. A study published in 2009 found that rather than a 1-to-1 ratio, as would be expected if the climate were not warming, the ratio has been closer to 2-to-1 in favor of warm temperature records during the past decade (2000-2009). This finding cannot be explained by natural climate variability alone, the study found, and is instead consistent with global warming.

The study used computer models to project how the records ratios might shift in future decades as the amount of greenhouse gases in the air continues to increase. The results showed that the ratio of daily record highs to daily record lows in the lower 48 states could soar to 20-to-1 by mid-century, and 50-to-1 by 2100.