Scientific Method / Science & Exploration

2012 was hottest on record in the US, warm everywhere else

Snow took the year off in some regions of the country.

Of the 118 years on record, 2012 came out on top in 19 states.

When 19 of the contiguous 48 states have the warmest years on record, it's pretty much guaranteed to be a record year for the country as a whole. But saying that 2012 had record heat doesn't truly capture how much of an aberration the year was for the US, according to the annual report from NOAA. Another 10 states had their second-warmest years on record; seven more had their third. The end result was a year that was a full degree Fahrenheit above the previous record holder, and  1.8°C (3.3°F) above the average from last century.

Most of the warmth came during the early part of the year, which was accompanied by significant drought conditions across a wide section of the country. NOAA's "climate extremes index," which incorporates temperature, precipitation, and other trends shows the entire South (from Texas to the East Coast) experienced its most extreme weather in the spring; everything but the West Coast was much above average during that time. The index shows that 2012 was either the most extreme on record or came in second to 1998, depending on whether you included hurricane activity.

For some regions of the country, snow took the year off. As NOAA puts it, "The 2011/12 winter season was nearly non-existent for much of the eastern half of the nation." The result was the third-lowest snow cover on record in the 48 contiguous states, with only a relatively heavy winter in the Pacific Northwest keeping matters from being worse. The low snowpack contributed to the drought conditions that prevailed over most of the summer.

Hurricane activity was a mixed bag. With 19 named storms, ten hurricanes, and one major hurricane, it was tied as the third most active season on record. At the same time, none of the storms reached Category 4 or 5, which has only happened two other times since 1995. The sole Category 3 hurricane, Michael, only sustained that status for six hours. The New York area will take little comfort from the fact that Sandy wasn't even a hurricane when it made landfall, allowing the US to have its seventh year without a hurricane making landfall.

It's obviously very tempting to assign this extreme year to climate change (and a number of people have). In the case of some specific facets, it's easy to do so; for example, the flooding caused by Sandy was exacerbated by the fact that climate change has driven a small increase in sea levels over the last century. But attribution is hard (at least from a scientific perspective) and takes a significant amount of time, so the studies that link specific events to climate change usually take a year or two to appear.

The global picture

Globally, however, 2012 doesn't seem to have been an extreme year. Every year since 2000 has been warmer than every other year on record except 1998, which is a strong outlier from the rest of the last century. Although the final numbers for the year have yet to be posted at any of the sites that track them, data from the earlier months suggest that 2012 will be no exception, and it has a fair chance of winding up on the top-10 list of warmest years. But, if it does, it's likely to end up somewhere in the middle of the list.

Part of that is the hangover from the recent La Niña event in the Pacific. Even though there was a relatively weak El Niño (the warmer phase of the El Niño-Southern Oscillation) in 2010, it was enough to push that year to the warmest on record. Since then, we've mostly been in a relatively strong La Niña, which has moderated global temperatures.

The ENSO index shows that the tropical Pacific has mostly been in a cold phase recently.

That started to change this year, but there tends to be a lag between ocean temperatures and the response of global temperatures. So far, at least, the El Niño has been very weak, and it quickly dropped back toward a neutral state. There's no good way to predict whether this represents a temporary dip, or whether the El Niño will strengthen over the course of 2013.

Although 2012's global temperatures won't be setting any records, that doesn't mean that they were insufficient to drive extreme weather events. It will certainly be well above the 20th century's average temperature and will likely be one of the 10 warmest years on record to date. But it will take some time before scientists can actually crunch the numbers to see whether any particular weather trends can be ascribed to the year's warmth.

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