The 2012 U.S. heat wave has made considerable news. So none of this should come as any particular shock, perhaps just a little extra shock to what most of us in the U.S. have experienced so far this year. The average temperature for the contiguous U.S. during June was 71.2°F, which is 2.0°F above the 20th century average, which placed June 2012 as the 12th warmest June on record, as the NCDC announced Monday. June 1933 was the warmest June for the U.S. on record due to Dust Bowl conditions. It was also the tenth driest June on record, even with record precipitation in Florida as a result of Tropical Storm Debby. As I wrote earlier, my state of Colorado experienced its warmest June on record. Seven nearby states experienced top-ten warmest Junes.
The Jan-Jun 2012 period is the warmest such period in U.S. history, as the following graphic displays:
Figure 1. The five warmest years for the contiguous U.S. compared to 2012 as of the end of June 2012.
You can see that the first three months of the year were much warmer than average, but it was March that really pushed conditions to an extreme level: +6F. Since then, the year-to-date average anomaly has edged back down to “just” +4.5F. 1998 was the warmest year on record, largely due to the strong 1997-1998 El Nino event. Note that 2006 also makes this list – and that was without the aid of a strong El Nino. 2012 is clearly much more anomalous than any other year to date. The only way it won’t finish as the warmest year on record is if much cooler than normal conditions blanket most of the country for the remainder of the year. It’s not impossible, but conditions would need to be quite different from what we’ve experienced so far this year.
Meteorologists and climatologists look at other time periods that aren’t calendar years. For instance, the past 12 months (Jul 2011 – Jun 2012) just set a new record for the warmest 12-month period on record in the U.S., squeaking past the record that the end of May (Jun 2011 – May 2012) set, as the following graphic shows:
Figure 2. The set of warmest 12-month periods for the contiguous U.S. in the modern-era.
The past 12 months were 3.23F warmer than the long-term average, which is only slightly higher than the 3.18F anomaly for the preceding 12-month period. These two values are both higher than the 2.83F anomaly for the preceding 12-month period (May 2011-April 2012) and the 2.61F anomaly for April 2011-March 2012. Thus, out of the 12 warmest 12 consecutive month periods in contiguous U.S. history, 1/3 of them have occurred in just over the past year. The odds of this occurring randomly is just 1 in 1,594,323. Thus, until 124,652 AD, we should only see one more 13-month period so warm, and that assumes the climate is staying the same as it did during the past 118 years. Needless to say, such an assumption looks incredibly weak.
Looking further at the graph, you can see that 21st century periods dominate the top-12. That is one important difference from the previous graph which showed 1934 and 1921 and the 3rd and 5th warmest years on record. Those years had stretches of time that were shorter than 12 consecutive months over the entire country that were anomalously warm. The heat that is occurring now is spread over a larger area than previous heat waves. Specific heat values for a location or during just one month might not hit record highs, but overall conditions are warmer now than during previous warm periods in the U.S. In other words, the background climate is warmer than it was in 1921 or 1934, enough so that heat records and long stretches of very warm conditions are a little likelier each year to occur.
Another kind of graph might help the reader visualize this:
The CEI: “summariz[es] and present[s] a complex set of multivariate and multidimensional climate changes in the United States so that the results could be easily understood and used in policy decisions made by nonspecialists in the field.” It shows the percentage area of the U.S. with top 10% extremes. Obviously, January-June 2012 set a new record at 44%.
If the planet continues to warm throughout the 21st century, which is more likely to occur the longer we continue emitting heat-trapping greenhouse gases, months, seasons, and years of time that break records today could be considered cool by comparison to conditions at the end of the century. The implications are wide-ranging and profound for human societies and ecosystems. The world won’t end, but it certainly won’t be the same as today either.