Record highs vs. lows reveal shifting climate
Climate change has long suffered from a perception problem. Studies show that people tend to view it as a long-term phenomenon divorced from their more immediate and pressing concerns, rather than something that is already reshaping the planet in many ways. Part of the explanation for this may have to do with the statistics typically cited when scientists discuss climate change, such as the nearly one degree Fahrenheit increase in globally averaged surface temperature during the past century.
This statistic, while profoundly alarming to many scientists, is at the same time rather lacking, because it is not readily relatable to the local level.
A forthcoming study in the journal Geophysical Research Letters offers an innovative and useful metric to show how climate change is affecting the United States, and it is one that may help people spot the fingerprint of long-term warming when they get their daily weather information, whether it's from this site or elsewhere.
The study examines the changing relationship between record daily high maximum temperatures and record low minimum temperatures across the United States between 1950 and 2006. It finds that record highs are now much more common than record lows, averaged across the country, compared to the middle of the 20th century. The ratio of record highs to record lows has increased from about one to one at the beginning of the period, to two to one in the present decade.
Absent a warming climate, the ratio of record daily highs to lows should be about even, the study states.
The researchers went on to conduct computer model simulations of future climate, which showed that by 2050, record highs would dwarf record lows by a ratio of 20 to one, reaching 50 to one by 2100, as the model's climate warmed in response to increased emissions of greenhouse gases such as carbon dioxide and methane. The simulation assumed emissions would rise according to a roughly "business as usual" scenario.
The study is the result of a joint effort between public- and private-sector scientists, including a Weather Channel meteorologist and a researcher from Climate Central, a nonprofit climate science communications group. ** Disclosure: I am working with Climate Central on a climate science reporting project, but was not involved with this research. **
If you fancy raw numbers rather than ratios, consider this: the study found that between Jan. 1, 2000, and Sept. 30, 2009, the lower 48 states chalked up 291,237 record highs compared with 142,420 record lows.
On a shorter time scale, between the beginning of this year through the end of September, there had been 11,711 record highs compared to 7,449 record lows. This translates to a ratio of just less than two to one. However, the record lows may gain some ground once October is taken into account, since it was the third coolest October on record in the United States.
In a press release, lead author Gerald Meehl of the National Center for Atmospheric Research emphasized that the study translates long-term climate change into daily weather terminology. "Climate change is making itself felt in terms of day-to-day weather in the United States," Meehl said. "The ways these records are being broken show how our climate is already shifting."
The study found that the shift in the ratio between record highs and lows would be extremely unusual under a climate that was not warming. "The behavior of the ratio in recent years is very, very, very unlikely under stationary conditions," said co-author Claudia Tebaldi of Climate Central.
Interestingly, the study found that record cold is becoming a more rare occurrence with time, not that record heat is occurring much more frequently.
However, Tebaldi cautioned that cold snaps are not going to disappear entirely.
"...I think it is important to highlight the fact that record lows still happen here and there, every now and then, which is something people seem to be surprised by and take as a sign that global warming should be doubted," Tebaldi said in an email conversation. "So in a way the study makes global warming relevant on a daily scale, but also explains that on daily scales other counter-intuitive effects can still manifest themselves without necessarily being at odds with the idea of average warming."
The views expressed here are the author's alone and do not represent any position of the Washington Post, its news staff or the Capital Weather Gang.
Other reactions to study:
* More Record Highs and Far Fewer Lows
* U.S. Record Temperatures--A Closer Look
* Record high temperatures far outpace record lows across U.S.
* Comments On Meehl Et Al 2009 On Trends In Record High And Low Temperatures
* A Critique of the October 2009 NCAR Study Regarding Record Maximum to
| November 18, 2009; 10:45 AM ET
Categories: Climate Change, Freedman, Science
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