Heat waves in St. Louis -- more frequent and related to climate change?
WASHINGTON – Heat waves are as much a part of St. Louis summers as cold beer and Cardinals baseball. But a new study suggests that hot spells are more common now than they were a half century ago, when families cooled off by blowing fans over ice blocks.
A new analysis of heat waves in St. Louis conducted by the Union of Concerned Scientists found that the average annual number of such hot spells is twice as high now as it was 60 years ago, with the area grilled about four more hot spells a year than it was in the 1940s. A heat wave is defined as three or more consecutive days of “dangerously hot” air masses.
When a dry, tropical air mass hovers over St. Louis these days, temperatures at 3 a.m. – an indicator of a true heat wave – are on average about 4.4 degrees F higher than they were when our grandparents suffered summers six decades ago, scientists say.
“Deadly heat waves have become more common in St. Louis because the city’s weather has changed,” said Larry Kalkstein, a University of Miami geography professor who was a lead author of the report. He said the scrutiny of weather data found “more hot, dry air masses from the Southwest and hot, humid air masses from the Gulf of Mexico.”
The report – “Heat in the Heartland: A Look Back at 60 Years of Warming in the Midwest” – analyzed weather data in St. Louis and four other big Midwestern cities: Chicago, Cincinnati, Minneapolis and Detroit. The heat-wave trends were clearest in St. Louis and Detroit, but the study found that, on average, summer weather in all five cities is changing in ways that increase heat-related health risks. (Click here for a summary of the St. Louis findings.)
While the UCS study makes no direct claim on global climate change, it says that "such sticky, steamy, uncomfortable weather is poised to become even more common as our climate warms." Critics of such assertions say that scrutinizing just a few decades in a specific region can lead to misleading conclusions, especially considering that many high temperature readings in the Midwest occurred during the severe drought years of the 1930s.
For example, atmospheric scientist John R. Christy, who directs the Earth System Science Center at the University of Alabama, told a U.S. Senate panel Wednesday that "the occurrence of the records by decade makes it obvious that the 1930s were the most extreme decade and that since 1960, there have been more all-time cold records set than hot records in each decade."
Kalkstein is known for devising a system for identifying oppressive air masses – the umbrella of air over a city as characterized by temperature, humidity, wind and cloudiness – that the National Weather Service uses to assess the need for heat advisories and warnings.
Rather than defining "hot humid days" with specific temperatures and humidity levels, the study refers to types of air masses. For example, a Moist Tropical+ air mass (MT+) is "the most oppressive subset of moist tropical air . . . most often associated with heat-related mortality. It is present when both morning and afternoon temperatures are above the mean on a moist tropical day."
While the temperature and dewpoints (a measure of humidity) used to define a MT+ air mass change as the summer progresses, Kalkstein told the Beacon that in mid-July, "the average overnight MT+ temperature is in the upper 70s and the afternoon temperatures are about in the mid-90s. Dewpoints average in the low to mid-70s."
In the study, "hot dry days" refer to the Dry Tropical air mass, characterized in mid-July by "overnight temperatures about the same or a little lower than MT+, and afternoon temperatures approaching 100." Dewpoints are in the mid-60s.
Kalkstein said the report uses the terms "heat waves" or "excessive heat events" to represent three or more consecutive days of the MT+ or DT air masses, in any combination. He said the study defines "heat" as "any condition that can potentially lead to a negative health outcome in people."
The analysis found that the number of days when extremely hot, humid air masses that can cause health problems appeared over St. Louis doubled between 1946 and 2011, going from an average of six to 15 a summer. On average, the area also has four fewer cool, comfortable summer days than it did in the 1940s.
The study looked at Lambert airport weather-station data for afternoon and nighttime air and dew-point temperatures, air pressure, cloud cover, and wind velocity to determine the types of air masses that passed over the city during those years. While critics of the approach say the “heat island” impact of big cities contribute to the heat, the report also examined some smaller cities – including Columbia, Mo. – and found similar increases in the prevalence of hot air masses in June, July and August.
While Kalkstein said that "St. Louis is truly the hottest" of the cities analyzed in the study, he said in an email that the city "showed the strongest trends" -- along with Detroit -- toward more heat waves, including "the largest increases for overnight temperature" for air masses related to dangerous heat. "Some parts of the country show larger changes over the period than others; St. Louis apparently resides in one of these areas."
In a statement, Kalkstein said that, over the last 60 years, "these oppressively hot air masses have not only become more frequent, they have warmed significantly, which can threaten human health.”
A UCS news release quoted Pamela Walker, director of the St. Louis Health Department, as saying that the city is bracing for more heat waves. So far this year, two dozen people have died in the St. Louis region from illnesses related to heat. "We must treat prolonged periods of extremely high temperatures, as well as storms, as the new norm," Walker said in the statement.
Ed Smith, safe energy director with the Missouri Coalition for the Environment, said St. Louis has not recorded a record-low temperature since 2007. But this year alone, he said, city residents have endured 23 record highs during the daytime and 19 record-high "nighttime low temperatures."
Are record heat waves related to climate change?
The record heat and drought in Missouri and Illinois – as well as much of the nation – have contributed to the long-running debate over whether such events are related to global climate change and a trend toward global warming.
At a hearing Wednesday of the Senate Environment and Public Works Committee, a leading climate scientist – Christopher B. Field, director of the Carnegie Institution’s global ecology department and an environmental science professor at Stanford University – told senators that he saw a likely correlation.
“The trend in extreme hot temperatures is striking in the proportion of record-setting daily highs versus record daily lows in station data from the U.S. National Weather Service,” Field told senators.
If climate is not changing, Field said, experts would expect that -- for any given date -- "one should see approximately equal numbers of record high temperatures and record low temperatures."
What that was the pattern in this country from 1950 to 1989, Field said that since then “the proportion of record-setting highs has been growing and the proportion of record lows has been shrinking.”
But atmospheric scientist Christy sought to throw cold water on the connection between heat waves and global warming. He told the Senate panel that regional heat waves, droughts and record-high temperatures are not a good way to analyze global climate change.
"Recently, it has become popular to try and attribute certain extreme events to human causation," Christy said. "The Earth however, is very large, the weather is very dynamic, especially at local scales, so that extreme events of one type or another will occur somewhere on the planet in every year."
“Since there are innumerable ways to define an extreme event," Christy said, such an approach "assures us that there will be numerous 'extreme events' in every year because every year has unique weather patterns.”
Dismissing speculation that this year’s dry, hot weather in much of the country relates to global warming, Christy asserted that “extreme high temperatures are not increasing in frequency, but actually appear to be decreasing.”
Why? “The recent claims about thousands of new record high temperatures were based on stations whose length-of-record could begin as recently as 1981, thus missing the many heat waves of the 20th century," Christy said. "Thus, any moderately hot day now will be publicized as setting records for these young stations because they were not operating in the 1930s.”
“Extreme events are poor metrics to use for detecting climate change,” Christy said, mainly because of their rarity. He cited a 2006 analysis of Midwest droughts which found that “droughts have, for the most part, become shorter, less frequent, less severe, and cover a smaller portion of the country over the last century.” In other words, Christy asserted, "droughts have always happened in the Midwest and they are not getting worse.”
But Field, in his testimony, argued that record high temperatures are indeed significant. From 1990 to 2008, Field said, 63 percent of the daily records were high temperatures and only 37 percent were lows. In 2009, 55 percent of the records were record highs; in 2010, 69 percent were record highs; in 2011, 73 percent were record highs.
And so far this year, Field said, 92 percent of the daily temperature records in the United States were record highs. “Through the first 23 days of July, there were 20 record high temperatures for every record low.”
The committee’s chairman, U.S. Sen. Barbara Boxer, D-Calif., said that the nation’s drought and several other “recent events make it clear that the climate continues to change and the likelihood of extreme events is growing greater, which puts our nation and our people at risk.” As evidence, Boxer cited:
- The National Oceanic and Atmospheric Administration reported in June that the previous 12 months had been the warmest 12-month period the nation had experienced since recordkeeping began in 1895.
- As of July 3, 56 percent of the nation was enduring moderate to exceptional drought conditions. Scientists at NOAA have indicated that this year’s record-breaking Texas drought appeared to be influenced by climate change.
- The National Aeronautics and Space Administration reported last month that an iceberg twice the size of Manhattan broke off of Greenland.
But the committee’s ranking Republican, U.S. Sen. Jim Inhofe, R-Okla. – one of the most outspoken climate-change skeptics on Capitol Hill – contended that the drought and heat waves were natural climate fluctuations. He has accused climate-change scientists of “a hoax” and he argued that "the global warming movement has completely collapsed.”
Field said studies have not shown direct links or trends related to localized severe weather events as tornadoes, and the evidence related to the prevalence of hurricanes is mixed. However, he contended that “for other categories of climate and weather extremes, the pattern is increasingly clear. Climate change is shifting the risk of hitting an extreme.” That is especially clear in relation to the increased risk of heat waves, he said.
“These findings about risk do not speak directly to the role of climate change in any particular event,” Field said. “In this sense, the increase in risk of a weather extreme from climate change is parallel to the increasing risk of an accident from speeding in a car.
"The evidence pointing to the driving force behind the extra risk (either the climate change of the excess speed) can be strong, but it is still difficult to predict exactly when and where disaster might occur.”