Has global warming made heat waves more lethal in Sweden? That’s the conclusion of a study by Swedish scientist Daniel Oudin Åström and colleagues, published last October in Nature Climate Change (NCC). The researchers examined the association of mortality and extreme temperatures in Stockholm. They found that the number of “heat extremes” increased from 220 in 1900-1920 to 381 in 1980-2009. After adjusting for urban heat-island effects, they conclude that climate change was responsible for 288 out of 689 heat-related deaths in the latter period.
Why mention this now? Last week, NCC published a rebuttal by Chip Knappenberger, Patrick Michaels, and Anthony Watts. The authors also posted commentaries on their respective blogs.
One thing that puzzled me right off the bat is Åström et al.’s definition of “heat extreme”: any two-day period when the temperature exceeds 67.2ºF. To some of us who hail from the Sun Belt, 67°F is still sweater weather.
Knappenberger et al. find two major flaws in the Åström study. First, the Swedish scientists mistakenly assume that all warming not due to urban heat islands must be due to anthropogenic climate change. But Stockholm’s climate is also affected by a natural mode of climate variability called the Atlantic Multidecadal Oscillation. The AMO was primarily in its negative (cold) phase during 1900-1929 and primarily in its positive (warm) phase during 1990-2009. The difference between the two phases “is likely to be responsible for some portion of the increase in extreme-heat events identified by Åström et al. and inappropriately attributed to global climate change,” Watts writes.
Second, and more importantly, the Åström team ignores a relevant finding from another Åström et al. study on extreme temperatures and mortality in Stockholm. The key concept here is “relative risk” — an estimate of how much likelier an individual is to die from exposure to a particular risk factor relative to individuals who are not exposed.
In that study, Åström and colleagues found that the relative risk of dying from extreme heat in Stockholm was about 20% in the beginning of the 20th century. But in the NCC study, they estimate that the relative risk of dying from extreme heat in 1980-2009 was 4.6%. In other words, people in Stockholm today are only about one-fourth as likely to die during heat waves than was the case in the early 20th century. [click to continue…]




