Iowans who live long enough likely will have to get used to fewer nice days, according to the authors of a new climate change study.
Unlike most such studies, in which scientists endeavor to predict how climate change will affect droughts, floods, fires and other cataclysmic events, the new study — the first of its kind — predicts how climate change will affect the frequency and location of mild days. The authors define “mild” as days having temperatures between 64 and 86 degrees F., with less than a half-inch of rain and dew points below 68 degrees F., indicative of low humidity.
Because of rising heat and humidity, scientists with the National Oceanic and Atmospheric Administration and Princeton University say the annual global average of 74 nice days a year will drop to 70 in the next two decades and to 64 after 2081. Some days will be too warm, some too humid and some will be both.
The largest decreases in nice days will occur in the tropics, where some regions could lose as many as 50 nice days per year by the end of the century, the study found.
The projected global change is nearly identical to that foreseen for Iowa.
In an analysis done specifically for Cedar Rapids, lead author Karin van der Wiel said the current number of nice days per year, 76, is expected to decline to 72 per year in the period 2016 through 2035 and to 66 per year in the period 2081 to 2100.
In the June-through-August summer period, Cedar Rapids’s current average of 31 nice days per year is expected to fall to 26 in the 2016-through-2035 period and to 17 in the 2081-through-2100 period, according to van der Wiel.
That long-term loss of 14 nice summer days will be somewhat offset by a gain of four nice days spread across winter, spring and fall.
Not surprisingly to most Eastern Iowans, Cedar Rapids’s current average of nice days in the December through February winter season is zero. While that is not expected to change in the next 20 years, the authors expect Cedar Rapids residents to enjoy an average of one nice winter day per year at the end of this century.
Asked about the findings, Iowa State University climate scientist Gene Takle said: “Humidity is what’s going to get us here in Iowa.”
Since 1970, absolute humidity in the June-August summer months has increased 13 percent in Des Moines, he said.
Besides detracting from what otherwise would have been nice days, the increased humidity has provided moisture and energy for the extreme rainfall events that many consider the hallmark of climate change in Iowa.
“Dr. Takle is right that humidity increases will cause a lot of days to be too humid to meet the criteria for mild weather in Iowa,” van der Wield said.
Though significant in many ways, precipitation changes in Iowa do not impact mild weather much, she said.
“Because of rising temperatures, more days in winter will be warm enough to be counted, but that is counteracted by changes in summer in which more days will be too warm,” she said.
Two U.S. cities well known for fine weather — San Diego and Los Angeles, with, respectively, 180 and 157 nice days per year — are projected to gain a few more in the remainder of the century.
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Of major American cities, Miami is expected to lose the most nice days during that period, falling from 97 per year to 69.
Among leading cities worldwide, London is expected to gain 24 nice days, while Lima, Peru, currently with 326 nice days per year, is expected to lose 114 of them by the end of the century.