In particular, I am going to declare 70ºF as a critical threshold in comfort, with minimum temperatures below that value generally yielding reasonable comfort, whereas minimum temperatures above that level generally yielding uncomfortable sleeping conditions. Going a step further, I'll define the human misery index in Salt Lake City as being equal to the number of days with a minimum temperature at or above 70ºF.
Looking at historical data for the Salt Lake City Area stretching back to 1874, we see a rapid change in the July human misery index beginning around Y2K. In particular, around that time a clear upward trend began and we have had five Julys since 2007 with 15 or more days with a minimum at or above 70ºF, something that never happened previously.
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| Source: NOAA Regional Climate Centers |
This trends has a variety of implications for everything from human and environmental health to electricity demand. Cooling degree days in July, for example, show a gradual increase for the periods prior to 2000, but since then, we've been experiencing a new normal, with the average number of days clearly higher and several years with values near or above 600.
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| Source: NOAA Regional Climate Centers |
Development and associated urban heat island effects, global warming, and recent circulation patterns are possible contributors to these recent trends, although I am unaware of a study that has attempted to quantify the contributions of each of these effects. I suspect it is likely that that both the urban heat island and global warming are contributing significantly to the upward trends. It is possible that instrumentation moves and local conditions around the airport might also be contributing.
Personally, I'm starting to find all of this quite depressing. Mother Nature might perturb the jet stream some summer to give us an unusually cool July, but it is difficult to imagine the long-term trend not being towards an even hotter future. By the end of the century, Salt Lake could be the new St. George. Good luck with that.
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By coincidence (?), the Times today ran an equally depressing story on the increase in average low temperatures: https://www.nytimes.com/interactive/2018/07/11/climate/summer-nights-warming-faster-than-days-dangerous.html?hp&action=click&pgtype=Homepage&clickSource=story-heading&module=second-column-region®ion=top-news&WT.nav=top-news
ReplyDeleteTo your list of reasons for the recent increase in number of days with minimum above 70 I would add the drought we have been discussing the past few months. There is speculation climate change is causing or at least intensifying the drought. Recalling the DeRose et al data for the Bear River, we would expect the decline in flow since 2000 to be associated with the increase in overnight lows we have been experiencing in July in Salt Lake. My guess is Julys in Salt Lake when the Anasazi disappeared during the 1200s were similarly miserable to what we are now experiencing. My hope, and we'll see how futile it is as the future plays out, is that we are not in the beginning of a 50 or 100 year drought and that we will cycle into a wetter cooler period in the next few years. We can only pray, eh
ReplyDeleteThere are two primary factors contributing to the ongoing persistent drought over the southwest. The first is a lack of precipitation. The second is the high temperatures, which yields greater evapotranspiration (and also has effects on snowpack and runoff). Most of the evidence that I have seen suggests that the lack of precipitation is probably a reflection of internal variability, or what you call a "cycle". It may have happened with or without climate change. However, the warmth is being amplified by climate change and this is essentially a "drought multiplier" due to more rapid evapotranspiration.
DeleteIn the past there is strong evidence of persistent drought over the southwest. However, the warming that has been and will continue to occur means that we need to be cautious using past droughts as inferred from proxy records (e.g., tree rings) as an analog for future drought. In particular, during dry periods, drought will come on faster and be more severe in the future.
Jim
praying gets ya nothing.
ReplyDeleteIt may be useful to compare this trend in minimum temperatures to the corresponding (or perhaps just the daily maximum) dew point values, if a graph of dew point trends can be obtained. A lot of nocturnal cooling may be related to surface evaporation, especially in irrigated areas in a dry climate, and therefore to the amount of watering around the measurement site and in the area as a whole. I would speculate that a large-scale change from irrigated grass to rockscapes, etc is a big deal for urban area minimum temps. SLC is a good place to observe these trends because of its dry climate and the location of KSLC relative to noctural prevailing winds over the urban area.
ReplyDelete