Guest essay by Eric Worrall

A new study claims that global warming will deliver 4 fewer mild days per year by 2065, and 10 fewer mild days by 2100 – out of an annual average of 74 mild days per year. But the study’s rigid definition of a “mild day”, which the author uses as the basis of speculation about the impact of projected climate changes on human wellbeing, takes no consideration of people’s ability to adapt to different climates.

Shifting patterns of mild weather in response to projected radiative forcing

Karin van der Wiel, Sarah B. Kapnick, Gabriel A. Vecchi

Abstract

Climate change has been shown to impact the mean climate state and climate extremes. Though climate extremes have the potential to disrupt society, extreme conditions are rare by definition. In contrast, mild weather occurs frequently and many human activities are built around it. We provide a global analysis of mild weather based on simple criteria and explore changes in response to radiative forcing. We find a slight global mean decrease in the annual number of mild days projected both in the near future (−4 days per year, 2016–2035) and at the end of this century (−10 days per year, 2081–2100). Projected seasonal and regional redistributions of mild days are substantially greater. These changes are larger than the interannual variability of mild weather caused by El Niño–Southern Oscillation. Finally, we show an observed global decrease in the recent past, and that observed regional changes in mild weather resemble projections.

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The aim of this study is to investigate mild weather. Mild weather is weather that is neither too hot, too cold, too humid nor rainy–weather that could also be described as being “pleasant”. Mild weather occurs frequently in most parts of the world. It does not disrupt society the way climate extremes do, instead many human outdoor activities are enhanced by or depend on mild weather. Examples of such activities include picnics, football games, dog walks, bike rides, and outdoor events such as music festivals or weddings. Furthermore, the absence of mild weather during construction work, infrastructure projects, road works, landscaping projects, air travel, and rail or road transportation may cause delays with significant negative economic consequences. This relationship with recreational and industrial human activity makes mild weather a relevant meteorological condition for society.

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Here, we use a global definition of mild weather, based on simple meteorological criteria that should be relatively easy to relate to:

Daily maximum air temperature between 18 and 30 °C.

Daily total precipitation not exceeding 1 mm.

Daily mean dewpoint temperature not exceeding 20 °C.

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3.1 Present day global distribution of mild weather

The global distribution of days with mild weather in the control experiment is shown in Fig. 1a. Globally, an annual average number of 74 days with mild weather is found; this translates to 20% of all days in a year or, accounting for population density, 89 days per year per person.

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3.2 Changes in response to radiative forcing

In response to 21st century radiative forcing (RCP4.5), the global distribution of mild weather is projected to change (Fig. 2a). The tropics and subtropics are projected to have fewer days with mild weather; the extratropics are projected to have slightly more days with mild weather.

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4 Discussion

We have presented the first analysis of mild weather occurrence and projected changes with respect to climate change. The analysis is based on simple criteria that may be refined or extended in future work. Such refinements could include additional variables, for example, cloudiness or wind speed, or by defining spatially or seasonally varying criteria. Furthermore, the uncertainty of the projections related to scenario choice could be part of a future assessment. The current analysis is based on RCP4.5, the projected changes are likely to be larger in response to RCP8.5 that prescribes stronger radiative forcing.

In conclusion, the global mean number of mild days in a year is projected to decrease in response to radiative forcing. …