Cold weather events to persist as planet warms

Current projections estimate that the globe will be about 2-5°C (3.6-9°F) warmer by the end of the century, depending on future emissions of greenhouse gases. It's sometimes assumed that climate changeimplies a smooth, linear march up the thermometer, with each season slightly warmer than that of the previous year, without exception. Seen this way, any abnormal cold snap would appear to falsify the predictions of climate science. Obviously, that's a simplistic view of the climate, so what do climate models really project the future to be like? A recent paper published in Geophysical Research Letters takes a look at how cold weather events are expected to change in a warming climate.

Climate projections are based on models that incorporate every bit of physics and chemistry we are able connect to Earth's climate, and they do the difficult work of calculating everything out for each grid cell on the planet at each time interval. A number of research groups (such as NASA and the National Center for Atmospheric Research) develop their own models, so the average of multiple models is often used to generate more robust projections, like those published by the Intergovernmental Panel on Climate Change. The authors of this recent paper used nine of those models to compare cold events during the last decade of the 21st century with those of the last decade of the 20th century, based on a middle-of-the-road greenhouse emissions scenario (IPCC scenario A1B).

The team characterizes "extreme cold events" for each year in three ways: intensity, duration, and frequency. Intensity is calculated as the lowest 3-day average of maximum daily temperatures. Duration is defined as the greatest number of consecutive days that dip below freezing each year. Frequency is simply the total number of days that reach freezing temperatures in that year.

Model projections for the years 2091-2100 were compared to the average annual intensity, duration, and frequency for the decade of 1991-2000, and the researchers counted the number of times (for each individual grid cell) that a year exceeded that 1991-2000 average.

In the end, they find that while the frequency of cold events decreases everywhere, the duration and intensity of cold events in the future don't look that much different from the present. Roughly half the globe experiences at least one year in the 2090s with cold events more intense than the 1991-2000 average, and portions of the planet (particularly North America and Europe) go through events of greater duration. The exact details of the number of years or land area affected have a pretty wide spread among the models, so the authors focus on a more general conclusion: we'll still need to be prepared to deal with cold weather events even as we focus on mitigating the effects of warming.

This is a good example of the complexity of the climate system. Climate warming is ultimately measured by global mean annual temperature. At smaller spatial and temporal scales, there is bound to be variation—weather is a dynamic thing. The relevant message from the paper is this: the same models that project a warming planet also project the continuation of regional extreme cold weather events.

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