A common perception holds that temperatures will become more variable in a warming world, with “higher highs and lower lows”, or at least that temperatures will vary over a wider range over time. Surprisingly, this idea has little basis in the science.
Evidence instead suggests that temperatures have become slightly less variable as Earth has warmed, and climate models also predict a modest reduction in variability with warming over the coming century. The reduction in variability seems to be concentrated primarily in the winter months, and it is modest enough that it will have little impact on the increasing frequency of extreme heat events in a warming world. However, it is good news that increasing variability seems unlikely to further increase the frequency of extreme heat.
The mean and the variability are both important aspects of global temperatures. The mean is simply the spatially-weighted average of individual station anomalies over time (you can think of it as simply the average of all weather stations over time). The variability measures how much the temperature changes day-to-day or month-to-month within a single year at individual stations.
The figure below, from the IPCC’s Third Assessment Report, shows how temperatures would change if the mean increased, if the variability increased, or if both increased. Mean increases lead to more record hot temperatures over time (compared with the present), and fewer record cold temperatures. Increasing variability without a change in mean would lead to more record hot temperatures and to more record low temperatures. If both are increasing, it would lead to an even greater increase in extreme heat events but less change for extreme cold events, perhaps the least desirable outcome.
Evidence from daily and monthly land surface temperature records over the last 50 years suggests that variability has been flat or slightly decreasing. This conclusion is based on looking at data from around 7,000 stations worldwide that report daily minimum and maximum temperatures. Over the same period, global land temperatures have warmed by almost 1 degrees C (1.8 F). The figure below shows global mean temperature by year in red and average annual variability across all stations in blue. Despite the rapid rise in temperatures, variability (shown in blue) has been decreasing.
This decline in land temperature variability is most pronounced in the winter; summer variability, where extreme heat events are more of a concern, has been essentially flat. These results are similar to those found in a paper last fall by Huntingford et al published in the journal Nature. Huntingford and colleagues looked at both land and ocean temperature records and found no evidence of increasing variability. They also analyzed the outputs of global climate models, and reported that most climate models actually predict a slight decline in temperature variability over the next century as the world warms. The figure below, from Huntingford, shows the mean and spread of variability (in standard deviations) for the models used in the latest IPCC report (the CMIP5 models).
This is good news overall; increasing mean temperatures and variability together would lead to even more extreme heat events. But “good news” is relative, and the projected declines in variability are modest, so rising mean temperatures by the end of this century will still push the overall temperature distribution well outside of what society has experienced in the last 12,000 years.