More Temperature Variability in a Warming World? Not So.

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A little ‘good news’ at least relatively, as scientific evidence provides little basis for supporting a commonly held perception of more temperature variability in a warmer world.

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.

Zeke Hausfather

Zeke Hausfather, a data scientist with extensive experience with clean technology interests in Silicon Valley, is currently a Senior Researcher with Berkeley Earth. He is a regular contributor to Yale Climate Connections (E-mail: zeke@yaleclimateconnections.org, Twitter: @hausfath).
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8 Responses to More Temperature Variability in a Warming World? Not So.

  1. RB says:

    Some random thoughts. For the greenhouse effect, an electrical analog would be that of a train of current pulses (from the sun) striking the land/ocean/atmosphere system which can be modeled as a resistor/capacitor network. As more CO2 is added to the atmosphere, its resistance increases. The temperature at the surface (voltage in the electrical analog) would fluctuate with the turning on/off of the solar (current) input. But the average voltage (temperature) in the electrical analog has to rise so that average current in = average current out. Now, if the resistance increases, the expectation would also be for greater filtering at the output of the RC-network resulting in reduced voltage (temperature) fluctuations. It seems like this analogy holds in this context. But then again it doesn’t have to necessarily since if one assumes that surface temperature oscillations are produced by equivalent LC-networks (ocean currents etc), a greater energy in the system should result in a greater amplitude of oscillation i.e. temperature fluctuations.

  2. Its worth pointing out that this article refers to the global mean. There are some areas of the world where variability has increased over the past 50 years, though its unclear the extent to which it might be externally forced.

  3. AJ says:

    I’m a little surprised at the “flatness” of your variability over time plot. I would have expected it to have agreed with Hansen et al 2012. This is curious. Personally, I was able to replicate Dr. Hansen’s post-1950 findings, but my pre-1950 findings indicated there was no long term change in variability.

    https://sites.google.com/site/climateadj/gistemp-var

    (my apologies for not “finishing” the above post)

  4. AJ,

    Hansen et al actually wasn’t claiming that variance was increasing (as Jim told us when we asked him about it). The flattening of the frequency density functions over time was an artifact of the baseline period chosen. This is discussed in the Huntingford and Jones et al paper linked in the post.

    You can see more about it here: http://tamino.wordpress.com/2012/07/21/increased-variability/
    And here: http://tamino.wordpress.com/2012/07/23/temperature-variability-part-2/
    And here: http://static.berkeleyearth.org/memos/wickenburg-hansen-memo.pdf
    And here: http://static.berkeleyearth.org/memos/hausfather-hansen-memo.pdf

  5. Aaron says:

    Well, duh.

    The GHE affects lows far more than highs. It warms the poles more than latitudes . And stratifies the verticle atmosphere.

    Common sense tells us it is a stabilizing force.

  6. Chip Knappenberger says:

    Zeke,

    When the IPCC TAR came out with that graphic, we were highly critical of it for not including a declining temperature variability case (instead preferring the display the more dramatic cases)–especially in that there was plenty of evidence at the time (including some we published) cited by the IPCC that temperature variability was declining. Heck the IPCC even wrote in the TAR:

    “Consequently, there is now little evidence to suggest that the interannual variability of global temperatures has increased over the past few decades, but there is some evidence to suggest that the variability of intra-annual temperatures has actually quite widely decreased. Several analyses find a decrease in spatial and temporal variability of temperatures on these shorter time-scales.”

    It looks to me like the evidence for declining variability has continued to mount.

    Maybe the IPCC should consider playing things straight.

    -Chip

  7. Nullius in Verba says:

    As I understand it, there are changes in both mean and variability on decadal time scales going in both directions. (See the orange/blue graphics here.) Also, the transitions are quite sharp, which suggests the influence of discrete transient events – the 1998 El Nino is particularly clear, and there’s some other event around 1930-1950.

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