December 5, 2012

If you’re 27 or younger, you’ve never experienced a colder-than-average month

The United States National Oceanic and Atmospheric Administration (NOAA) released a usual monthly State of the Climate Global Analysis in October 2012 which was anything but usual. Buried in the report was this astounding factoid: “This is the 332nd consecutive month with an above-average temperature”. What that means is, if you’re 27 or younger, you’ve never experienced a colder-than-average month. I find that phenomenal, a trend which if it continues might allow many to become ‘old timers’ who can “recollect the ‘good old days’ when a monthly temperature could be below average” (i.e. prior to March 1985).

This statement is certainly headline-grabbing, although there is some devil in the detail. Specifically, what is the ‘average’ which the NOAA benchmark against? A bit of research reveals that the NOAA use a reasonably robust 3-decade (1981-2010) average for their graphics, but the phrasing of the paragraph in question suggests that in this case they are comparing to the 20th century average. If it was the 3-decade dataset then the months Jan 1981 – Feb 1985 would have had to have been exceptionally cold to skew the average so low that it could be exceeded 332 times consecutively.

The field of time series and calculating moving averages (and variabilities) is fascinating, and no doubt with sufficient data-mining, as in any field (imagine sporting statistics), a number of other shocking statistics could be extracted. Nonetheless a 332 month run is still impressive (or incredibly concerning). Interpretation of climate change will more and more require punters to be comfortable with interpreting both running averages and changing variabilities. We can have extremely cold snaps in a month (variability) while still having an above average month for temperature.

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James Russell is a quantitative ecologist jointly appointed in the School of Biological Sciences and the Department of Statistics. He was the 2012 Prime Ministers Emerging Scientist prize recipient. See all posts by James Russell »

Comments

  • avatar

    Surely the headline is simply wrong?

    Last July (2011), I was living in Karori, Wellington. It snowed. Some roads were closed for a week to buses and other traffic.

    The claim appears to be suggesting one of two things:

    Either the temperatures I experienced overall that July in Karori were higher than the average temperature (since?). This would, I suspect, require several days of summer-type weather to even out the average temperature to be above average.

    Or

    I did not experience any weather in Karori, but experienced weather globally.

    This former simply did not happen, and the latter is simply stupid.

    That aside, even if we take the assertion that we experience weather globally at face value, your analysis (and theirs) should be a moving average. Our experiences are at the time they occur, so you need to consider the average at the time each monthly temperature was taken. If the average slowly increases over time, then months earlier in the series will be months that are now below average, but could have been – at the time – above average.

    2 years ago Reply

    • avatar
      David Hood

      The paragraph the sentence is taken from is for average temperature across land and ocean surfaces, so the global average temperature, and the 332nd month makes sense within that context (the globe is the subject). Written long-form the headline should be “If you’re 27 or younger, you’ve never lived on the globe when the globe has had a month of below average temperatures”

      2 years ago Reply

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