Mike Hulme, Professor of Human Geography at the University of Cambridge, has a somewhat bizarre article published in Academia Letters called Climates Multiple: Three Baselines, Two Tolerances, One Normal. It’s basically a discussion of the recent World Meteorological Organisation (WMO) decision to re-define the present day climate as the period 1991-2020, replacing the period 1961-1990.
The article starts by suggesting that this means that
Climate will ‘change’, one might say, in an instant; the world’s climate will ‘suddenly’ become nearly 0.5°C warmer. It is somewhat equivalent to re-setting Universal Time or adjusting the exact definition of a metre.
Well, from the mid-1970s to the early 2000s we actually have warmed by about 0.5oC. This has nothing to do with how the baseline is defined. It’s also hard to see that it’s equivalent to adjusting the exact definition of a metre. I also wonder if Mike Hulme has got this the wrong way around. If we make the baseline period more recent, then the anomaly values actually go down, not up. You might argue that the change in baseline has caused the world to suddenly become 0.5oC cooler, rather than warmer (it hasn’t, obviously, but the change has reduced the anomaly values by about 0.5oC).
The rest of the article discusses the various baselines (present day, pre-industrial, historical) and what we might mean by a climate normal, but I don’t really get the overall point. Clearly we have to be careful about how we discuss climate change, be clear about what baseline we’re using, and be aware that what might be regarded as normal is changing. But this is a feature of the topic; it’s not something that can really be avoided.
It may be also technically true that
The adoption of particular baselines and tolerances is an overtly political process with geopolitical, ethical and technological consequence
but it’s also the case that none of these decisions change physical reality. Changing the baseline does not change how much we’ve warmed, how fast we’ve warmed, and how much we will warm if we continue to emit greenhouse gases into the atmosphere. If this is carefully communicated, it’s hard to see how these changes have any real political significance (on top of the political significance of climate change itself, of course).
In some sense, Mike Hulme’s article seems to be doing the very thing it’s cautioning against. The only way that changing the baseline, or what we regard as a climate normal, can have any broader political significance is if people overplay the significance of making these changes. Suggesting that redefining a baseline has geopolitical implications would seem to be an example of doing so.