“Climate departure” means…

… next to nothing!

I have recently seen this term “Climate departure” cropping up all over the place, made popular by a paper in Nature, entitled “The projected timing of climate departure from recent variability”. Climate departure in itself is an old concept, and is used “for studying how “normal” or “unusual” a particular year or event is compared to the long-term average for the region under consideration.” (Suckling, 1987)

The authors, already in the abstract, note that “Unprecedented climates blahblahblah, highlighting the vulnerability of global biodiversity and the limited governmental capacity to respond to the impacts of climate change. Our findings shed light on the urgency of mitigating greenhouse gas emissions if climates potentially harmful to biodiversity and society are to be prevented.”

And then the lovely Daily Mail caught its eyes on this, and entitled a new article: ‘Apocalypse Now: Unstoppable man-made climate change will become reality by the end of the decade’. One of the sentences that is oh-so-beautifully-written but clearly has nothing to do with climate departure is: “The Earth is racing towards an apocalyptic future in which major cities such as New York and London could become uninhabitable because of irreversible man-made climate change within 45-years.”

Needless to say, other newspapers follow that apocalyptic approach. Here is National Geographic: “To put it simply: The coldest year in New Guinea after 2020 will be warmer than the hottest year anyone there has ever experienced.” argh…

The article from Washington Post is somewhat better. But they also push the bounds of interpretation: “Lagos, Africa’s largest city, with a population 21 million and rising, is already vulnerable to flooding. It’s got only 16 years before it hits climate departure.” Presented as if those two issues have something in common. Or this: “The fact that these cities pass climate departure so soon is a scary reminder of how rapidly they’re going to feel the effects of climate change.” Nope, absolutely not. Climate departure is a specific, rather uninformative, measure of climate change but tells us nothing about the effects of climate change.

So what can the concept “climate departure” actually tell us?

Apart from the fact that it gets hotter in some places, not much! And that it is getting hotter is something that we clearly know from the reports like the IPCC provides. The reason for why climate departure is a problematic concept is clearly that it is an arbitrary measure. As an illustration, take a cup of water. In the morning it is 20 degrees, left in the sun the whole day it might go up to 22 degrees. Assuming variability does not change, then climate departure means that from a certain poin in time onwards your cup of water will be between 22 and 24 degrees.  If you put your finger inside, you will not notice a difference. Noone will notice any difference.

DIfferences arise however, if your water before was just less than 0, or just below 100 degrees. Then a small increase in temperature matters a lot, since then there is an actual change in the state of the water itself. It will become liquid or gaseous. For you this will matter a lot, since you cannot walk on the water anymore once it becomes liquid and you cannot touch it any longer if it becomes gaseous. And here the concept of climate departure is only meaningful since we are dealing with tipping points. Outside of tipping points, the concept of climate departure has not much meaning.

Thus, contrary to what the Daily Mail or the Washington Post try to make us believe, climate departure has nothing to do with apocalyptic, uninhabitable, irreversible, or how rapidly we’re going to feel the effects of climate change.

Also, contrary to what the authors in Nature claim, this has little to do with “highlighting the vulnerability of global biodiversity”. Yes, biodiversity responds to climatic changes, but for that we need a scala that says somethings about an absolute change and how local biodiversity is likely to react.

And it also says nothing about “the limited governmental capacity to respond to the impacts of climate change”, simply because this paper in Nature only gives an idea as to how temperature will change during the next years, but says nothing whatsoever on the impacts of climate change. And especially nothing about the governments ability to react to climate change.

In my opinion, one has to be very careful with this kind of apocalyptic approach. While it seems to sell well – congrats for the authors to get their paper in Nature, a highly accredited journal – it should not become scientific practice to jump to conclusions that are outside of the story that one’s actual study is able to tell.

Some trivia and update: Interesting in itself is, of course, what climate departure should mean for Oymyakon. In Oymyakon, the minimum mean temperature is -46 degrees, and the maximum mean temperature is 15 degrees. If variability does not change, then the new maximum mean temperature in Oymyakon will be 71 degrees. hmmm, not enough for a good Finish sauna, but close… Or imagine we take the 100 year average min/max temperature – it will not be the same as the 1 year min/max temp. And for climate change, we may want to look at the 1000 or 10000 year min/max average temp. So scale matters and gives really different results for climate departure. We may end up with a Finish sauna, or not much change at all. It seems to me that the way the IPCC presents things, in terms of 2°C warming and its effects on biodiversity etc, provides a more qualitative useful approach:

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