2023 has set a new all-time record for the global mean 2m-temperature. This in itself is nothing sensational; it was more or less to be expected, as we got a new record approximately every 4.5 years since 1980. What is surprising is the extent of this overshoot: we have remained a hair’s breadth below the 1.5 degree mark.
A lot of recent news have raised eyebrows of scientists and interested public, of which some are listed here as examples:
- Sea surface temperatures in the North Atlantic and globally have risen extremely, ending the drought in Europe and turning it into flooding.
- The forest fires have destroyed 11 times more forest area than was newly planted (article in Nature, unfortunately behind paywall).
- The floating ice around Antarctica has greatly diminished, which has led to a destabilisation of the West Antarctic Ice Sheet.
- The Earth’s albedo, i.e. the proportion of sunlight that is immediately reflected, has decreased measurably, leading to a higher rate of energy absorption.
These pieces of the mosaic fit into a larger picture: the acceleration of warming. Since 1980, apart from fluctuations, warming has been astonishingly linear, with a very constant rate of 0.2 °C/decade. This linearity is most likely over.
Tamino, a climate blogger that I follow, is a seasoned statistician. In one of his recent blog posts, he looked at the HadCRUT temperature dataset, one of the five analyses of global temperature data published by well-known institutions, and calculated the slope of the smoothing function chosen by him for each year. (With a 20-year-averaging to determine it.)
Over the last ten years, the expectancy value of the slope rose from 0.2 °C/decade to 0.27 °C/decade, which is a clear deviation from the linear increase. Towards the end though, the uncertainty range increases sharply, so that a continued linear increase is still within it. One could argue that a rate increase is this not proven. But a much higher rate of 0.34 °C/decade is now just as likely as the previous value of 0.2 °C/decade.
Furthermore, he has determined and eliminated known fluctuations that are not dependent on the greenhouse gases. These are: the variation in solar radiation, the effect of volcanic eruptions and the large-scale South Pacific weather pattern known as the El Niño Southern Oscillation (ENSO). When you do this, you can see the influence of greenhouse gases (and other unnamed feedback mechanisms) more clearly.
The result is that the expectancy value of the increase rate of this adjusted temperature is now 0.3 °C/decade and that the previous 0.2 °C/decade is no longer within the uncertainty range – i.e. that the acceleration of warming over the last ten years can be regarded as almost certain.
This is only a statement about the past. What statements about the future, what predictions can be made from this? It is reasonable to extrapolate such a temperature curve. For the extrapolation we have to make certain assumptions, e.g. that the frequency distribution of the warming rate and its change, which can be obtained from the known data, will stay about constant, and that data from the recent past should be weighted more than data from the more distant past.
With these assumptions, we have an implicit climate model: we consider the Earth system as a black box and assume certain properties of this data source to be constant. If important input variables change (emissions) or non-linear feedbacks occur (tipping points), we reach the limits of an implicit model: the prediction quality deteriorates. But it is good enough for the next 20 years.
If we assume that the rate cannot fall faster than it has risen, we must expect faster warming at least until 2040, even under the most optimistic assumptions. However, in view of the certain reduction in the Earth’s albedo, there is no physical basis for such optimism.
In other words, we must accept as a fact that global warming is accelerating and act accordingly.
Comment
What does this mean for our politics and our private behaviour? No panic, but even more consistency, focus, awareness, persuasion, action, effectiveness and efficiency. Less distraction by side issues. A clearer vision of a profound decarbonisation of life.
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This text appeared in German on piqd.de and my German mirror blog.
Translation based on text generated by deepl.com.
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