Science Day, COP27
By Professor Dave Waltham, Department of Earth Sciences.
Relevant United Nations’ SDG goals:
It is hard to imagine how anyone in 2022 could still be in doubt that climate change is real. Globally, we know that climate scepticism varies around the world.
It’s very hard to argue climate change is not happening. The scientific consensus is overwhelming. Each decade sees more wildfires, more floods, more droughts, smaller glaciers, more coral-reef bleaching, less sea and glacial ice in the summer, higher sea levels and record temperatures across the globe.
I find suggestions, that these changes are not humanity’s fault, almost as implausible as the idea they’re not there at all. The animated figure, below, shows the very close link between industrial greenhouse gas emissions and global mean temperature rise. This is spectacular “experimental confirmation” of quantitative predictions going back, not just to the 1990s but, to the 1890s!
Temperatures have risen, since 1880, as the total emissions into the atmosphere have gone up. Temperatures are from NASA (compared to the 1950-1980 average) and emissions are from “our world in data”.
We’ve therefore known, for 130 years, how much climate change to expect from the level of atmospheric fly-tipping we’re indulging in. But our understanding of its seriousness is much more recent.
I’ve been aware of the debate over anthropogenic climate change since the 1980’s but, for most of that time, it was widely assumed that a sensible target was to keep warming to 2 or 3 ⁰C. But, at the 2015 COP in Paris, small island developing nations argued that we need to do better and keep the rise to 1.5 ⁰C.
The resulting “Paris Accord” was backed up, the following year, by an explosive report from the IPCC (Intergovernmental Panel on Climate Change) comparing the consequences of a 1.5 ⁰C rise to those of a 2⁰C rise. Every issue, from fire to flood and from poor harvests to biodiversity loss, was bad at 1.5 ⁰C but catastrophic at 2 ⁰C. The societal impact of the report was almost instant with, for example, “Extinction Rebellion” and “Fridays for the Future” both arising at that time.
However, even the IPCC reports may turn out to be optimistic. Its predictions are largely based on the assumption that climate change will be relatively orderly even predictable—that, as my animation shows, steady increases in greenhouse gasses will produce steady rises in temperature. That may not be true. There may be “tipping points”—thresholds at which sudden, larger jumps in temperature occur.
Earth’s climate system has such thresholds. For example, we’ve spent the last few million years repeatedly switching between intensely cold “ice-ages” and warmer “interglacials”. You can see the most recent switch, 12 thousand years ago, on the graph below but it’s also interesting to see the evidence for other sudden jumps in temperature, both up and down, over the last 50 thousand years.
Average temperatures, in central Greenland, over the last 50 thousand years (after Alley, 2000). Note step-like changes such as the one, 12 thousand years ago, which marked the end of the last ice-age.
The science of, when let alone if, tipping points occur is not yet well developed but of one thing we can be sure; the probability of hitting a tipping point will mount as temperatures increase. Tipping points are therefore more likely at 2 ⁰C than they are at 1.5 ⁰C and that’s a good reason to try to keep rises as low as possible.