An important paper was recently published in Science on “A 485 Million Year History of Earth’s Surface Temperature” by Emily Judd and others. This long-term record of Earth’s temperature over virtually the entire history of complex life on our planet is important for understanding Earth’s history, putting present-day climate change in context, mapping the relationship between climate and the evolution/extinction of life, and tracking the chemistry of the atmosphere and oceans. This research reveals how relatively small changes can push the planet through tipping points.
Over the half-billion years studied, Earth’s surface temperature varied from 11.1 to 36.1 degrees. The authors divide Earth’s temperature into five phases – cold (11-18 degrees), cool (18-22 degrees), transitional (22-25 degrees), warm (25-28 degrees) and hot (28–36 degrees). Over the half-billion years studied, the percentage of overall time spent in each temperature category was: cold (13 per cent), cool (18 per cent), transitional (27 per cent), warm (23 per cent), hot (18 per cent). Transitional means transitioning between warm/hot and cool/cold phases.
The average global surface temperature today is 13.9 degrees which is unusually low in historical terms – global temperature was lower only twice in the past, and one of these intervals was brief. Earth is firmly in the cold category now and even the worst-case global warming scenario of a 3-degree increase in Earth’s temperature, bringing us to 16.9 degrees would still leave us in the cold category. There is obviously much scope for Earth to get a lot warmer than this as demonstrated by Earth’s temperature history.
At least two mass extinctions of life correlate with marked short-term changes in global temperature. One (Ordovician) was about 440 million years ago when temperatures dropped by 7 degrees in a few million years, precipitating a relatively brief ice age killing 80 per cent of living species. And 250 million years ago, temperatures jumped by 12 degrees in fewer than one million years, probably exacerbating the largest mass extinction in Earth’s history, the Permian-Triassic extinction, when more than 90 per cent of all living marine species and over 70 per cent of land species died.
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Earth’s temperature has suddenly shut up or down in a short period of time at least four times over the half-billion-year history recorded in this paper. These incidents are interpreted as triggered “tipping points”, when a relatively minor perturbation disturbs a finely balanced system, precipitating a sudden sharp change. The situation is analogous to a two-pan balance in equilibrium, with substantial equal weights on each pan. Just a small extra weight placed on either pan will immediately disturb this equilibrium.
Although the study indicates that atmospheric carbon dioxide concentration was the dominant driver of temperature changes over the period studied, relatively minor changes in any of several parameters – atmospheric greenhouse gas concentrations, sun’s energy output, water flow through the Gulf Stream, extent of Arctic ice cover, etc – could move us past tipping point transitions, potentially transforming Earth into a hothouse or precipitating an Ice Age.
Unfortunately, we don’t know how close to a tipping point we might be at any given time or what parameter might act as a trigger. For example, there is evidence that a nearby gamma ray burst severely depleted Earth’s protective ozone layer, allowing levels of ultraviolet radiation lethal to life to reach Earth and this may have triggered the Ordovician mass extinction.
We can only work on what we know about and we do know that carbon dioxide is a warming gas that has been increasing in concentration in our atmosphere since the start of the Industrial Revolution and that this increasing concentration is warming Earth – the greenhouse effect. This increasing temperature is an obvious potential candidate for a tipping point trigger. If this trigger were pulled it might precipitate a runaway greenhouse warming. Efforts are afoot to tackle this problem with a view to eliminating emissions of greenhouse gases to the atmosphere by 2100, but it looks like this target will not be met.
For some years now we have been continually exposed to gloomy predictions in the media about climate change and its consequences. Climate anxiety is now a recognised condition that is caused by this relentless barrage of bad news, and this condition can be quite intense in young people. Psychologists tell us the best antidote is to join a group that is taking action to combat climate change. The worst option is to sit alone fretting.
William Reville is an emeritus professor of biochemistry at UCC