Past warming shows 2°C brink may be close

Once again, past warming warns of the power of climate change. The surprise is that it doesn’t take much warming to raise sea levels six metres.

Even if the world’s nations keep their promise to contain global warming to within 2°C, past warming shows that the Earth will still change visibly – and perhaps sooner than science currently expects.

Sea levels could rise by six metres.  Large tracts of the polar ice caps could collapse. The Sahara could become green. The edges of what are now tropical forests could turn into savannah, to be seared and maintained by regular outbreaks of fire. The northern forests could move 200 km nearer the north pole and, ahead of them, the tundra.

That is what will happen, if the past is a sure guide to the present. A 2°C rise in temperature is the maximum agreed by 195 nations when they met in Paris in 2015, a promise that can be maintained only by reducing carbon dioxide emissions, chiefly by switching rapidly from fossil fuels to renewable sources such as solar and wind power.

Three times in the last 3.5 million years, the planetary thermometer has risen, to up to 2°C higher than those temperatures humans enjoyed for most of the last 2,000 years. And three times the global climate has changed in response.

What is less certain is the rate of change: a six metre rise in sea levels fuelled by the thermal expansion of the oceans and the loss of the world’s glaciers, and the retreat of the Greenland and Antarctic ice caps, could take thousands of years. But once such changes began it would be very difficult to halt or reverse them.

“The carbon budget to avoid 2°C warming may be far smaller than estimated, leaving very little margin of error”

All geology is based on an axiom that the present is the key to the past: landscape around us tells a story of the conditions under which the rocks were formed. It follows that the past should also foretell the possibilities of the future, and researchers from 17 nations report in the journal Nature Geoscience that they looked again at three recent intervals when the world was warmer.

One of these began at the close of the Ice Age, 9000 to 5000 years ago; one between the last two ice ages 129,000 to 116,000 years ago; and one from a warm period known as the mid-Pliocene 3.3 to 3 million years ago. The first two were responses to very subtle but predictable shifts in the planet’s orbit.

But the oldest of these warmings was driven by an increase of carbon dioxide in the atmosphere to between 350 and 450 parts per million. These are levels that match those of today, as a consequence of 200 years of fossil fuel exploitation.

The research raises questions about the completeness of the climate models now used by scientists to predict future change.

Slow to act

As ice retreats and vegetation cover changes, so does the traffic in carbon between living things and the rocks, ocean and atmosphere. And the catch – for climate modellers – is that although the world’s nations promised to act, action so far has been slow. Fossil fuel is still “business as usual”. And this inevitably will play into the calculations in unpredictable ways.

“While climate model predictions seem to be trustworthy when considering relatively small changes over the next decades, it is worrisome that these models likely underestimate climate change under higher emission scenarios, such as a ‘business as usual’ scenario, and especially over longer time scales,” said one of the scientists, Katrin Meissner, of the University of New South Wales, in Australia.

And Hubertus Fischer, of the University of Bern, in Switzerland, who led the study, said: “Observations of past warming periods suggest that a number of amplifying mechanisms, which are poorly represented in climate models, increase long-term warming beyond climate model projections.

“This suggests the carbon budget to avoid 2°C warming may be far smaller than estimated, leaving very little margin of error to meet the Paris targets.”