The last time ocean temperatures were this warm, sea levels were up to nine metres higher than they are today, according to the findings of a new study, which were described as “extremely worrying” by one expert.
The researchers took samples of sediment from 83 different sites around the world, and these “natural thermometers” enabled them to work out what the sea surface temperature had been more than 125,000 years ago.
This revealed that over the course of some 4,000 years the oceans had got about 0.5C warmer, reaching about the same temperatures as are found now – after a similar increase achieved largely as a result of human-induced climate change in little over a century.
Previous research has established that sea levels at the time were between six and nine metres higher. This gives an indication of what sea levels might be like once the vast oceans expand and ice sheets melt over the course of the next centuries and millennia.
If sea levels were to increase by nine metres, parts of London and New York, almost all the Netherlands, huge chunks of China, including Shanghai, and much of Bangladesh would be just some of the places that would be lost to the sea.
But the bad news does not end there.
For the computer models used by scientists to predict what the climate will be like in the future had failed to pick up on the rise in temperatures 125,000 years ago.
This suggests the models could be missing a key warming effect that might be about to kick in, sending temperatures higher than currently expected.
Another recent study suggested the sensitivity of the climate to greenhouse gases could be much greater than previously thought, potentially putting the world on course for more than 7C of warming by 2100 — a prospect described as “game over” for life as we know it.
Dr Jeremy Hoffman, of Oregon State University, lead author of a paper in the prestigious journal Science about the new research, told The Independent that sea levels some 125,000 years ago might give a rough indication of what could be expected over the next few centuries as the warmer temperatures slowly take effect.
But he stressed the reasons for the global warming then and now were very different – the former was natural, the latter caused by humans – so the world’s last major warm period could not be viewed as a simple way to predict the future.
“There are a lot of things that have happened over the last century that far outpace the natural world,” Dr Hoffman said.
“It’s not just the warming, it’s the release of carbon from reservoirs [of fossil fuels] in the planet that have been around for millions of years.
“We’re talking about something that took millions of years to form and we’re removing it in decades.
“The Earth would need to have an eruption like Mount St Helen's happen every 2.5 hours … to keep pace with the emissions we are producing.”
He said perhaps the most significant implication of their research was that current computer models of climate change were failing to pick up on the warming 125,000 years ago.
“If we are missing some process that would give rise to additional warming [at that time] … that would only work to be under-estimating the future climate as well,” Dr Hoffman said.
Commenting on the research, Andrew Watson, a Royal Society research professor at Exeter University, said: “Sea level responds directly to global temperatures, but slowly, so that the full extent of sea level rise will only be apparent over thousands of years.
“The study suggests that in the long term, sea level will rise six metres at least in response to the warming we are causing.
“The good news is that with luck it will continue to rise slowly, so that we have time to adapt, but the bad news is that eventually all our present coastal city locations will be inundated.”
Professor Richard Allan, a climatologist at Reading University, said: “The result that present global sea surface temperatures are indistinguishable from those at the last interglacial 125,000 years ago is extremely worrying since sea levels were six to nine metres higher then compared to present.”
He said that heating up the “depths of our vast oceans” to the point where sea levels reached that point would take thousands of years “so sustained and substantive cuts in greenhouse gas emissions from energy-intensive activities remain vital and beneficial to societies”.
And Professor Michael Mann, a renowned climate scientist from Pennsylvania State University, described the studies findings as “sobering”.
“It indicates that we may very well already be committed to several more metres of sea level rise when the climate system catches up with the carbon dioxide we’ve already pumped into the atmosphere,” he said.
“That is actually consistent with some model simulations. The important thing to recognise is that the climate system hasn’t yet come into equilibrium with the increase in carbon dioxide, so it is misleading to compare the historical sea level rise we’ve seen so far with the sea level rise 125,000 years ago, because the latter indicates the full response [to the warming effect].”