More than 100,000 years ago, a period of unusually warm sea surface temperatures saw the oceans rise up to 9 metres higher than they are today.
The bad news? We might be looking at the same thing happening again, after researchers discovered that the heat extremes of that ancient, natural phase of global warming were actually similar to the temperature trends we're seeing today.
A new analysis led by researchers from Oregon State University has found that sea surface temperatures (SST) during the last interglacial period (LIG) – also called the Eemian – bear a striking resemblance to the rise in SSTs that have taken place over the last 150 years or so.
The LIG occurred between 129,000 and 116,000 years ago, and was one of the warmest periods of the last 800,000 years.
During the interglaciation, Earth's climate warmed due to a shift in the tilt of the planet, causing sea levels to rise between 6 and 9 metres higher than they are at present.
Now, after analysing marine sediment cores from 83 sites around the world, the researchers have reconstructed global sea surface temperatures during the LIG, and matched it with temperature data sets covering the years 1870–1889 and 1995–2014.
At the outset of the LIG, some 129,000 years ago, the global sea surface temperature was already similar to the average sea surface temperature for the 1870–1889 period.
Temperatures warmed very slowly during the LIG, with the global sea surface temperature increasing by about 0.5°C by 125,000 years ago.
But while that change occurred slowly over 4 millennia during the LIG, the 1995–2014 average shows the sea surface is at the same temperature now – and the rise took less than 150 years this time around.
"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 6 to 9 metres higher then compared to present," says climate scientist Richard Allan from the University of Reading in the UK, who was not involved with the study.
Even though the sea surface temperature now is about the same as it was 125,000 years ago, there's still a lot we don't know about how sea levels will rise this time, since the time-scale over which these temperature increases took place – and are still taking place – is so different.
During the LIG, the warming occurred so slowly that sea ice melting would have been a gradual, drawn out process. This time around – with a much faster increase in sea surface temperatures due to human activity – it's unclear how long the effect will take.
"[D]ue to the length of time it takes to heat up the depths of our vast oceans and to melt giant ice sheets it would take thousands of years before sea level could potentially rise to such levels," says Allan.
"[S]o sustained and substantive cuts in greenhouse gas emissions from energy-intensive activities remain vital and beneficial to societies."
While current global efforts are aiming to rein in carbon and keep global warming temperature increases to less than 2 degrees Celsius above pre-industrial levels, it's not great news that – as it stands – human intervention can speed up natural global warming processes to such a worrisome extent.
"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," lead researcher Jeremy Hoffman told Ian Johnston at The Independent.
"We're talking about something that took millions of years to form and we're removing it in decades. 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."
Despite the sober outlook the research provides, scientists say we need to use this knowledge and act while we can, to mitigate the impact on generations in many centuries from now.
"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," researcher Andrew Watson from the University of Exeter in the UK, who wasn't involved with the study, said in a statement.
"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."
The findings are reported in Science.