Sounds like there’s new food to calibrate our oceans’ sea level sensitivity. In red the image shows inundations around the Gulf of Mexico under Eemian sea levels. That’s ‘bye bye Houston, New Orleans, Miami.’
Two days ago we looked at the Pliocene, an epoch with CO2 concentrations around as high as they’ll be in 4 years time, 2-3 degrees higher temperatures and with average sea levels 25 meters above today’s levels.
But our regulars may also remember another important paleoclimate comparison, with the Eemian, in between Pleistocene glaciations – when temperatures we thought[!] were ‘more than 2 degrees Celsius higher’ and -we thought- sea levels bumped 4-6 meters.
In February we paid attention to a new paper by James Hansen, who argues the Eemian temperature difference could have been ‘smaller than one degree’ and who uses the Eemian sea levels to illustrate that our path to 2 degrees warming is too much – as it will manifest itself in multiple meters of sea level rise.
Ice sheets: from unstable to unstabler?
Hansen could be more right than he thought back then. The new study, conducted by a team of earth and atmospheric scientists of Arizona University, and upcoming in Geophysical Research Letters, has tried to better reconstruct the Eemian ocean temperatures from the fossil record. They find the temperature difference was even smaller, just +0.7 degrees Celsius.
They also use an adjusted figure for the Eemian sea level, which they state was [at the peak about 125,000 years ago – sea level height is much easier to reconstruct from sediments than temperature] 8-8.5 meters higher than today.
As thermal expansion is a physical characteristic of water, they can also conclude which percentage of the 8 meters was caused by thermal expansion: only 5 percent, or 40 centimeters.
The rest can only be one thing: meltwater. So something big must have happened to the Earth’s ice sheets. Perhaps Greenland and Antarctica are (even) less stable than we think?
A minimum sea level rise of meters
The authors think Antarctica contributed most during the Eemian: 4.1 to 5.8 meters. They also say the warming and melting consequences of the current rise in atmospheric greenhouse gases is largely irreversible and that we are already en route to at least 4-6 meters of sea level rise over the coming centuries [centuries, because of thermal time lags between atmosphere, oceans, and the larger ice masses].
Eemian paleo climate comparison
There is still one problem we have with the Eemian comparison. It was just 120,000 years ago, right before the last ice age. This means geologically, physically, we are discussing almost exactly the same planet Earth. Almost.
The ‘big’ difference between the Eemian and the Holocene Earth could be in the phase of Milankovitch cycles [although even these resemble, as they are the forces that make both the Eemian and Holocene inter-glacials, instead of ice ages]. During the Eemian the tilt of the Earth’s axis [obliquity] was slightly bigger, as was the Earth’s orbit deviation [eccentricity], both factors increasing seasonal variability. Although this leads to long, dark and extra cold polar winters, it also exposes the Arctic and Antarctic ice sheets to summer sunshine, when melting occurs – and the albedo feedback can be activated.
But could this explain 8 extra meters of sea level? Don’t ask us – let’s research.
Elephant in room: not 8, but 25 meters
And while at it, let’s not ignore the Pliocene elephant that is standing 25 meters high right behind us. Because we’re not aiming for Eemian, we’re aiming for Pliocene temperatures: +2 to +3 degrees Celsius – at minimum. And by the way, that’s the sort of warming where even thermal expansion translates to more than a meter of sea level rise – without any of the ice melting.
© Rolf Schuttenhelm | www.bitsofscience.org