In March we learned ice sheet formation is two-sided. On Sunday, sipping coffee whilst reading the latest Nature Geoscience, we learned the same might go for ice sheet melting. If so, melting would likely accelerate over this century and sea levels would rise faster than previously projected.
The new insights are derived from the combined use of 19 different climate models, which were tuned to better incorporate temperature changes of ocean waters surrounding glaciers and icecaps – and study the effect of the surrounding water temperature on the ice.
Boiling the frog slowly
With respect to global warming oceans trick us. They not only -for a while- absorb a chunk of the CO2 we emit – they are also very slow to warm up, simply by being as large and massive as they are [thereby creating a delay of several decades before any emitted tonne of CO2 reaches its maximum atmospheric warming potential and fooling us to think the current CO2 concentrations should be linked to current global temperatures].
This oceanic temperature delay however is not uniform. The deep seas may keep cool for many decades or centuries [let’s hope the latter, don’t wake the methane clathrates just yet] the upper layer of the oceans have direct interaction with the atmosphere and warm faster.
And that’s what the University of Arizona researchers have focused on, running the climate models on the mid-range forecast scenario for increase in atmospheric greenhouse-gas concentrations [so an underestimation of (extrapolating) current emission trends].
Warmer seas around the ice
They find the subsurface oceans surrounding the two polar ice sheets at depths of 200–500 m will warm much faster than thought. The subsurface seas around Greenland for instance would warm by 1.7-2.0 degrees Celsius. Around Antarctica the emissions scenario would translate to 0.5-0.6 degrees warming at a depth of 200-500 m, also more than previously thought.
The temperature difference between Greenland and Antarctica is explained through their different positions on the thermohaline circulation, with Greenland much more exposed to tropical warm waters due to the proximity of the Atlantic Gulfstream, whereas the Antarctic Circumpolar Current keeps Antarctica’s shores somewhat isolated.
Nevertheless, also for the Antarctic icesheets, the effect of ocean warming has been underestimated, the researchers conclude. Although the water will not warm as fast as the air above the ice sheets, the heat carrying capacity of water is much larger and therefore the energy can penetrate deeper into the ice. Also, at a depth of several hundreds of meters, the warming waters can undermine glacier fronts and perhaps destabilise entire glaciers, leading them to flow faster towards the ocean – and loose more ice.
Especially in Antarctica this process of glacier undermining can continue for a very long time, as much of the ice cap is land ice sliding over the ocean floor, which means it could be lifted or lubricated by a wedge of intrusive warm water.
“This paper adds to the evidence that we could have sea level rise by the end of this century of around 1 meter and a good deal more in succeeding centuries,” says co-author Jonathan Overpeck, a geosciences professor at the University of Arizona.
Bridging the paleo gap
As clever climatologists and mathematicians keep improving climate models – and testing and calibrating them in rerun simulations of witnessed climate events – these models may finally get to exactly the same conclusion as paleoclimate studies: just 2 degrees warming leads to multiple meters of sea level rise. It would also help explain current Greenland melting.
© Rolf Schuttenhelm | www.bitsofscience.org