2 degrees warmer climate in late Pliocene meant 12-32 meters higher sea levels

That would likely mean that also the official UN climate goal of limiting the average world temperature rise to no more than 2 degrees Celsius – a target linked to 450 ppm CO2 equivalent stabilisation scenarios (practically ambitious, theoretically weak) – will eventually lead to many meters of global sea level rise.

This is what a research team comprised of 10 Earth scientists led by Rutgers University estimate, based on a paleoclimatic study into the Pliocene. Their findings were published today in Geology.

Pliocene climate temperature
The Pliocene climate was warmer than today’s climate. Note the decreased temperature difference between equator and poles, as also observed and projected under the modern climate change. World temperature anomaly map [by Wikimedia Commons author Giorgiogp2] is based on USGS temperature reconstruction data, as shown below the world map, for the late Pliocene period between 3.3-3.0 Ma. The new publication in Geology focuses on a slightly younger timeframe of 3.2-2.7 Ma, in which the Pliocene climate was already somewhat cooler.

The Pliocene, which lasted from 5.3 to 2.6 million years ago, was the last relatively warm period before the onset of the Pleistocene [‘age of ice ages’]. In the late Pliocene sea levels were on average about 22 meters* higher than today, the Rutgers team has calculated examining benthic foraminiferal fossils collected in Virginia, New Zealand and the North Pacific Ocean – let’s call that decent geographical coverage.

[*) The 68 percent confidence interval (‘likely’) lies between 17-27 meters sea level rise – extremely likely, 95%, between 12-32 meters.]

According to the researchers it is most probable that in the Pliocene both the Greenland Ice Sheet and the West Antarctic Ice Sheet were completely gone – and next to that the East Antarctic Ice Sheet would have to have been smaller than it is today.

Although the Pliocene climate comparison does tell us something about ice sheet climate sensitivity, it does not tell us very much about the speed at which such a large sea level rise could occur. The scientists keep that in a margin of ‘between centuries and several thousands of years’.

In the portion of the (late) Pliocene climate that the new study has focussed on (3.2-2.7 Ma) the atmospheric CO2 concentrations were around as high as they are today – and temperatures were about 2 degrees warmer.

[That makes more sense than you may think. If we stopped emitting carbon per tomorrow and halted the rise in CO2 concentrations the world’s measured temperature rise would likely double still, due to ocean climate inertia mostly.]

According to Kenneth Miller, lead author and Earth Science professor at Rutgers University, “the natural state of the earth with present carbon dioxide levels is one with sea levels about 20 meters higher than at present.”

From West to Greenland to East Antarctica

We sort of knew we had that coming, didn’t we? The Eemian was just 0.8 degrees warmer, while Eemian sea levels were 8-8.5 meters higher. Probably the biggest contributers to the Eemian sea level rise was the West Antarctic Ice Sheet.

[*) And for instance the MIS11 interglacial was hardly any warmer and had sea levels 6-13 meters higher than today’s world.]

We also just learned it probably takes just 1.6 degrees warming to completely melt the Greenland Ice Sheet. That’s a definite 5 meters sea level rise, already in the pocket. And as a finishing touch there is of course thermal expansion of sea water – which leads to about one meter sea level rise for every one degree of ocean warming – so that is two more.

The East Antarctic Ice Sheet is really quite stable, multiple studies have shown. But then again it is so big, that any melting along its marine boundaries would easily contribute significant additional amounts of sea level rise.

In fact the new study is merely reaffirming science and therefore shouldn’t shock anyone who’s paid good attention during paleoclimate class. A 2009 publication in Phylosophical Transactions of the Royal Society A for instance estimated Pliocene sea levels were about 25 meters higher than today.

Further Pliocene climate characteristics
It is better to compare, as the researchers have indeed done, the late Pliocene climate system to our own, than to try and do that with the early Pliocene, some 2.5 million years before. That is because on a timescale of several millions of years continents and oceans may not move much, but plate tectonics can still have rather large climatic impacts. The most notable thing to occur was the formation of the Panama Isthmus 3 million years ago, connecting North and South America, but separating the Pacific and the Atlantic Ocean. In a couple of articles we’ve paid attention to the ecological importance of isthmuses, and breaking these, by digging canals, like the Suez Canal. There are however also large climatic implications. After it closed, approximately 3 million years ago, ocean currents changed considerably, with for instance an increasing salinity difference between the Pacific and Atlantic Ocean.

As another consequence the downwelling in the North Atlantic increased and the entire Meridional Overturning Circulation (MOC) got to a higher gear.

This may also have had large implications for temperature distribution within Earth’s climate. Without a strong interconnected system of ocean currents, most of the planet’s heat exchange takes place through the general circulation of the atmosphere. This leads to warmer tropics and better isolated, so colder poles. Therefore ice sheet sensitivity to global temperature could have been quite different in the early Pliocene, compared to today’s world, with stronger ocean circulation. Typical temperature reconstructions for the late Pliocene however [see one at the top of this story – 3.3-3.0 Ma] already show an Earth in which a warmer climatic state is indeed [through for instance ice albedo feedbacks] relatively strong around the poles, and (on average) weaker around the equator, exactly the pattern that is monitored under the current climate warming.

[The reason the Pliocene ended and made way for the Pleistocene increased polar glaciations could paradoxically also lie in this same strengthening of the MOC. Increased ocean circulation is good for marine biomass production and carbon absorption in general. As a consequence atmospheric CO2 levels dropped, after which the Pleistocene started. So when you transport enormous amounts of warm tropical waters to the poles for about 400,000 years, you end up with ice ages, which after a while may shut down the MOC again, further increasing the polar cooling, as for instance happened at the Younger Dryas. Meanwhile though carbon concentrations rise steadily, again ending the ice age. Why is it Mr Milankovitch had to ruin our nice little all-explaining climate theory?

© Rolf Schuttenhelm | www.bitsofscience.org

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