“There are some recent modeling efforts that now show you could get a section of the Antarctic ice sheet, several meters worth of sea level rise, to go in a decade. We used to think it was centuries.” — Andrea Dutton Geochemist at the University of Florida.
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Recent reports out from Climate Central and supported by the work of experts show that a sea level rise of at least 6 meters could already be locked in. And as bad as that sounds, a six meter sea level rise from the warming already set in motion by high atmospheric greenhouse gas levels and likely to come from further human emissions could be a best-case or even unrealistic scenario.
To get an understanding as to why so much water may be heading toward the coastal cities of the world, enough water in a 6 meter rise to set off a mass migration of hundreds of millions away from the world’s coasts (just 1.1 meters is enough to flood out 150 million people), it helps to take a good, hard look at paleoclimate. In studying past, warmer, climate states, we can get an idea how much additional sea level rise might be in store. When looking at these past climates for comparison, the key readings to keep in mind are — temperature, greenhouse gas level, and related sea level.
A Question of Whether We Lock in Greenhouse Gas Levels Comparable to Past Climates
Starting with the current climate that is now being rapidly warmed by human fossil fuel burning, we find that this year peak monthly CO2 levels hit near 404 parts per million. It’s a value fast approaching the top of this key greenhouse gas’s range during the Pliocene around 3.5 million years ago. A time when temperatures were 2-3 degrees Celsius hotter and sea levels were between 25 and 75 feet higher than they are today.
(What Virginia Beach looks like after 6 meters of sea level rise. Notably, about everyone I knew as a child or who still lives in VB now is under water in this scenario. Image source: Climate Central.)
Looking at the climate situation in this way tends to elicit a bit of an ‘oh crap’ response. And it should. For all other things being equal, if CO2 levels were to remain so high over the course of a few Centuries, that’s where we’re headed. Toward a world with 2-3 C hotter temperature and 25 to 75 foot higher seas.
But the atmosphere of today is only a rough allegory to that of Pliocene times. In addition to CO2, our airs now host expanding volumes of other greenhouse gasses — exotic and common. A vast majority of which are emitted as a result of fossil fuel burning, extraction, and industrial processes. So to compare our atmosphere to that of the period around 3.5 million years ago and expect the same results with regards to temperature and sea level would be unrealistic. Current methane readings alone — in excess of 1800 parts per billion — now hit levels likely twice that of the Pliocene. And methane is a greenhouse gas with a global warming potential equal to 20 to 120 times that of CO2 over timescales relevant to current human civilization.
As a result of this additional accumulation of methane and other gasses, this year’s atmosphere is a closer allegory to past atmospheres containing an equivalent of about 484 parts per million CO2 (CO2e). Such times, occurring 15-25 million years ago, hosted sea levels that were more than 100 feet (and possibly as much as 200 feet) higher than today.
It is for this reason that we should view Climate Central’s recent and excellent report on sea level rise — based on Paleoclimate and predicting that at least 20 feet of sea level rise could already be locked in — with a bit of concern. At issue with the report are two factors. The first is that the study bases its findings on predicted temperature increases for the 21st Century only. A process established by IPCC-based studies in which it is assumed that 2 degrees Celsius warming over the course of this Century is, perhaps, the best possible target we can hit through a pretty rapid transition to a zero or near-zero carbon civilization. Implied in this IPCC approach is limiting global CO2 accumulation to 450 parts per million or less. A level that also implies a 530 to 550 parts per million CO2e when other gasses are added in unless all the methane overburden falls out due to its short atmospheric lifetime (about 8 years). A dicey assumption at best considering that at least some and possibly all of that overburden could be maintained by feedbacks now at play in the Arctic and in the world’s land and ocean systems.
(At six meters of sea level rise, Miami is completely submerged. Image source: Climate Central.)
In worse cases, we could see the methane overburden expand in the event that the Arctic carbon stores are less stable than we’d hoped. So while 450 parts per million CO2 might limit us to between 2 and 2.3 C warming this Century, 530 to 550 parts per million CO2e gets us to 2.2 to 2.9 C.
The second issue is that we are only looking at warming for the 21st Century. Due to the long term warming impact of CO2 and other greenhouse gasses on the climate system in total, each 1 C worth of warming this Century implies about 2 C worth of warming long term (ESS sensitivity). So hitting the 2 C target by 2100 gets you to 4 C after many Centuries. And hitting a 550 parts per million CO2e threshold means about 2.7 to 2.9 C 21st Century warming and 5.5 to 5.8 C long term warming. An upper range that is nearly enough to melt all the land ice on Earth and raise sea levels by nearly 240 feet.
How Fast Could Sea Levels Rise?
At least 6 meters indeed! In the 550 parts per million CO2e case, we have one of the better global human carbon emissions scenarios meeting with one of the somewhat more pessimistic Earth Systems response scenarios (but not the worst case) for an absolutely terribly catastrophic outcome. An outcome made even more terrifying by the fact that it is in the mid-to-low range of overall projected greenhouse gas forcings for this Century. In other words, 2 C warming this Century can start to look like a pretty bad outcome for the long haul and we’d probably best be trying to hit well below the implied 450 ppm CO2 target (as Hansen and others have warned). And to this point, we had better move very fast on emissions reductions, because the longer even current greenhouse gas levels are maintained the more likely we hit ice sheet destabilizations that push world ocean levels closer and closer to the Pliocene’s or Miocene’s swollen seas.
(Just 1 C worth of global warming from 22,000 years BP to 15,000 years BP was enough to set off rapid sea level rise during the end of the last ice age. We are fast approaching the 1 C warmer than 1880s thresholds now. Image source: Commons and Livescience.)
Which brings us to a final question hinted at in the header — how fast could sea levels rise if human forced warming approaches 2 C or more this Century? The modelling efforts Dutton hinted at shows that West Antarctica alone can contribute meters of sea level rise over the course of just decades. And going back to paleoclimate studies of the end of the last ice age we find hints that somewhere between 1 and 2 C worth of warming can trigger very large and rapid glacial outbursts (that then increased sea levels by as much as 16 feet per Century). Finally, recent glacier surveys from Antarctica to Greenland have found extensive and expanding destabilization. Efforts and evidence that imply the 39 inches of sea level rise predicted by IPCC this Century may be quite conservative, even under the better case emissions scenarios.