1.4: Impacts - sea levels

Some baseline scenarios for sea level rise

For more general reading on climate impacts I reccommend the IPCC technical summary, NAS report on climate, and a summary of the IPCC report on the oceans.Before we look at the sea level projections I just want to show some graphs that explain what the different scenarios it refers to are. RCP stands for Representative Concentration Pathway, with higher numbers being a higher emission pathway. The IPCC report looks mainly at these four RCP’s, and below are the PgC/yr, the ppm and the estimated temperature changes associated with these pathways.emissions and ppm

projected temperature changes

Okay so what do sea levels look like under these scenarios?

sea level scenarios

And here is a map of the expected sea level rise globally in those same scenarios:

local sea level changes

According to Michael Mann’s course, at 1 meter global average sea level rise, 145 million people would be directly affected, and there would be on the order of a trillion dollars of damage. From a recent paper: “We estimate one billion people now occupy land less than 10 m above current high tide lines, including 250 M below 1 m”. Note that the rise at certain coastal areas will be higher than the global average. This paper provides some specifics:

“By 2040, with a 2 °C warming under the RCP8.5 scenario, more than 90% of coastal areas will experience sea level rise exceeding the global estimate of 0.2 m, with up to 0.4 m expected along the Atlantic coast of North America and Norway. With a 5 °C rise by 2100, sea level will rise rapidly, reaching 0.9 m (median), and 80% of the coastline will exceed the global sea level rise at the 95th percentile upper limit of 1.8 m. Under RCP8.5, by 2100, New York may expect rises of 1.09 m, Guangzhou may expect rises of 0.91 m, and Lagos may expect rises of 0.90 m, with the 95th percentile upper limit of 2.24 m, 1.93 m, and 1.92 m, respectively.”

Implications of potential tipping points in ice melt

The above plots of mean/expected sea level rise do not strongly reflect specific, apparently-much-less-likely scenarios in which we would see catastrophic tipping-point collapse of the Arctic or Antarctic ice.

To be up front, I have limited understanding of how likely this is to happen and what the real implications would be. But to try to understand this, it looks helpful to separate major sources of sea level rise due to ice melt into at least four groups:

  • Sea ice (Arctic and Antarctic)
  • Greenland ice sheet
  • West Antarctic ice sheet
  • East Antarctic ice sheet
Ideally ice melt would go slowly, e.g., a century or more, giving some time to respond with things like negative emissions technologies (next post) and perhaps some temperature stabilization via solar geo-engineering (next next post). But from my understanding, it is at least conceivable that it could happen in just a couple of decades, for instance if a series of major tipping points due to in positive feedbacks are crossed, e.g., methane release en masse from the permafrost

Now, a question for each of these is whether they can be reversible, if we brought back the temperature via removal of CO2 or via solar radiation management.

This seems to be a concept used frequently in popular discussions of this topic, i.e., people ask “is it already too late to stop sea level rise by limiting emissionsThere is both a weak and a strong form of the question of irreversibility of harm due to emissions. The weak form is asking if there is a self-reinforcing process of warming and sea ice melt that emissions reduction would not be enough to stop. For instance, as ice melts, ocean albedo lowers, melting more ice. This means we may already have locked in ongoing sea level rises with our current emissions and (non)actions.

The strong form of this question asks could we ever get the ice formations back to what they once were even with negative emissions & or solar radiation management?

Sea ice

We are already seeing significant loss of summer sea icein the Arctic. In Antarctica, the sea ice normally doesn’t survive in summer, but loss of the winter sea ice could significantly disrupt ocean circulation.

Another paper called “The reversibility of sea ice loss in a state-of-the-art climate model“, concludes similarly suggests its model implies that “future sea ice loss will occur only insofar as global warming continues, and may be fully reversible”A 2012 modeling paper called “How reversible is sea ice loss” looks at this question and the authors conclude that, “Against global mean temperature, Arctic sea ice area is reversible, while the Antarctic sea ice shows some asymmetric behavior – its rate of change slower, with falling temperatures, than its rate of change with rising temperatures.We find no irreversible behaviour in the sea ice cover”. In other words this modeling paper suggests, yes, if you bring temperatures back down there is no reason to expect you couldn’t get the ice formations back.

So it seems like sea ice may not have the irreversible tipping points we are worried about here. Although it should be noted that the concept of ‘reversibility’ discussed above unfolds over multiple centuries! So maybe the sea ice cover, as such, is reversible with temperature — albeit with a delay: their notion of reversibility is defined in a simulation that unfolds over multiple centuries!


If Greenland melted in full, sea level would rise around 7 meters.

There is another kind of ice formation though, one that is formed above land, like Greenland, West Antarctica or East Antarctica. This seems much more worrying with a paper in PNAS stating:

“We also discuss why, in contrast to Arctic summer sea ice, a tipping point is more likely to exist for the loss of the Greenland ice sheet and the West Antarctic ice sheet.”

The PNAS paper goes on to say that : “…there is some evidence from general circulation models that the removal of the Greenland ice sheet would be irreversible, even if climate were to return to preindustrial conditions (54), which increases the likelihood of a tipping point to exist. This finding, however, is disputed by others (55), indicating the as-of-yet very large uncertainty in the modeling of ice-sheet behavior in a warmer climate (56).”

Especially worryingly they also note that: “In this context, it should be noted that the local warming needed to slowly melt the Greenland ice sheet is estimated by some authors to be as low as 2.7° C warming above preindustrial temperatures (64, 65). This amount of local warming is likely to be reached for a global warming of less than 2°C (39).”

There is some slightly more hopeful, or terrifying, research suggesting that solar geoengineering could be very useful in slowing or preventing the collapse of Greenland’s ice sheet.A fascinating paper suggests a different primary driver of melting of Greenland, however:

“In an analysis of recent changes over Greenland, Hofer et al. (2017) found that the substantial reduction in cloud cover over Greenland in the past 2 decades is the likeliest cause for the accelerated mass loss from the ice sheet over this period. To arrive at this result they simply calculated how much melt would result from the change in downward surface shortwave energy received over the melt season as a result of the change in cloud cover and compared this against the other contributions to melt and accumulation. They find that the ∼ 10 % reduction in summer cloud cover over Greenland in the past 2 decades led to a ∼ 4000 Gt loss of mass making it the dominant driver of surface mass balance change in this period.

So just as a recap, it looks very possible that the melting of Greenland’s ice formations could be irreversible and that what is looking more and more likely to be a temperature path we go down (2℃) would likely be enough to trigger high enough local warming for this to happen.

West Antartic

If West Antarctica melted in full, we’d be seeing several meters of sea level rise.

West Antarctica looks, if anything, worse. Notably, there is at least the possibility of bistable dynamics or very strong hysteresis whereby Antarctic melting, once it got underway, could not be reversed even if we brought the temperature and atmospheric CO2 back to current levels. It is apparently only recently that quantitative modeling of the ice dynamics has made major strides.

In particular, the West Antarctic Ice Sheet is vulnerable to a so-called Marine Ice Sheet Instability, which some suggest may already be occurring, and which depends on the specific inward-sloping geometry of the land underlying that ice formation. It is possible that this dynamic is already starting to happen.

See this paper for moreConceivably, though not by any means with certainty from what I can tell, even solar radiation management and negative emissions, which would artificially bring the temperature back down might not be able to stop the melting when it is driven by such unstable undersea processes

All is not completely lost in that case, to be sure. There are at least some proposed local glacier management schemes that could directly try to limit the ice loss, including buttressing the ice sheet underwater, as well as spraying snow on top. In short, given enough urgency and time humanity will likely find a way, like Dyson’s sea kites, that could actively reduce sea levels and counteract sea level rise regardless of its exact cause.

But seriously, we want to avoid major tipping points in the climate system. If we cross major tipping points in sea or land ice melt, it will be very challenging to deal with at best.

Let me know if you have something really good on thisNone of this is to say exactly how likely or unlikely it is that such tipping points would be reached. I simply don’t know, and I haven’t found much online to ground an answer in a specific temperature or time frame bbb ut there is some real possibility that self-accelerating processes of ice melt in Greenland and West Antarctica are already happening

East Antartic

If all of Antarctica, including the comparatively-likely-to-be-stable East Antarctica, melted we’d be seeing perhaps >60 meters of sea level rise.

To summarize:

  • Sea ice: hopefully fairly quickly reversible
  • Greenland melt: unclear, maybe reversible by solar radiation management or aggressive negative emissions
  • West Antarctica: may not be reversible at all except by things like active snow dumping via altering air flows, or by preventing the melt through things like direct buttressing of the ice sheet
  • East Antarctica: let’s hope it doesn’t melt… fortunately it seems unlikely to do so