What is not new

Ultimately sea levels will rise several feet, given the present levels of CO2 in the atmosphere. We already knew this by examining paleo data, and finding periods in the past with similar surface temperatures and/or similar atmospheric CO2 levels as today.

I put a graphic from a paper by Gavin Foster and Eelco Rohling at the top of the post. It does a good job of summarizing the paleo data.

If we keep pumping CO2 into the atmosphere at current, or even somewhat reduced, levels for a few more decades, the ultimate increase in sea levels will be significant. Find the 400–500 ppm CO2 range on the map and notice that the average sea level rise in times past, indicated by the horizontal orange-reddish line, is 14 meters.

Let me rephrase that to make it clear. We have already caused something like 14 meters of sea level rise. Like the horrifically sad words uttered by a movie or TV character who has received a fatal wound and turns to the killer, uttering "You've killed me" (then they die), we've done this. It is just going to take some time to play out. But it will play out.

A conservative estimate is that likely sea levels will rise 8 meters or more, quite possibly considerably more. But generally, people who talk about sea level tend to suggest that this will take centuries. Part of the reason for that is that it takes a long(ish) time for the added CO2 to heat up the surface, then it takes a while for that heat to melt the ice sheets. However, there is no firm reason to put a time frame on this melting.

A new paper that is making a great deal of news, and that is still in peer review, suggests that the time frame may be shorter than man have suggested. We may see several meters of sea level rise during the lifetime of most people living today.

What is not known

We don’t really how long this will take. Looking at the paleo record, we are lucky to get two data points showing different ancient sea levels that are less than a thousand years apart. There are a few moments during the end of the last glaciation where we have data points several centuries apart during which sea levels went up several meters. We don't have a good estimate for the maximum rate at which polar ice caps and other ice can melt.

The current situation is, notably, very different from those periods of rapid sea level rise. The amount of CO2 in the atmosphere is approximately double the Pleistocene average, and the rate at which CO2 levels and temperatures have gone up has not been seen in tens of millions of years. Whatever rate of sea level rise over the last several tens of thousands of years must be regarded as a minimum, perhaps a very low minimum.

What is new

The new paper argues for more sea level rise, ultimately, than many others have suggested, but it is still within the range of what we had already guessed from the paleo record. Most current research on the rate of glacial melting show relatively slow levels compared to what the new paper suggests. In particular, the new paper suggests that this is wrong, and that we may see three meters of sea level rise over the next fifty years.

The paper is very complex and covers a lot of ground that I will not attempt to address here. The tl;dr is that the researchers model the current melting of ice, and finds that the rate is accelerating over time. This means that current rates are a gross underestimate of the rate of sea level rise.

Here are two interviews with Michael Mann on the new work.

See also this by Joe Romm. Climate Crocks has Hansen, lead author, on CNN, here. See also this explainer by the paper's first author.

Mann talks about some of the effects of sea level rise, including global effects. We are already seeing food prices being affected now and then by climate catastrophes. Consider the fact that much of the rice grown in southeast Asia is grown on land that will be inundated by this sea level rise. This applies to the US as well. This combined with increased drought in places that are not flooding, and social unrest such as occurred in Syria when crops fail – causing further agriculture in those areas to simply stop happening – will cause a major food crisis in the near future. Our children and grandchildren will be hungry, at war, living in a post-civilization world. That is the world those who deny climate science and stand in the way of taking action are causing.

Here is the abstract of the paper:

There is evidence of ice melt, sea level rise to +5–9 meters, and extreme storms in the prior interglacial period that was less than 1°C warmer than today. Human-made climate forcing is stronger and more rapid than paleo forcings, but much can be learned by combining insights from paleoclimate, climate modeling, and on-going observations. We argue that ice sheets in contact with the ocean are vulnerable to non-linear disintegration in response to ocean warming, and we posit that ice sheet mass loss can be approximated by a doubling time up to sea level rise of at least several meters. Doubling times of 10, 20 or 40 years yield sea level rise of several meters in 50, 100 or 200 years. Paleoclimate data reveal that subsurface ocean warming causes ice shelf melt and ice sheet discharge. Our climate model exposes amplifying feedbacks in the Southern Ocean that slow Antarctic bottom water formation and increase ocean temperature near ice shelf grounding lines, while cooling the surface ocean and increasing sea ice cover and water column stability. Ocean surface cooling, in the North Atlantic as well as the Southern Ocean, increases tropospheric horizontal temperature gradients, eddy kinetic energy and baroclinicity, which drive more powerful storms. We focus attention on the Southern Ocean’s role in affecting atmospheric CO2 amount, which in turn is a tight control knob on global climate. The millennial (500–2000 year) time scale of deep ocean ventilation affects the time scale for natural CO2 change, thus the time scale for paleo global climate, ice sheet and sea level changes. This millennial carbon cycle time scale should not be misinterpreted as the ice sheet time scale for response to a rapid human-made climate forcing. Recent ice sheet melt rates have a doubling time near the lower end of the 10–40 year range. We conclude that 2°C global warming above the preindustrial level, which would spur more ice shelf melt, is highly dangerous. Earth’s energy imbalance, which must be eliminated to stabilize climate, provides a crucial metric.

And here is a key graphic:

The reference for the paper: Ice Melt, Sea Level Rise and Superstorms: Evidence from Paleoclimate Data, Climate Modeling, and Modern Observations that 2°C Global Warming is Highly Dangerous, by James Hansen, Makiko Sato, Paul Hearty, Reto Ruedy, Maxwell Kelley, Valerie Masson-Delmotte, Gary Russell, George Tselioudis, Junji Cao, Eric Rignot, Isabella Velicogna, Evgeniya Kandiano, Karina von Schuckmann, Pushker Kharecha, Allegra N. Legrande, Michael Bauer, Kwak-Wai Lo.