Evidenced-based Sea Level Rise Projections Remain Low

Global Science Report is a feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

When it comes right down to it, the biggest potential threat from a warming climate is a large and rapid sea level rise. Everything else that a changing climate may bring we’ve seen before (or at least the likes of it), recovered from, and are better off for it (i.e., gained experience, learned lessons, developed new technologies, etc.). In fact, the more often extreme weather occurs, the more adaptive is our response (see for example, decreasing mortality in heat waves). So in that sense, climate change may hasten our adaptive response and reduce our overall vulnerability to it.

A large and rapid sea level rise is a bit of a different story—although perhaps not entirely so.

While we do have a large amount of infrastructure (e.g., big cities) in low-lying coastal regions, it is completely wrong to show them underwater in the future—a typical device used by climate activists. What will happen is that we will act to protect the most valued portions of that infrastructure, as shown in a recent report from leading experts (including from the U.S. Environmental Protection Agency) on sea level rise and response.

But, while targeted action will save our big cities, there is still a lot of real estate that will be lost if sea level rises a large amount in a short amount of time (say, by more than a meter [a little more than 3 feet] by the end of the 21st century).

We therefore keep a vigilant eye on sea level rise research. And what we’ve concluded is that sea level rise by the year 2100 is very likely to be quite modest, say about 15 inches—an amount that should allay concerns of a catastrophe. We’ve detailed literature in support of our conclusions here, there, and elsewhere.

This week, a new paper has come to our attention that further supports our synthesis.

In the journal Quaternary Science Reviews, researchers Nicolás Young and Jason Briner summarized the extant scientific literature on the size of the ice sheet covering Greenland during warm periods in the recent geologic past, with a special emphasis on the middle Holocene—a multi-millennial period centered some 3,000–4,000 years ago during which the temperatures across Greenland were about 1°–3°C higher than the 20th century average. They note this is similar to conditions projected to occur there by about the year 2100.

What Young and Briner found was that the size of the Greenland ice sheet—especially the best observed portions covering the west and southwestern parts of Greenland—during the mid-Holocene was smaller than it is today—but not by a whole lot. They wrote:

[W]e suggest that despite some degree of inland retreat, the West and Southwest [Greenland ice sheet] margin remained relatively stable and close to its current position through the Holocene thermal maximum.

The implication is that despite a period of warmer-than-present temperatures in Greenland lasting some 2,000 years, the ice sheet did not shrink to such an extent as to result in a whole lot of sea level rise. This is good news as to what to expect from future warming—the Greenland ice sheet seems pretty stable in the face of rising temperatures.

This is consistent with the remarkable findings of Dorthe Dahl-Jensen’s research team concerning the warmest era in the last 1.5 million years or so—the first 6,000 years of the last interglacial period, known as the Eemian.  It began 128,000 years ago. 

The ratio of two different isotopes of oxygen (18O/16O) in air trapped in ice provides an accurate measure of local temperature, and because snow compacts every year, it’s fairly straightforward to count backward, year-by-year, as one drills down through the Greenland icecap. Up until Dahl-Jensen’s report, no one had gotten completely through the Eemian.  And, up until then, it was thought that temperatures in those 6,000 years were some 2°–4°C warmer than in the current era. (Greenland’s temperatures were pretty flat during the 20th century.) Dahl-Jensen’s work shows that Greenland was an astounding 8° +/– 4° warmer! Over that 6,000 years, Greenland lost approximately a quarter of its ice, contributing to 2 meters of sea-level rise.

Young and Briner, along with Dahl-Jensen, provide strong evidence that Greenland’s ice will be disturbed very little by what humans are likely to do to the atmosphere. Let’s use the top-end of Young and Briner’s warming by 2100 (3°C) and jack it up to 5° for the next hundred years. Then, let’s make the plausible assumption that we haven’t a clue about society’s energy structure 200 years from now, so we’ll stop things there and let the warming damp back to 20th century levels in 500 years. The integrated heat applied to Greenland (we’ll provide gory details on request) works out to 1,500 degree-years.  What Young and Briner found was that the Holocene maximum provided, on average, 4,000 degree-years (2,000 years multiplied by 2°), over twice what humans can contribute. And Dahl-Jensen showed it took a whopping 36,000 degree-years to melt only a quarter of the ice there, 24 times what we can do. In other words, we can’t change the climate enough to ever cause a massive sea level rise from melting Greenland’s ice.

Young and Briner also find that the models tend to overdo the mid-Holocene ice sheet retreat. Examining a leading ice sheet model (described by Lecavalier and colleagues), Young and Briner conclude:

The modeled minimum ice sheet in Lecavalier et al. (2014) at 4 [thousand years ago] equates to a 0.16 [meter] sea-level contribution, but considering minimal inland retreat of the ice margin based on geological reconstructions, we suggest that this value may be a maximum estimate of the [Greenland ice sheet] contribution to sea level in the middle Holocene.

Lecavalier’s estimate of 0.16 meters equates to 6.3 inches—which Young and Briner think should represent the worst-case result of 2,000 years of projected end-of-the-century temperatures across Greenland. This also comports well with the estimates from the United Nations Intergovernmental Panel on Climate Change (IPCC), which, in its Fifth Assessment Report, projected the sea level rise from Greenland as being between 0.07 and 0.21 meters (2.8 to 8.3 inches) with a median value of 0.12 meters (4.7 inches) even under its highest greenhouse gas emission scenario.

Like we said, our view that future sea level rise will be modest is now firmly established by the scientific literature, contrary to nonscientific alarmist claims.

References:

Dahl-Jensen, D., et al., 2013.  Eemian interglacial reconstructed from a Greenland folded ice core.  Nature, 493, 489-494.

Lecavalier, B.S., et al., 2014. A model of Greenland ice sheet deglaciation constrained by observations of relative sea level and ice extent. Quaternary Science Reviews, 102, 54-84. DOI: 10.1016/jquascirev.2014.07.018.

Young, N.E., and J.P. Briner, 2015. Holocene evolution of the western Greenland Ice Sheet: Assessing geophysical ice-sheet models with geological reconstructions of ice-margin change. Quaternary Science Reviews, 114, 1-17, DOI: 10.1016/j.quascirev.2015.01.018