New Research On The Rising Sea

Human caused greenhouse gas pollution has locked us into a situation where the global sea level will rise, at an unknown rate, high enough to inundate most major coastal cities and vast areas of agricultural land in low lying countries, and wipe out thousands of islands. Entire countries (small, low lying ones, and pacific ocean nations) will either disappear entirely or be made very small. Even as we head towards a likely limit in global food production in relation to increasing demand, large productive agricultural areas will be destroyed. As far as I can tell, there is nothing to stop this from happening, though reducing our greenhouse gas pollution to zero over the next several decades may prevent the global ocean from rising to its absolutely maximum amount.

So sea level rise is important.

The surface of the Earth comes in two forms: Ocean bottom and continent. They are totally different geologically, with the ocean bottom consisting of relatively heavy basaltic rock formed at the margins between spreading plates, and continents of lighter rock, generally formed from below.

The global ocean sits mainly on the oceanic plates, but at its edges (except in a very few special locations), it rests against those continents. Over time the sea rises and falls. When the sea is at its lowest point, with a good amount of its volume reduced because it is trapped in glacial ice, most of the continents are exposed. When the sea is at its highest point, vast areas of the continental margins are inundated. At present, the ocean is pretty high, covering much of the continental margin that it ever covers, but there is room to grow, with large areas of the coastline subject to future inundation.

Rising surface temperatures caused directly or indirectly by human release of greenhouse gas pollution melt glaciers and warm the ocean, both of which are causing the global sea level to rise. This is a long and complicated process. We add greenhouse gas, mainly CO2, to the atmosphere, and this causes warming, enhanced by various positive feedbacks that either cause an increase of additional greenhouse gases such as water vapor, methane, and more CO2, or reduce the ability of certain natural systems to absorb these gasses. The greenhouse gas causes warming, which causes more greenhouse gas, which causes more warming. Meanwhile, most of this extra heat is actually trapped in the ocean where it only contributes a little to melting glaciers, but does contribute to expanding the volume of the ocean. The ultimate amount of heating, and the ultimate amount of sea level rise, takes a long time to be realized, and the rate of this change is only roughly estimated.

What we have already done to the atmosphere will cause sea level rise to continue for a very long time, possibly many centuries, possibly thousands of years. We have increased the amount of CO2 in the atmosphere from the mid 200s parts per million (ppm) to 400ppm, and we expect that increase to continue for decades. Evidence from the past, through the science of paleoclimatology, tells us that when the atmosphere holds between 400ppm and 500ppm of CO2, the global sea level is many meters above the present level.

Understanding sea level change is therefore critically important to understanding the impacts of climate change. We can measure current sea level rise and assume that steady increase over time (even if it is a bit variable) is mostly caused by global warming, heating the ocean and melting glacial ice. But there are problems with these measurements and associated estimates. Recent research has shown that Antarctic, which holds most of the world's ice, is or could or will contribute a very large amount of water to the sea. But, other recent studies show that some of the expected reduction in glacial size might not be happening at the rate previously estimated. At the moment, sea level is rising at a certain rate, and some research explains a good amount of that increase from melting ice, but other research takes that melting ice out of the equation and leaves that portion of the sea level rise unexplained, for now.

Past sea level change (up or down), prior to the industrial revolution when we started releasing all this greenhouse gas pollution, should give us a baseline against which to assess modern day measurements, and is an essential part of the process of understanding this critically important system. But it is difficult to measure sea level, at present or in the past. We can measure the current position of the sea at a given part of the continental margin by just going there and measuring it. Sea level over recent decades, going back in some places a few centuries, can be estimated using tide gage records. We can sink cores (or trenches) in relatively protected areas (such as behind barrier islands) and find organic material that would have been formed just below the surface of the sea, measure its elevation and date it, to give an estimate of sea level in the past. We can put the tide gage data and the coring data together and get a rough estimate of sea level change.

But that estimate is not just rough, but almost useless, without a lot of careful further study. As the organic material representing older sea levels is buried by later organic material or other sediment, it tends to be compressed and lower in elevation. The study of this process is many decades old, and this can be adjusted for, but it is complicated. The actual sea level at a given point along the coast depends partly on how big the ocean is at the moment (obviously) but also on the position and strength of major currents. At present, and many times in the past, the North Atlantic ocean is bunched up way out at sea because of the movement of currents. This lowers the sea level along the coast in many areas. But if these currents either move or change in their strength, this effect changes, and the coastal sea level goes up or down independently of the global sea level.

Wherever there were large glaciers, the land has been pushed down by the weight of the ice. After the glaciers melt away, the land rebounds. Where this happens along the coast, estimating global sea level from local sea level becomes quite complicated. Meanwhile at the outer edge of the glacial mass, the land is actually pushed up to compensate for the depression caused by the massive glaciers. This is called "forebuldge." Forebuldge makes the sea level look lower than it should, until the forebuldge reduces and flattens out. Indeed, the rebound effects of enormous glaciers in Canada are still happening, changing the position of the shoreline of Hudson's Bay fast enough that cabins built on the shore a century ago are now a long walk from the sea.

This is all manageable, and people have been working on collecting these data and figuring out how to use it since the 1960s. But now, this week, what may be the first research project to put most of these data together to provide a pretty good estimate of sea level variation over the last 3,000 years, has been published.

The key result from this paper is the graph at the top of this post.

Robert E. Koppa, Andrew C. Kemp, Klaus Bittermann, Benjamin P. Horton, Jeffrey P. Donnelly, W. Roland Gehrels, Carling C. Hay,b,k, Jerry X. Mitrovica, Eric D. Morrow, and Stefan Rahmstorf's paper, "Temperature-driven global sea-level variability in the Common Era" (PNAS) does this:

We present the first, to our knowledge, estimate of global sea-level (GSL) change over the last ∼3,000 years that is based upon statistical synthesis of a global database of regional sea-level reconstructions. GSL varied by ∼±8 cm over the pre-Industrial Common Era, with a notable decline over 1000–1400 CE coinciding with ∼0.2 °C of global cooling. The 20th century rise was extremely likely faster than during any of the 27 previous centuries. Semiempirical modeling indicates that, without global warming, GSL in the 20th century very likely would have risen by between −3 cm and +7 cm, rather than the ∼14 cm observed. Semiempirical 21st century projections largely reconcile differences between Intergovernmental Panel on Climate Change projections and semiempirical models.

So now we have a much better idea of the nature of global sea level rise for a couple thousand years prior to human greenhouse gas pollution, and we have a firm demonstration of the effects of this pollution on sea level over the last century or so.

We are fortunate that one of the authors of this paper, Stefan Rahmstorf, is a blogger at Real Climate, where he wrote this post summarizing the original paper (though the original paper, linked to above, is pretty readable!).

Climate Central produced this graphic based on the paper:

Of this, Rahmstorf says, "The fact that the rise in the 20th century is so large is a logical physical consequence of man-made global warming. This is melting continental ice and thus adds extra water to the oceans. In addition, as the sea water warms up it expands."

How much will sea level rise by the end of the century?

In his post, Rahmstorf brings in a second study on sea level rise, also just published (see the RC post for more details). That research attempts to estimate the amount of sea level rise expectd by 2100. There are four separate studies, each using three different (RCP) assumptions about future human caused climate change, and each combination of study and model provides a range. In centimeters, the lowest numbers are around 25 (close to the amount that has already happened over the last century) and the highest numbers are around 130-150 (so, up to about five feet).

Rahmstorf appears to agree with my thinking on this, which is that these estimates don't account for catastrophic deterioration of ice sheets and subsequent increase in melting, if such a thing results from what appears to be increasing instability of some of those glacial features. For example, huge parts of the Antarctic ice sheet are in the form of vast glacial rivers pinned in place by a precarious "grounding" of ice on rock near the mouth of those rivers.

If that grounding falls apart, the entire river can start to march to the sea very quickly, establishing a new grounding line upstream. It is possible that such a new grounding line is way upstream. As all that ice falls into the sea, it would likely expose high vertical cliff that would then start producing ice bergs at a very high rate for many years. There may be other features currently deep under the ice that would be exposed, such as pre-melted water near warm spots. In other words, the drainage of meltwater will not be made less efficient by such a collapse, but rather, more efficient, regionally and for a certain period of time. The point is, the impact on the rate of glacial melt of such events is pretty much unknown and very difficult to estimate.

Rahmstorf notes, "The projections on the basis of very different data and models thus yield very similar results, which speaks for their robustness. With one important caveat, however: the possibility of ice sheet instability, which for many years has been hanging like a shadow over all sea-level projections. While we have a pretty good handle on melting at the surface of the ice, the physics of the sliding of ice into the ocean is not fully understood and may still bring surprises. I consider it possible that in this way the two big ice sheets may contribute more sea-level rise by 2100 than suggested by the upper end of the ranges estimated by Mengel et al. for the solid ice discharge, which is 15 cm from Greenland and 19 cm from Antarctica. (The biggest contributions to their 131 cm upper end are 52 cm from Greenland surface melt and 45 cm from thermal expansion of ocean water.)"

He backs this up by reference to other recent studies showing that ice sheets have in the past broken up at surprisingly high rates.

One and a half meters, or five feet, of sea level rise within the lifetime of those born today is possible. Half of that is extremely likely. Double that may even be a possibility. This is expected to continue for centuries, even millennia, or until all the ice melts, whichever comes first.

How many things in your life originate from some thing that happened in the past? The invention of agriculture (that happened many times from about 10,000 to 4,000 years ago), the invention of writing (again, multiple times, thousands of years ago), the modern western system of government and law (depending on where you live, the Magna Carta, the US Constitution) hundreds of years ago. If you are religious, it is likely that your religion's roots are thousands of years old. The establishment of property rights, water rights, all of that.

If human civilization exists, with some continuity with the present, 1,000 years from now, such a list will include the release of fossil carbon in the form of greenhouse gasses by the people of the 19th, 20th, and 21st centuries. That was the event that caused the sea to rise and engulf so much of the fertile land, causing a major (if possibly slow moving) exodus of most of the settled people of the world. In a thousand years, after we've either stopped using fossil fuel, or didn't but just used it all up, people will still be measuring for the rise of the sea that we are causing right now.

I don't think they will be thanking us.