A lot of the focus of the Copenhagen negotiations has been on the expectations for what the climate will look like at the end of this century. It's a convenient milestone, and it gives people something that they can wrap their heads around, but it's a very arbitrary and unscientific time point, as some of the impacts of current carbon emissions may take centuries to fully register. This is especially true when it comes to sea level; current estimates of a rise of about a third of a meter by the end of the century seem manageable, but it's not as if the oceans will stabilize on January 1, 2100. A paper published in Nature on Wednesday now provides some sense of just how badly off that estimate may be, by showing that a period with similar temperatures saw ocean levels that were probably over six meters above the ones we currently experience.
The problem with picking an arbitrary time point, like the end of the century, is that there are significant time lags in geologic systems. When it comes to the climate, the oceans provide an enormous heat sink, so it takes many decades for the global temperature to equilibrate to a given level of greenhouse forcing. It can then take far longer for other systems to equilibrate to the new temperatures; for example, it might take decades more of turnover before a given ecosystem shifts north in response to warmer climes.
This is especially true for sea level changes. Some of that will come from the thermal expansion of the oceans, which will track the slow rise in their temperatures. But a significant portion may come from the melting of ice sheets, and it takes a lot of energy to melt ice. So, although it's important to understand the short-term trajectory of sea level rise, it's also essential to have some sense of where it will ultimately stop after the planet's climate stabilizes.
To that end, the recent paper suggests that the last interglacial (LIG)—the previous period of warming between ice ages—may provide a valuable model. Earth's orbital eccentricity during that period was more than twice what it has been during the current interglacial, which caused temperatures of roughly one or two degrees Celsius above present ones. Those would put it at the low end of current projections for likely temperature rises this century.
Data from a number of sites have suggested that sea levels were quite a bit higher during the LIG, but a variety of factors make individual, local readings poor proxies for the global oceans. Individual sites may be subject to different geological influences, and the melting of large ice sheets has a dynamic effect on ocean levels. For example, they exert a gravitational pull that draws water to them at the same time as their weight pushes the local land down. Melting them both allows the water to be more evenly distributed, and sets off a tectonic rebound that raises the land. All of this shifts enough mass around to alter the Earth's rotation.
As the authors of the paper put it, "Contrary to an analogy commonly taught in introductory classes, adding water from melting land ice to the ocean is not like pouring water into a bathtub."
In short, the new paper builds a better bathtub. Geological data on local uplift is incorporated in the model, as are dynamic gravitational and tectonic influences based on the changing ice sheets. The model was then fed a series of data, with varying temporal resolution, from a global collection of sites. These include data from erosion at former shorelines, areas where coral grew in shallow waters, deposits of oceanic foraminifera, etc. The model accommodates uncertainties in both the accuracy of the measurement and timing when generating the uncertainties in its results.
Running the model reveals a sea level peak at about 124,000 years ago, near the start of the period. But the startling figure is where the levels appear to have peaked: 7.2 meters above the present levels, with a 67 percent confidence interval of �1.3m. More generally, there's a 95 percent probability that they went over 6.6m above present levels at least once during the LIG, and a 67 percent chance that they exceeded 8m above present levels.
To see whether any of the specific data was throwing the model off, the authors performed runs of the model in which they excluded specific data sources, like coral or erosional features. In every case, the reconstructions peaked above six meters.
To get a sense of the time lag involved, they examined how fast ocean levels rose during this period. Their models only allowed resolution of these rates on the millennial scale, but the values simply aren't that startling: less than twice the current rate, which is in the neighborhood of 3mm a year.
The models also provide some indication of where all that water is coming from. At one point or another, both the Arctic and Antarctic ice sheets (primarily Greenland and the West Antarctic sheets) lost about 2.5m worth of ocean level. That would imply a substantial reduction compared to their current volumes.
Current estimates are that we've got 145 million people and about a trillion dollars worth of infrastructure within one meter of the present sea levels, which provides some sense of the scope of adjustment that might be required over the long term. The most important extension to this work will probably be incorporating better temporal data. The current data only allows the rate of rise to be resolved on millennial scales, so it's impossible to know whether we might have decades or centuries to adjust to the changes.
Nature, 2009. DOI: 10.1038/nature08686