Estimates for higher sea-level in coming decades are one of the major “take home” messages from the newly released IPCC report.
Projected sea-level rise had been a particularly controversial aspect of the 2007 IPCC Fourth Assessment Report, with many scientists decrying the absence of modeling of dynamic ice sheet movements and arguing that models had underestimated historical sea-level rise. As a result, many had expected treatment of sea-level rise to be the focus of much attention on the new report, and that has been the case.
Understanding why sea-level rise now is forecast to be greater, and why substantial uncertainties remain, requires examining the different causes of sea-level rise and the different ways of estimating future sea-level rise using both physical models and evidence from Earth’s geologic past.
Global sea levels have increased fairly steadily over the past 130 years, with some acceleration evident over the past few decades. The figure below shows observations both from tide gauges (from 1880 to 2009) and satellites (1993 to 2013), with measurement uncertainty reflecting incomplete coverage of tide gauges shown in grey.
Revised Scenarios Complicate Direct Comparisons
Direct comparisons between the numbers from the IPCC 2007 Fourth Assessment Report (AR4) and the newly released Fifth Assessment Report (AR5) are difficult, because underlying scenarios have been significantly revised. Projecting future warming and also sea-level rise depends primarily on two factors: climate sensitivity (how much warming from a given amount of atmospheric greenhouse gas) and emission scenarios (how much greenhouse gases will be emitted).
The new IPCC report does not significantly change estimates of climate sensitivity, but IPCC created new emission scenarios. The chart below shows a comparison of temperature and sea-level rise projections from the scenarios that are most similar. But because the scenarios are not identical, some of the differences relate to the scenarios themselves rather than to any differences in model physics or climate sensitivity.
Projected warming is similar in AR4 and AR5, but projected sea-level rise is notably larger in the new AR5 report. This increase results both from incorporation of more rapid ice sheet dynamics and from improved modeling of thermal expansion and ice melt. The former had been excluded from consideration in AR4: IPCC maintained in that report that “Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude.”
Impacts of Different Causes of Sea-Level Rise
Sea-level rise is caused by a number of different factors, including thermal expansion of water, melting of glaciers and ice sheets, and changes in ice sheet flows (rapid dynamics). Somewhat surprisingly, thermal expansion tends to be a larger contributor than direct melting. As water warms up, it expands, and as the oceans absorb extra heat as a result of the accumulation of greenhouse gases in the atmosphere, sea level will rise.
Glacial melt makes up the second largest contributor to sea-level rise, though it is somewhat limited in the long run by the relatively small total mass of glaciers in the world. Melting of the Greenland and Antarctic ice sheets is projected to be relatively small over the next century, though melting of those ice sheets could become the largest contributor to sea-level rise a few centuries from now if warming persists. Finally, rapid dynamic change in Greenland and Antarctic ice sheets also is expected to contribute a relatively modest amount to the total sea-level rise over the next century. The projected contribution to sea-level rise of each factor by 2100 is shown for a particular future scenario (Representative Concentration Pathways or RCP6.0) in the figure below.
Different Places, Different Amounts of Sea-Level Rise
One often-overlooked facet of sea-level rise is that it will not be the same everywhere. Some parts of the ocean are projected to rise much faster than others. For example, sea-level rise off metropolitan New York City is expected to be about 50 percent more than that off San Francisco. Some of this difference is the result of the rising or subsidence of different land areas, and some the result of gravitational or ocean dynamics. The figure below from the new IPCC report shows the best estimate for 2081-2100 sea level compared to those from 1986-2005 for the four major RCP scenarios for different areas of the world.
These geographic differences in sea-level rise can prove to be highly important. While slow increases in sea level may allow countries to adapt by constructing sea walls or relocating homes and other property, the real damage occurs in areas of the world prone to storm surges. In New York City, for example, storm surges like those associated with Hurricane Sandy would be even more disastrous in a world with a few additional feet of sea-level rise.
Latest Sea-Level Rise Estimates Still Too Conservative?
Despite the higher sea-level rise projections in the latest IPCC report and improved ability of models to reproduce historical rates of sea-level rise, some respected scientists maintain still that even the new AR5 numbers are too low.
Their argument primarily boils down to two different approaches: process models, which use climate models and physics to estimate sea-level rise, and semi-empirical models, which include evidence of past sea-level rise (mostly during the last ice age) to help determine the relationship between temperatures and sea level. Process models tend to produce lower estimates, empirical models higher ones.
The recently released IPCC Working Group I report discounts empirical models, saying that “Many semi-empirical model projections of global mean sea-level rise are higher than process-based model projections (up to about twice as large), but there is no consensus in the scientific community about their reliability and there is thus low confidence in their projections.” Researchers like Potsdam University Professor Stefan Rahmstorf, who have worked with semi-empirical models, argue that a wholesale discounting of these approaches is erroneous, and represents an overly conservative approach from the IPCC.
For now at least, it is still unclear just what exactly future sea-level rise will be, as uncertainty ranges remain large. At the same time, it is clear that the best estimates of future sea-level rise are considerably higher now than they were just five years ago. The level of sea-level rise by 2100 will mostly depend on which future emissions scenario societies follow, and it’s clear that significant efforts to reduce future emissions, coupled with well thought-out adaptation efforts, can significantly reduce future sea-level rise and its damaging impacts.