What is a reasonable forecast of sea level in 2100?

Question

The IPCC 2013 Physical Science Basis Report says 0.4m - 0.6m. That is somewhat dated and now http://www.atmos-chem-phys-discuss.net/15/20059/2015/acpd-15-20059-2015.html seem to have contested it. Is it safe for local governments to plan on the basis of 0.4 - 0.6 meters, or is there another value that would be better?

Their abstract is summarized below for reference.

There is evidence of ice melt, sea level rise to +5–9 m, 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.

Answer 1

An important aspect of this is location. Sea-level rise will not be exactly uniform across the globe and tides/storms will cause a lot of regional variability in terms of coastal change. Local governments should not necessarily be planning for an average global sea-level rise but instead develop a regional understanding of the coastal effects they will likely experience. For instance, see California's recent CALIFORNIA COASTAL COMMISSION SEA LEVEL RISE POLICY GUIDANCE here: http://documents.coastal.ca.gov/assets/slr/guidance/July2015_Full_RecFinal.pdf which says

Sea level at the San Francisco tide gauge has risen 8 in (20 cm) over the past century, and the National Research Council (NRC) projects that by Year 2100, sea level in California may rise by 4 to 56 in (10 to 143 cm) for areas north of Cape Mendocino and 17 to 66 in (42 to 167 cm) for areas south of Cape Mendocino (NRC 2012). While the California coast regularly experiences erosion, flooding, and significant storm events, sea level rise will exacerbate these natural forces, leading to significant social, environmental, and economic impacts.

The estimate of 0.6 meters has increased as recent research shows Greenland's melt rate is a bit faster than previously estimated and ocean warming, which causes expansion, is also happening more rapidly.

Below is some useful information from the 2014 US National Climate Assessment ( http://nca2014.globalchange.gov/report/our-changing-climate/sea-level-rise ).

Global sea level has risen by about 8 inches since reliable record keeping began in 1880. It is projected to rise another 1 to 4 feet by 2100.

Projecting future rates of sea level rise is challenging. Even the most sophisticated climate models, which explicitly represent Earth’s physical processes, cannot simulate rapid changes in ice sheet dynamics, and thus are likely to underestimate future sea level rise. In recent years, “semi-empirical” methods have been developed to project future rates of sea level rise based on a simple statistical relationship between past rates of globally averaged temperature change and sea level rise. These models suggest a range of additional sea level rise from about 2 feet to as much as 6 feet by 2100, depending on emissions scenario.

Recent projections show that for even the lowest emissions scenarios, thermal expansion of ocean waters and the melting of small mountain glaciers will result in 11 inches of sea level rise by 2100, even without any contribution from the ice sheets in Greenland and Antarctica. This suggests that about 1 foot of global sea level rise by 2100 is probably a realistic low end. On the high end, recent work suggests that 4 feet is plausible.,,,, In the context of risk-based analysis, some decision makers may wish to use a wider range of scenarios, from 8 inches to 6.6 feet by 2100., In particular, the high end of these scenarios may be useful for decision makers with a low tolerance for risk

In the next several decades, storm surges and high tides could combine with sea level rise and land subsidence to further increase flooding in many of these regions. Sea level rise will not stop in 2100 because the oceans take a very long time to respond to warmer conditions at the Earth’s surface. Ocean waters will therefore continue to warm and sea level will continue to rise for many centuries at rates equal to or higher than that of the current century

The discussion paper, which is still undergoing peer-review at the time of this post, brings into question the semi-empirical methods used to calculate the rate of sea-level rise on the decade-scale. As an alternative, they use paleo-climate data and doubling times to make their own approximation, which predicts faster sea level rise. Let us be clear, though, the amount of possible net sea level rise over the next few hundred years or so is not really in question. The timing of how fast the sea will rise over the coming century is what is less certain. So, a coastal community doing long term planning could consider sea-level rise even further in the future. Most of Greenland could melt over the next few hundred years, which would contribute nearly 6 meters of sea level rise. If we are strictly talking about the year 2100, though, that is a difficult forecast to make. For planning purposes, it would be wise to assume at least 1 meter of sea level rise by 2100, with definite continued increases.