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Sea level projections

21st Century

• Introduction
• Projections for the 21^st Century
• Time series for the 21^st Century

Introduction

Sea-level rise is a central element in detecting, understanding, attributing and correctly projecting climate change. During the 20^th century, the oceans have stored well over 80 per cent of the heat that has warmed the earth. The associated thermal expansion of the oceans, together with changes in glaciers and ice caps, will likely dominate 21^st century sea level rise. However, there is increasing concern that the ice sheet contribution may be larger than previously estimated, and on longer time scales, the ice sheets of Greenland and Antarctica have the largest potential to contribute to significant changes in sea level.

Projections for the 21^st Century

During the 21^st century, sea level will continue to rise due to warming from both past (20^th century and earlier) and 21^st century greenhouse gas emissions. The most robust projections of 21^st century sea-level rise are the Assessments of the Intergovernmental Panel on Climate Change (IPCC) of 2001 and 2007.

Projected sea-level rise for the 21^st century: The projected range of global averaged sea-level rise from the IPCC 2001 Assessment Report (Church et al. 2001) for the period 1990 to 2100 is shown by the lines and shading. The central dark shading is an average of models for the range of SRES greenhouse gas emission scenarios. The light shading is the range for all models and all SRES scenarios and the outer bold lines include an allowance for land-ice uncertainty.

The updated AR4 IPCC projections of 2007 for the SRES scenarios (Meehl et al. 2007) are shown by the bars plotted at 2095. The magenta (lighter) bar is the range of model projections (90% confidence limits). Ocean thermal expansion and melting of glaciers and ice caps are the largest contribution to this range. The red bar is a potential but poorly quantified additional contribution from a dynamic response of the Greenland and Antarctic ice sheets to global warming. Note that the IPCC AR4 states that "larger values cannot be excluded, but understanding of these effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea-level rise."

Estimates of the ocean thermal expansion are made with coupled climate models for the range of SRES greenhouse gas emission scenarios. Recent estimates indicate that non-polar glaciers and ice caps may contain only enough water to raise sea level by 15 to 37 centimetres (Lemke et al. 2007). Melting of glaciers at lower altitude and latitude in a warming climate will eventually result in significant reduction of the sizes of the glaciers and thus a reduction in their contribution to the rate of sea-level rise. The largest contribution is from large glaciers in regions with heavy precipitation, such as the coastal mountains around the Gulf of Alaska, or Patagonia and Tierra del Fuego in South America. Many of these glaciers flow into the sea or large lakes and melt quickly because the ice is close to melting temperature.

For Greenland, both glacier calving and surface melting contribute to mass loss. Over the last few decades surface melting has increased and now dominates over increased snowfall, leading to a positive contribution to sea level during the 21^st century (Lemke et al. 2007).

For the majority of Antarctica, present and projected surface temperatures during the 21^st century are too cold for significant melting to occur and precipitation is balanced by glacier flow into the ocean. In climate change scenarios for the 21^st century, climate models project an increase in snowfall, resulting in increased storage of ice in Antarctica, partially offsetting other contributions to sea-level rise. However, an increase in precipitation has not been observed to date (Lemke et al. 2007).

In addition to these surface processes, there are suggestions of a potential dynamical response (sliding of the outlet glaciers over the bedrock) of the Greenland and Antarctic ice sheets. In Greenland, there was a significant increase in the flow rate of many of the outlet glaciers during the early 21^st century. One potential reason for this is increasing surface melt making its way to the base of the glaciers, lubricating their flow over the bed rock, consistent with increased glacier flow rates. Another effect which may be becoming more important is that, as the ice shelves around Antarctica and Greenland melt or break up (e.g. Larsen B) they allow the glaciers behind them to flow faster, leading to increased flow into the ocean.

Time Series of Sea-Level Projections for the 21^st Century

Unlike the IPCC TAR (2001), the AR4 (2007) does not provide time series of sea-level projections through the 21^st century, but does provide maximum and minimum projections for the decade 2090-2099 (here termed '2095') and for the potential dynamic response of the Greenland and Antarctic Ice Sheets. For 2095, the TAR and AR4 projections agree well at the upper limit and but not so well at the lower limit, as shown on the above figure. To estimate a time series of the maximum and minimum IPCC AR4 projections, Hunter (submitted) scaled the equivalent TAR projections (from Table II.5 of the IPCC TAR, pp. 824-825). The resulting scaled maximum and minimum values are in the tables below (Hunter, 2008).

References

Church, J.A., J.M. Gregory, P. Huybrechts, M. Kuhn, K. Lambeck, M.T. Nhuan, D. Qin and P.L. Woodworth (2001), Changes in Sea Level. Climate Change 2001: The Scientific Basis, J.T. Houghton et al., Eds., Cambridge University Press, 639-694.

Hunter, J.R. (2008), Estimating Sea-Level Extremes Under Conditions of Uncertain Sea-Level Rise. Submitted to Climatic Change)

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