Climatic Research Unit : Information sheets

10: Sea level rise

Sarah Raper

Introduction

On the decade-to century timescale, changes in global mean temperature result in changes in sea-level mainly through thermal expansion of the ocean and the melting of land ice. Over the past 100 years the world has warmed by about 0.5°C (see Information Sheet 1). Due to increasing atmospheric concentrations of carbon dioxide and other greenhouse gases, the prospect for the future is an even warmer world. Has sea-level also risen over the last 100 years? If so, is there evidence that the rise is climate-related? What are the projected rates of global warming due to anthropogenic forcing and how fast is sea level likely to rise? These are the questions addressed below.

How much has sea level risen over the last 100 years?

Sea level change over the last 100 years can be estimated from tide gauge data. These data reflect both water level changes and vertical land movement and the two have to be separated in order to estimate changes in sea level. Another problem is that the tide gauge data are not evenly distributed, but show a bias towards the continental coastal margins of the Northern Hemisphere, in particular, North America, Europe and Japan. Despite the uncertainties, global mean sea level is thought to have risen substantially over the last 100 years. Based on available analyses the rise over the last 100 years has been in the range 10-20 cm.

Factors contributing to sea level rise.

Global warming is thought to change sea level mainly through thermal expansion of the ocean and by changing the net mass balance of glaciers and ice sheets. In addition, the stability of the West Antarctic Ice Sheet and uncertainties due to the pre-industrial state of the mass balance of Greenland and Antarctica must be considered. Estimates of sea level rise from these sources over the last 100 years are very uncertain and the total gives an estimate which is less than that based on the tide gauge data as can be seen from the table below.

Table: Estimated contributions to sea level rise over the last 100 years
Component contributions Low Middle High
Thermal expansion 2 4 7
Glaciers/ice caps 2 3.5 5
Greenland ice sheet -4 0 4
Antarctic ice sheet -14 0 14
Surface water and ground water storage -5 0.5 7
Total from above -19 8 37
Total based on tide gauges 10 15 20

How do changes in sea ice and floating ice shelves affect sea level?

Changes in sea ice extent and the extent of the floating ice shelves do not affect sea level directly. This is because they already displace the appropriate volume for water at the ambient temperature and it makes no difference whether the water is in a liquid or solid state.

What will happen to sea level over the next 100 years?

How much sea level will change in the future due to the continued burning of fossil fuels and other changes made by man such as changes in land use will depend on rather a large number of factors and for this reason estimates for the future are uncertain. The factors contributing to the uncertainty include the following -
  1. the assumed future greenhouse gas and aerosol emissions,
  2. the resulting greenhouse gas concentrations in the atmosphere, which depends on the various sources and sinks of the various gases,
  3. the effect of the changed concentrations on the radiative forcing of the atmosphere,
  4. the effect of this forcing on the climate, in particular on temperature and snowfall,
  5. the rate of heat penetration into the ocean, which will affect the rate of thermal expansion of the ocean,
  6. the regional climate change over the glaciers, ice caps and ice-sheets (both temperature and snowfall),
  7. the response of the glaciers, ice caps and ice-sheets to this climate change.

The user-friendly software MAGICC addresses all these factors in a consistent way. The present version gives results which are compatible with the IPCC Second Assessment Report. An example of a sea level rise projection from MAGICC is shown in the Figure below.

graph
Figure: Modelled sea level rise from 1765 to 2100 using the simple models in the MAGICC package. The future scenario used is the ‘business as usual’ scenario IS92a. The uncertainty range is for uncertainty in the climate sensitivity only.
(Postscript version)

References:

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Last updated: July 2000, Sarah Raper