The rise in sea level due to thermal expansion will also become exponential
The increases in global land and sea temperatures are expected to cause huge changes in local climate and weather, and some of the predicted changes are detailed in subsequent posts. For now, we consider only the effects that the global temperature change has on sea level.
We can easily estimate the amount of sea level rise due to its temperature increase alone, because this rise comes from the expansion of the volume of water in the ocean. This expansion results from the increase in the ocean temperature, which lags the increase in the global surface temperature. Despite a large amount of publicity, there has so far been a relatively small contribution from the melting of ice and glaciers on land. (Note that the melting of floating ice changes surface salinity but does not change the sea level.)
Therefore, we can use the observed rise in sea level up to now, and combine it with the expected global temperature change, to estimate the expected increase from this effect. The future contribution from melting ice, mostly from the Greenland ice sheet, can be added afterward.
Sea level rise does lag the surface temperature increase, of course, because it takes time for heat to transfer through the water column. Because amount of sea level expansion depends linearly on the temperature, however, and because the temperature has an exponential increase, the sea level rise eventually must become exponential with the same time dependence (same rate) as the temperature.* An exponential extrapolation is shown in the figure, matched to the measured rise since 1975 (when the global temperature started its exponential rise).
Overall, the sea level rise by 2050 is predicted to be about a half meter (19") from thermal expansion alone. While this may be small compared to the other factors considered to this point, this is not an insignificant rise in terms of wetlands damage and coastline erosion.
At this point, we have made the more secure of the predictions that can be made. The remaining sections will summarize some of the more complex issues involving more detailed and less certain climate and weather predictions.
Contributions from the Greenland ice sheet
The Greenland ice sheet is thought to be the major contributor to sea level rise from melting, now and in the near future. Recent research suggests that it has contributed 25% of the observed rise to date. Complete loss of the Greenland ice sheet will result in a global rise in sea level of 7 meters (22 feet). Current projections, however, are that it may take more than 300 years for the melting to be complete (see the "tipping point" projections in the next section). The actual melting rate will depend on many factors, including the temperature rise in the future.
With all these uncertainties, at this point we just note that if the time for complete melting was as much as 600 years, in the next 40 years we should expect to lose about 7 meters x (40/600) or a half meter. This would suggest that by 2050 we would have a total sea level rise of about a meter (3 feet), half from thermal expansion, half from melting ice. It is quite likely that the contribution from the Greenland melting will be more than this amount; it is unlikely to be less.
Next: Dangerous tipping points ahead.
DETAILS
The observed rise in sea level (black line) was taken from a Wikipedia review. The predicted exponential rise (red line) is the best fit to the data starting in 1975, when the temperature started its current rapid rise, and is the extrapolation which has the same time dependence as both the global temperature and CO2 levels. The estimate of the amount of sea level rise due to the melting of the Greenland ice sheet is from a (2009) report by S. H. Mernild, et al.
* The proof is the same as for the temperature lagging behind the CO2 level when it has an exponential rise, discussed in the last section.
No comments:
Post a Comment