Well this is a nice pickle! It seems that sea level rise won't stop even if we aggressively mitigate global warming and keep global average atmospheric temperatures beneath 2 degrees celsius. The reason, it seems, is there is enormous inertia in the world's oceans and we are now warming the deeper ocean which will continue for hundreds of years.
Simple physics really. You heat a liquid and it expands. This thermal expansion is responsible for about a third of the currently observed rate of sea level rise. And it will continue for several hundred years as deeper cooler layers of the ocean take up the heat from the upper layers. Even if we take aggressive action to reduce carbon emissions, and go carbon negative, such action may only slow the rate of sea level rise as the deeper ocean continues to expand.
The authors say:
There is a commitment to further sea-level rise even if temperatures stabilize. This is because of several factors, the most quantifiable being thermal expansion of sea water. That is, as warming temperatures make their way deeper and deeper into the ocean through mixing processes in various ocean regions, an ever-increasing volume of water warms and expands, thus producing ongoing rises in sea level. There would also be commitment in the melting of ice sheets and glaciers that would contribute to further sea-level rise.
Of course current sea level is already out of balance with atmospheric temperatures. Scientists warned in March (and earlier) that Global Warming means 20 Metre sea level rise is in the pipeline according to looking at the paleo-climate response to temperature. Scientists have pointed out that the climate and sea level are more sensitive to small changes in temperature and that the paleoclimate record points towards potential rapid climate change.
The big issue is how fast we melt the Greenland and West Antarctic Ice sheets, which are already losing mass at the rate of more than 300 Gt yr making up another third of the current observed sea level rise. Ground water mining has accelerated over recent decades and is also contributing a small but growing proportion to sea level rise, although this is partially offset by dams and other above ground water storage us humans have implemented for industrial, mining and agricultural use.
Here in Australia we debate the issue of processing 10,000 refugees - boat people - largely fleeing war and persecution. Yet we ignore the impact that sea level rise will have in the Asian region which will produce environmental/climate refugees swamped by the rising seas we have contributed to through our coal emissions. Some 10 per cent of the world’s population lives in low-lying areas that are prone to flooding and are threatened by sea level rise.
Sea level rise will also have a major impact on biodiversity and habitat loss as people migrate from coastal areas to higher ground, displacing natural habitats and species for human use of land.
Even though there is no way for us to stop this process of sea level rise that global warming has initiated, the authors stress that with aggressive mitigation action - cutting our carbon emissions and eventually going carbon negative with soil, forestry and blue carbon initiatives, "increases in sea level are likely to be much less than those in the less-aggressive scenario RCP4.5, and very much less than in RCP8.5. This is significant because aggressive mitigation buys time to enact adaptation measures."
Using a semi-empirical method, the authors show how possible higher estimate of sea-level rise could relate to climate change commitment. They carefully qualify the estimates saying "These values inform the upper range of the shading in Fig. 3 that encompasses the larger estimates. But the limit of the higher end of the shading is depicted as being indistinct to reflect that these are only estimates. There is no real way of knowing if these higher total sea-level rise values are credible, or if higher or lower values are more likely."
- Aggressive mitigation action RCP 2.6 - "a sea-level rise at 2100 compared with 1986–2005 of about 100 cm, with a further rise to roughly 195 cm at 2200 and nearly 280 cm by 2300." even though "global temperatures decrease after 2100 in that scenario"
- Moderate mitigation action RCP 4.5 - sea-level rise of nearly 115 cm by 2100 and 440cm by 2300.
- Low mitigation action RCP 8.5 - sea-level rise of nearly 145 cm by 2100 and 960cm by 2300.
Image Caption: Globally averaged sea-level rise anomaly (relative to 1986–2005) owing to thermal expansion (red line, as in Fig. 2), and the example from the IPCC AR4 (dashed green line) for RCP8.5 (a), RCP4.5 (b) and RCP2.6 (c). Note different vertical scales in the three panels; 1 m and 3 m sea-level rise values are grey dashed lines in each panel. Shading highlights uncertainty in future total sea-level rise projections, with lighter shading becoming less certain. Estimates calculated from a semi-empirical method lie near the upper limit of the shading that becomes less distinct with higher values.
Here is the abstract in full:
There is a common perception that, if human societies make the significant adjustments necessary to substantively cut emissions of greenhouse gases, global temperature increases could be stabilized, and the most dangerous consequences of climate change could be avoided. Here we show results from global coupled climate model simulations with the new representative concentration pathway mitigation scenarios to 2300 to illustrate that, with aggressive mitigation in two of the scenarios, globally averaged temperature increase indeed could be stabilized either below 2 °C or near 3 °C above pre-industrial values. However, even as temperatures stabilize, sea level would continue to rise. With little mitigation, future sea-level rise would be large and continue unabated for centuries. Though sea-level rise cannot be stopped for at least the next several hundred years, with aggressive mitigation it can be slowed down, and this would buy time for adaptation measures to be adopted.
You can read the full scientific paper published on 1st July 2012 - Relative outcomes of climate change mitigation related to global temperature versus sea-level rise at Nature Climate Change.
- Gerald A. Meehl et al, Nature Climate Change (2012) - Relative outcomes of climate change mitigation related to global temperature versus sea-level rise - doi:10.1038/nclimate1529
- Nancy Roberts, Care2.com, 1 July 2012 - Sea Level Rise from Global Warming Is Unstoppable, Say Scientists
- Image from Gerald A. Meehl et al, Nature Climate Change (2012) - Fig 3.