Cut greenhouse gas emissions and both global temperatures and sea levels will stop increasing. Right?

Not exactly. Everyone might want to reconsider ocean-front property, according to a group of researchers from the National Center for Atmospheric Research. In a paper recently published in Nature Climate Change, the team presents results from simulations showing that, while reducing emissions can stabilize global temperatures, sea levels will continue to rise for centuries.

Sea levels rising with increasing temperature makes sense. But why would this continue if the surface stops getting hotter? Well, the answer is a bit complicated, and there are multiple factors. According to the authors, the biggest contribution is actually from thermal expansion of sea water. Warm water at the surface slowly mixes downward, causing more water to heat up. It then takes a while for oceans to fully equilibrate to atmospheric conditions. As the temperature continues to rise, sea water expands—causing the sea level to rise. This continues even when the surface temperature stops increasing or begins to drop.

In order to get an idea of how much the sea levels might rise, the researchers took the well-known Community Climate System Model and analyzed a few climate change mitigation scenarios. This model predicts climate change and its effects by simulating the atmosphere and ocean as well as land and sea ice. The scenarios range from doing nearly nothing to reduce carbon dioxide emissions—which would result in warming of nearly five degrees C by the end of the century—to actually pulling enough carbon dioxide out of the air (negative net emissions) to begin cooling the planet by the end of the century. In between, the less-aggressive scenario has us stabilizing around three degrees C above pre-industrial levels by 2200.

According to the simulation results, even in the best-case scenario, sea levels will still be rising for another three hundred years.

Melting ice sheets and glaciers would also contribute to the sea level rise, but this is trickier to predict. In particular, we don't really know the physics of ice-sheet stability, although some empirical models exist. The authors used one of these, from the Intergovernmental Panel on Climate Change (IPCC)’s fourth assessment report (AR4), to estimate this contribution.

Taking contributions from glaciers and ice sheets into account (which the authors admit is highly uncertain) nearly doubles the predicted sea level rise in each scenario. Due to the uncertainties in these calculations, the actual numbers aren’t necessarily good quantitative predictions. However, limiting considerations to thermal expansion just gives a low estimate, since ice would definitely contribute to rising sea levels.

If we don’t do anything to reduce carbon dioxide emissions, the sea level could rise nearly two and a half meters at minimum by 2300. In the worst case, that could be more than ten meters.

In the best case scenario, where we invest heavily in renewables, nuclear power, and carbon capture and storage by 2070, we might still have to deal with a sea level nearly a meter higher than it is now. Due to uncertainties in the contribution from glaciers and ice sheets, this could be closer to three meters.

If the oceans are going to rise no matter what we do, why bother doing anything? For one thing, a rise of one meter is much better than ten meters. In addition, if we can aggressively cut carbon dioxide emissions, the sea level rise will happen slower—giving us more time to adapt.

Nature Climate Change, 2012. DOI: 10.1038/nclimate1529 (About DOIs)