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How high will sea levels rise? Let’s ask the experts.

Sea-level rise could prove to be one of the most far-reaching effects of global warming. Coastal regions may have to spend billions on defenses. Cities like Miami could find themselves submerged. Entire island countries like Kiribati could vanish entirely.

But here's the hitch: We still don't know exactly how fast or how much the oceans will rise in the coming century as the Earth warms. The various processes involved are tricky to model: The oceans will expand as they heat up, glaciers and ice caps will melt and add water to the oceans, ice sheets may become unstable. Studies and forecasts on this topic often disagree.

So here's one way to get a better sense for the broader debate: A new survey study in Quaternary Science Reviews simply asked 90 experts on sea-level rise for their projections, based on their work. This isn't brand-new scientific research, but it does give a useful overview of the current state of knowledge on the subject.

The results? The experts, on average, think global sea levels will rise somewhere between 0.7 and 1.2 meters by the end of the century if global warming continues unchecked (that's between 2.2 and 4 feet).

By contrast, they say, if we cut emissions significantly and avoid sharp temperature increases, we can likely limit that rise to somewhere between 0.4 and 0.6 meters (1.3 and 2 feet) by the end of the century:

(In the chart above, the red area shows a scenario in which greenhouse-gas emissions keep growing unchecked, known as RCP8.5. The blue area shows a scenario known as RCP2.6 in which we keep global warming below 2°C, likely requiring a 70 percent or more cut in cumulative global emissions by the end of the century.)

Below is an even more detailed table showing the range of sea-level rise forecasts for 2100 and 2300:

A few points stand out here:

-- These values are higher than the "consensus" view from the Intergovernmental Panel on Climate Change. The IPCC projected an 0.5 to 1.0 meter rise by 2100 in the case of unchecked warming. This survey offers higher values, with a 1.5 meter rise possible at the upper end. Why the difference?

Some back story: There are a variety of approaches that scientists take to model sea-level rise. First are "process-based" models that try to capture all the various physical phenomena that can cause ocean levels to creep upward (such as shrinking ice sheets in Greenland and Antarctica, groundwater pumping, the expansion of water due to extra heat). The problem? It's difficult to capture all of these processes precisely, particularly those involving ice.

Alternatively, there are "semi-empirical" models that look at how sea levels have changed with temperature in the past and try to project that forward. These models tend to be far more accurate in reproducing past changes in sea-level, which is a plus. They also tend to project even higher sea-level rise in the future than the process models. But they also have drawbacks: What if the relationship between temperature and sea level changes in the future?

The IPCC didn't rule these "semi-empirical" models out entirely. It simply concluded that it can't assess their reliability. That's why it ended up assuming — cautiously — a lower range for sea-level rise.

--There's not as much polarization among experts as you might think. The chart below comes from Stefan Rahmstorf, one of the authors of the paper. He notes over at Real Climate that there aren't two wildly opposing "camps" of sea-level experts, as press accounts have sometimes suggested.

The bulk of experts seem to think we'll see somewhere around 1 meter of sea-level rise by 2100 (some forecasts lower, some higher), if emissions continue unchecked. Then there are about a dozen pessimists worried about even larger rises:

("Let’s hope these outliers are wrong," Rahmstorf adds. "At least I don’t see a plausible physical mechanism for such a rapid rise.")

--Sea levels will keep rising even if we cut emissions drastically. This is a point we've discussed before, but it bears repeating. The carbon-dioxide that humans have already loaded into the atmosphere will likely have effects on the oceans for centuries to come. Some sort of adaptation will be necessary no matter what.

Most experts on this topic think there's no way to avoid at least some sea-level rise. Based on the paper above, even cutting cumulative emissions more than 70 percent will still lead to sea-level rise of 0.4 to 0.6 meters (1.3 to 2 feet) by 2100. The main choice here may be between "some" and "a lot."

Further reading:

-- Can we stop the seas from rising? Yes, but less than you think.

-- Sea level rise won't be uniform everywhere, thanks to winds, currents, and even gravitational effects. Here's a more detailed look at how sea-level rise will differ regionally. Cities like Tokyo have more to worry about than, say, Vancouver.

-- These 20 cities have the most to lose from rising sea levels.

-- Here's an excellent introduction to the science of modeling sea-level rise.