Things like extreme weather and droughts are the high-profile impacts of climate change—they are easy to see and understand. Sea level rise is much more subtle and slow-moving, but it's inexorable. Even if we stabilize our climate at a new, higher average temperature, the seas will continue to rise for centuries as the added warmth slowly melts ice and causes the water in the oceans to expand in volume.

However, since so much of human infrastructure is built right on the coasts, the rising ocean levels have the potential to cause more disruption than any other factor. Recently, some researchers attempted to quantify just how damaging sea level rise will be. At its high end, the costs are staggering: a touch over nine percent of the global GDP by the end of this century. However, that number assumes we'll keep building right on the coasts—and we're not really that shortsighted, right?

Projecting the costs involved with sea level rise by the end of the century is a difficult challenge. To begin with, the height of the seas will depend on the rate of warming, which will depend on the trajectory that emissions (and thus temperatures) take. So there are both cultural uncertainties—will we get our carbon emissions under control or not?—and scientific uncertainties about the rate of warming and how that rate will be reflected in ocean levels.

Once you have scenarios for the change in coastline, you have to start thinking about what that actually means. And that analysis depends on measurements of the current status, such as how much infrastructure we already have that will be at risk in the future. It also depends on models of the future: how much will we build near the coastline in the near term, and how much will we be willing to spend on protecting it in the longer term?

To give one practical example of the issues confronting the authors, it's instructive to consider one of the relatively simple factors involved in their study: the elevation of land near the coasts. One of the data sets available on land elevation was generated using the Space Shuttle. The Shuttle measured elevation with a technology that overestimated the true elevation because it registered the height of the tops of foliage and any ground cover. The authors generated estimates with both the Shuttle data and a separate measure derived from satellite sensing, and they included the difference as part of their uncertainty estimates.

(One of their data sets, used to measure the amount of infrastructure already in place at the coasts, comes from the delightfully acronymed Global Rural–Urban Mapping Project, or GRUMP.)

Based on these estimates alone, the value of assets within the reach of a 100-year flood event will range between $17 trillion and $180 trillion by the end of the century—and that's under an emissions scenario that's unrealistically low. Under business-as-usual emissions (the IPCC's RCP 8.5), the figures will range from $21 trillion to $210 trillion. We'll naturally lose some of that infrastructure to flooding each year. Even under the unrealistically low emissions scenario, the losses could reach up to five percent of the global GDP annually. For the business-as-usual, it ranges from a low of 1.2 percent to a high of over 9.5 percent.

The authors say that last figure is unrealistically high. Surely, they reason, we'll stop building so much near the coast and start relocating some vulnerable populations before the end of the century. But they also admit that in many areas of the globe, urbanization is leading to a coastward migration that is actually expanding the risk. In the US, state governments appear to be refusing to accept reality when it comes to sea level rise.

We can also make matters worse by continuing to pump ground water at unsustainable rates. This water will eventually end up in the ocean and, when pumped in coastal areas, can lead to ground subsidence that enhances the impact of rising oceans.

Of course, at least some areas can be protected by building infrastructure to keep the ocean out. Even under a low-emission scenario, costs for dike maintenance are expected to reach at least $12 billion a year by the end of the century; for higher emissions, the cost could reach up to $71 billion. Here, economics leads to a complicated trade-off: wealthy countries will put more (and more expensive) infrastructure near the coast, but their populations will likely demand more protection and be better able to pay for it.

Given the huge uncertainties, it might be tempting to ask what the value of this sort of study is. By showing a range of potential costs, the work provides some indication of what the scale of the problem will be, giving planners some sense of how to prioritize potential fixes. By incorporating multiple scenarios in their models, the authors have given themselves the chance to improve their estimates as data becomes more certain and as the path we're actually taking into the future becomes increasingly clear.

PNAS, 2014. DOI: 10.1073/pnas.1222469111 (About DOIs).