Humanity has trillions of dollars of infrastructure within a meter of the current sea level. Given that, it's rather important that we understand how long that sea level will stay at its current measurement and how quickly it will change. Unfortunately, we don't have very good data for this. Modern satellite records of sea level only go back a few decades; before that, we have about a century of tide gauge readings.

Unfortunately, those tide gauges weren't uniformly distributed throughout the globe. And, in many cases, the solid ground they are attached to has been shifting slightly. Attempts to account for these and other factors has produced a variety of estimates of past sea level rise, most being fairly similar but differing in a number of ways. Now, the latest attempt suggests that sea level rose more slowly early last century—which means it has accelerated dramatically in recent decades.

Doing the books

Ocean levels are rising due to a combination of added water (from the melting of glaciers and ice sheets) and the fact that water expands as it warms up. But that's not the only factor at play. Humanity has trapped a lot of water behind dams, and it has taken lots of groundwater and placed it back into the regular cycling of evaporation and precipitation. Collectively, these factors create what's called a "budget" for the sea level. If we knew how much they changed, we should have a good estimate of ocean levels.

But studies of past ocean levels have had a tough time matching that budget. That could be because our budget is off, and we've mis-estimated one or more of the contributing factors. But it could also be because our measures of ocean-level rise are a bit off. While tide gauges may be accurate, they're attached to land that can move. This motion could be the result of plate tectonics, or (in some regions) it can be the product of the land rebounding after the weight of a nearby ice sheet has been removed.

There can also be changes that affect entire regions. The currents that circulate water around the globe can end up pushing up the ocean's elevations in some areas. And large masses, like ice sheets or coastal mountains, can also have a gravitational attraction that draws in additional water.

Changes in any of these factors can influence local sea level, and thus the readings on tide gauges, without altering the global average sea level. Researchers know about these complications and have made a variety of attempts to compensate for them. But, as mentioned above, their efforts haven't matched each other's and haven't fit the budget defined by water additions and warming.

Rising tides

To get what they hope is a more accurate picture, the research team behind the new work focused on the tide gauges they understood the best, ones where we had measures of land motion with an uncertainty of less than 0.7mm a year. This cut the tide gauge set down to 322 stations. They then split these stations into six groups, meant to reflect the areas where currents normally redistribute water. The individual station readings were then combined within each region, and the regional sea level measures combined to produce a global measure.

Overall, this produces a trend of 1.3mm of sea level rise a year since 1902. If carried into the future, this would leave us with a very manageable rate of change. But it's not being carried into the future. Instead, since 1993, the data suggests ocean levels have risen at a rate of 3.1mm a year. That's similar to the value obtained by satellite readings, an important validation. In addition, the authors say it's a good match for the sea level budget set by water additions and warming. The rate of change in ocean levels is also highest in the two time periods where temperatures rose the most (The 1920s to '40s and 1970s to present).

Overall, the new reconstruction is similar to other work except during the early 1900s, when it shows ocean levels rising at a slower pace. As a result, it shows a larger acceleration of sea level rise: 0.018mm/year2, nearly twice as high as previous estimates.

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This won't be the final word on sea level rise, as someone is almost certain to think they have a better way of managing the uncertainty. But it should help us refine our expectations for the coming century, and therefore build new infrastructure more intelligently. Provided politicians pay attention, of course.

PNAS, 2017. DOI: 10.1073/pnas.1616007114 (About DOIs).