One major threat from climate change is the rising global sea level. At the coast, the rising seas will wipe out infrastructure and threaten wildlife. If ocean water moves deeper into landmasses, the salt will contaminate sources required for drinking water and agriculture.

A solid understanding of how quickly the sea level is rising, and the major contributing factors, is critical to developing practical plans to limit the problems and deal with the inevitable. Recently, a team of scientists has dived head-first into this challenge.

Effect of rising sea levels

The main factors influencing rising sea level have been well documented. First, climate change has led to increased global temperatures. As its temperature rises, the sea water expands (a process called steric expansion). In addition, ice sheets and land glaciers all over the world are shrinking through evaporation and melting. All of these factors contribute to rising ocean levels.

But the sea level doesn’t rise uniformly across the ocean—variations exist in different regions, and certain areas are more at risk.

Data collected from monitoring networks can be used to probe the mechanisms of sea level rise. In this investigation, scientists used satellite data from April 2002 to June 2014, which documented changes in mass (via gravimetry) and height (via altimetry) to account for the global and regional sea level rise budget. Because this combination of satellite data measures different contributions of the total sea level change, it can provide information on the overall change in the oceans' volume. These scientists compared their findings to previous studies that explored sea-level change due to water's expansion using analysis of temperature and salinity changes.

The team separated the sea level rise into several components: heat-driven expansion; glaciers and ice sheets; water movement, distribution, and quality (hydrology); and the rise of land as the glaciers above it melt off. They analyzed the sea level budget both globally and for a number of coastal areas.

Global sea level rise

Using their methods, they found that the global sea level rose by 2.74 ± 0.58 mm/y over this 12-year period. The volumetric contribution (1.38 ± 0.16 mm/y) was significantly larger than previous estimates made using temperature and salinity profiles. In addition, this contribution is twice as large as the value estimated using data collected from the uppermost 1,500m of the ocean.

They also found that the increase in sea level driven by the melting of ice sheets and glaciers (1.37 ± 0.09 mm/y) was offset by a negative contribution from the value that encapsulates water movement, distribution, and quality (-0.29 ± 0.26 mm/y). The remaining contribution required to close the budget was 0.22 ± 0.26 mm/y. This value represents other deep and shallow volumetric changes and internal ocean mass deviations, which are not accounted for elsewhere.

Regional sea level rise

When assessing regional changes in sea level, the team saw that the contributions of each component differed significantly from those of the global sea level. In the western Pacific and Indian Ocean, the rising sea level was dominated by the volumetric contribution (up to a whopping 75 percent). And it resulted in some dramatic local changes—it was as high as 14.7 ± 4.4 mm/y near the Philippines. The sea level was also strongly affected by the melting of glaciers and ice sheets, despite their distant location. In the central and eastern Pacific, heat-driven expansion contributions had a negative impact (-2.8 ± 11.5 mm/y) on the global sea level, although the margin of error was enormous.

Surprisingly, the coastlines around the Atlantic Ocean appeared to see significant changes that could not be attributed to either the expansion of the ocean or the melting of ice. This unknown contribution could be caused by either volumetric effects that are not properly picked up using these methods or from internal mass variations of the ocean.

The authors also found that the contribution attributed to water infrastructure did not change the global sea level. The contribution from changes in the glacial volume remained small at the coastlines except near the former ice sheets.

These findings are particularly useful for areas that are most susceptible to the rising global sea level like coastal cities—assuming we're willing to incorporate it into our planning.

PNAS, 2016. DOI: 10.1073/pnas.1519132113 (About DOIs).