Sea level rise

Global average sea level has risen 24 cm since 1850. Projections suggest that, by 2100, global mean sea level will rise between 54 cm under a “Paris Agreement” (RCP4.5) climate scenario and 74 cm under a “business-as-usual” (RCP8.5) climate scenario. Current sea-level research with GEUS focuses on understanding both the causes of sea-level rise, as well as regional variability in sea-level rise.

There are many causes of sea-level rise. Thermal expansion of seawater, or the decrease in water density as it warms, is a major source of sea-level rise. However, transferring land-ice, such as glaciers and ice sheets, into the ocean is the single biggest cause of recent sea-level rise. Dwindling Arctic land ice is responsible for 48% of global mean sea-level rise since 1850, with the Greenland ice sheet alone responsible for 11% of post-1850 sea-level rise. Recent Greenland ice loss is driven by both the runoff of liquid meltwater, and the discharge of solid icebergs. The ice loss caused by these processes, which changes through space and time, is assessed using a combination of satellite measurements, climate models, and in-situ observations.

Graph
Historical and projected components influencing global average sea level under the "business-as-usual" (RCP8.5) climate scenario (1850-2100). Sea-level change expressed relative to year 2006. Arctic land ice contributions highlighted relative to Antarctic land ice, thermal expansion and terrestrial water storage contributions to the global budget.
‘Scandinavian glaciers’ includes both Iceland and Svalbard. ‘Other glaciers’ includes all other land-ice sources. (AMAP, 2017. Snow, Water, Ice and Permafrost in the Arctic (SWIPA) 2017.)

Regional variability in sea-level rise is complex. To understand sea-level change means understanding not only the transfer of land ice into the ocean, but also, for example, how the gravitational field of Earth changes as massive water volumes shift around the planet. As the ice sheets diminish under climate change, so too do their gravitational fields. In the process, previously held ocean water is released to migrate elsewhere on Earth. Sea level actually falls in the vicinity of diminishing glaciers and ice sheets, not only due to the weakening of local gravity, but also the rapid removal of ice mass enhances rebound of the Earth’s crust out of the ocean.

In Scandinavia, sea-level changes measured by satellite since 1993 vary from no change at Reykavik, Iceland to about 150% of the rate of global average sea-level rise at Helsinki, Finland. It is perhaps counterintuitive to think that simply because Iceland is closer to the diminishing Greenland ice sheet that its sea-level change is extremely different from more distant Finland. But when it comes to the gravitational redistribution of ocean mass, it is the ice masses farthest away from a location that influence its sea-level rise the most. Looking towards the future, Scandinavian cities will similarly not experience identical sea-level changes. Take Oslo and Copenhagen as an example. Remarkably, only 600 km separates these cities but the year 2100 sea-level change projected at Copenhagen is about 300% of that at Oslo.

Understanding the origin of sea-level rise, especially the contribution from Arctic land-ice, as well as the regional variability in sea-level rise, particularly within the Baltic Sea, are therefore core themes of ongoing sea-level research at GEUS.

Map of sea level change
Recent sea-level change at five Scandinavian capitals. Background shading in the seas indicate the observed sea level change from 1993 to 2016 recorded by satellites (Map of sea leves trends). The charts show the sources of change between 2002 and 2016. “Non-ice processes” refer to processes like thermal expansion of warming seawater, persistent shifts in wind and ocean currents, and changes in water storage on land. Only Reykjavik, and to a very small degree Olso, have experienced local sea-level fall in response to ice loss from Greenland.

Jason Eric Box

Research Professor
Glaciology and Climate
Phone: +45 60124157

William Colgan

Senior Researcher
Glaciology and Climate
Phone: +45 91333814