July 26, 2012 | A reduction in greenhouse gas emissions could greatly lessen the impacts of climate change. However, the gases already added to the atmosphere ensure a certain amount of sea level rise to come, even if future emissions are reduced. A new "Perspective" study published in Nature Climate Change quantifies the impact on oceans of the "climate commitment" being made now by human activity.

As the atmosphere heats up, the world’s oceans absorb heat more slowly, due to the physical properties of sea water, the oceans' vast depth, and other characteristics. So there is lag time as the heat makes its way deeper and deeper into the oceans. This ever-increasing volume of warm water slowly expands—a process that occurs over centuries.

Thermal expansion accounts for about 0.8 millimeters per year of sea level rise since the 1970s, almost half of the estimated total sea level rise per year of 1.8 millimeters (0.075 inch). The other contributions come from melting glaciers and ice sheets, as well as other sources such as piping groundwater to the surface. If the most stringent measures to slow warming were put in place now, thermal expansion alone would still cause substantial sea level rise well into the future, the study finds.

NCAR scientist Gerald Meehl and colleagues analyzed future warming and potential sea level rise under four mitigation scenarios developed for its next report by the Intergovernmental Panel on Climate Change. In the most aggressive scenario, the world’s energy needs are met largely through renewable energy, nuclear power, and carbon sequestration—a combination that is technologically feasible, though difficult to enact. Under this scenario, there could be “negative emissions” of carbon dioxide by 2070, meaning that more CO₂ would be taken out of the atmosphere than emitted.

Even under such aggressive measures, which could result in global cooling by 2100, sea level still continues to rise for centuries. In this scenario, thermal expansion alone would push sea level beyond its 1986–2005 average by 14.2 centimeters (5.5 inches) in the year 2100 and by 24.2 centimeters (9.4 inches) in 2300. These values do not take into account additional sea level rise from melting of glaciers and ice sheets. In comparison, the scenario with least mitigation produces sea level rise of 32.3 cm (12.6 in) by 2100 to 139.4 cm (54.4 in) by 2300 from thermal expansion alone.

The wild card for future sea level rise is uncertainty about the contributions from the Greenland and Antarctic ice sheets, says Meehl. How fast the ice sheets could disintegrate has proven difficult to assess and model, making it challenging for researchers to precisely estimate how much sea level rise the world could be facing.

According to Meehl, the findings should not dampen efforts to rein in greenhouse gases but should revitalize those efforts. Under the most aggressive scenario, he points out, “Even though we’re already committed to a certain amount of sea level rise, it’s not going to go up nearly as much as if we don’t do anything.”

He believes the results should be taken into account when policymakers consider mitigation options. “There’s a sense of urgency to it—the longer we wait, the worse it’s going to get.” Adopting more aggressive measures sooner will slow the process, he points out, buying the world time to adapt to rising tides.

Gerald A. Meehl, Aixue Hu, Claudia Tebaldi, Julie M. Arblaster, Warren M. Washington, Haiyan Teng, Benjamin M. Sanderson, Toby Ault, Warren G. Strand, James B. White III, "Relative outcomes of climate change mitigation related to global temperature versus sea-level rise," Nature Climate Change (2012). doi:10.1038/nclimate1529

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