Past warming linked to rising sea level

A warming episode some 125,000 years ago significantly diminished polar ice sheets and drove sea level six to nine meters higher, a scientific review of recent data shows.

And while the cause of the warming was very different from today’s rapidly increasing greenhouse gas emissions, present-day warming is reaching comparable levels—suggesting the potential for a similar response in the decades to come by the great Greenland and Antarctic ice sheets.

The review, led by geochemist Andrea Dutton of the University of Florida (Dutton et al., 2015), examined the latest evidence for past sea level rise during so-called “interglacial” periods—warm interludes between ice ages—over the past three million years.

Of the four periods studied, including the Holocene epoch that we’re living through now, the bout of warming between 129,000 and 116,000 years ago might prove the most instructive to scientists, especially those attempting to predict the future effects of today’s warming trend.

Studies of past sea level rely on two main sources: the fingerprints of ancient climates left on tiny marine organisms, and sediments found along coastlines. By examining the shells of microscopic creatures called foraminifera, scientists can determine the ratio of two isotopes, oxygen 16 and oxygen 18. The ratio depends on seawater temperature, salinity and ice volume. Ice contains less oxygen 18; because of its lighter atomic weight, oxygen 16 allows water molecules to evaporate more readily from the ocean, later falling as snow that turns into ice. So the less oxygen 18 found in a sample of foraminifera shells, relative to oxygen 16, the more melted ice in the ancient ocean. The most accurate estimates come from deep-sea or “benthic” animals, which nullify the salinity signal.

In a similar way, the effects of past temperatures can be estimated from the ratio of magnesium to calcium in the foram shells.

Coastline sediments, especially shallow-water corals and salt marshes, can yield estimates of global mean sea level through radiocarbon (U-Th) dating. But because these reflect relative sea level—a kind of localized snapshot of sea level in relation to a particular coastline—scientists must make adjustments to obtain an accurate estimate of global sea level. They must factor in “loading,” the elasticity of the seafloor as water or ice masses shift, “self-attraction,” the gravitational changes brought about by increasing or decreasing ice mass, and “post-glacial rebound,” the slow rise of the Earth’s crust over thousands of years in response to the loss of ice mass as glaciers retreat.

Taken together, these two forms of proxy data show warming of about one degree Celsius during the interglacial period 125,000 years ago compared to preindustrial temperatures in our own era. But warming of Earth’s polar regions was far more severe: five to eight degrees Celsius higher for Greenland, three to five for Antarctica.

A melting Greenland likely contributed about a third of the estimated six to nine meter sea-level rise, with Antarctica adding a small amount and the balance from glaciers and the expansion of warming seawater.

And the rise in sea level didn’t happen all at once. A study of coastline sediments showed that melting rates oscillated between one and seven meters per thousand years, though the conclusion comes with large uncertainties.

The review authors also found that the cause of the ancient warming episode was mainly changes in solar radiation at high latitudes, driven by periodic orbital shifts.

Projections of future sea level vary widely, as discussed elsewhere on this site. Estimates range from 0.2 to 2 meters by 2100. But the study’s authors say understanding of past warming episodes and their effects on ice loss and sea level is improving. That includes more detailed reconstructions of past sea levels with a greater geographic distribution, and a better understanding of complex geophysical processes, such as post-glacial rebound, that affect sea level.