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Don Paul: There's nothing simple about rising sea levels

The mean global sea level is going up by 3.4 millimeters per year. The majority of that increase is tied to ongoing warming due to human activity, including the burning of fossil fuels, as well as deforestation.

The rising sea levels occur due to freshwater ice melt and the expansion of sea water due to warming (water expands as it heats). The largest sources of freshwater is directly related to ice melt from Greenland, Antarctica and glaciers around the world. That sounds simple enough, analogous to water trickling into a bathtub.

Like so many things in life, it isn’t.

The earth’s sea levels – note my use of the plural – are anything but simple to explain. With help from NASA’s tremendous body of research, I’ll give you a taste of the complexities.

The shape of the ocean basins themselves are constantly changing. We know when glaciers retreat, the land they had covered decompresses and rises, once all that weight is gone. That happened all over the portions of ice-covered North America when the last ice age retreated and ended. It’s happening now on Greenland as the rapid ice melt that is occurring there changes its mass, and it’s happening on the outer portions of Antarctica.

Conversely, large areas of land near the coast on the continents are subsiding, pushing surrounding waters upward.

The irregularities are fascinating. There are places in which the sea level has remained stable for the last few decades, as along the California coast. There are even a few places where sea level has declined. Greenland is a focal point in its region because ice melt there has generally accelerated, sending enormous quantities of fresh water into the salty North Atlantic:

NASA researchers note that it is important to know what portions of these quantities are due to melting from warming and what portions are due to chances in precipitation over different parts of Greenland. Some parts of Greenland are ice covered with little snow, while other parts have thick blankets of snow.

Elsewhere, measurement of melting from smaller ice masses such as glaciers and ice caps  with the larger Greenland-Antarctica melting contribute about 1/3 of the mean sea level rise in the last 20 years.

The freshwater ice mass of Greenland and Antarctica, however, loom large over the other ice source regions, which include melting from the Andes and the Himalayas. Recent findings via satellite data suggest the rate of ice loss from glaciers has been generally overestimated and accounts for less of sea level rise than had been estimated. More is due, and more will be due, to melting in Greenland and Antarctica over a long timescale.

By far, though, the biggest current contributor is the ocean’s absorption of atmospheric heat and the expansion of ocean waters due to that heating. In fact, more than 90 percent of heating due to increases in human-related greenhouse gas emissions is absorbed by the ocean, making thermal/heat expansion at this time the dominant cause of sea level rises.

In the meantime, the rise of land due to the disappearance of ice mass and the change in shape around sea basins related to that are already factored in to projected sea level rises. A lesser but not insignificant contributor is the rise in groundwater use by humanity, because that “used” water ultimately finds its way to the sea.

There is regional shorter-term variability in local sea levels due to ocean circulations and oscillations such as ENSO, the El Niño Southern Oscillation. When El Niño was at its height last winter, it transported expanded, heated sea water to the eastern tropical Pacific and lowered the sea level in the western tropical Pacific. Here is satellite-derived data from the two most recent major El Niños.

You can read about changes in regional gravity due to shifts in mass of sea waters in their basins in the link above.

We’ll skip the short-term changes in astronomical tides and storm surges except to say those changes are still critical. Sandy’s effects at landfall in New York and New Jersey were worsened by sea level rise. Miami and much of South Florida regularly experience more serious tidal flooding at full moon, and they face a greater threat with the approach of any tropical storm surge. Many island nations in the South Pacific are also critically threatened by rising sea levels.

The precise height of sea levels is now being measured with greater accuracy by satellites as well as new sophisticated floating instruments. While there are variations in the rate of sea-level rise on a regional scale, the mean is irrefutably rising and will continue to rise for the foreseeable future. Even if there were some magic switch to be thrown to stop ongoing mean global warming, it will take a great many decades for the ocean to give up the heat it has absorbed.

And, in doing so, it will be throwing that heat back into the atmosphere.

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