17: Sea Level Rise from Melting Ice

Overview

Glaciers and ice sheets in Greenland, Antarctica, and other parts of the world are melting as a result of climate change resulting from human-induced emissions of greenhouse gases. The rates of ice melt have increased by one order of magnitude in the last 40 years and will likely continue to increase rapidly in the next 40 years. At the current accelerated rate of ice melt, sea level will rise by about 1 meter by the end of the twenty-first century, but the actual number will depend on both the rate at which our climate continues to warm up and the rate at which ice sheets undergo catastrophic retreat and melt, especially in the marine-based sectors of the ice sheets. Large uncertainties remain in both processes. Meanwhile, paleoclimate records and basic physics dictate that the current regime of climate warming is unsustainable for ice sheets and that continued warming commits us to multiple-meter sea level rise in the coming centuries. A reduction in the rate of ice melt is possible but entails a massive curbing of our greenhouse gas emissions and the implementation of carbon sequestration to bring carbon concentration in the atmosphere back to lower levels.

The ice sheets in Greenland and Antarctica contain large volumes of ice, equivalent to a change in global sea level of 7 meters (43 feet) for Greenland and 56 meters (360 feet) for Antarctica if all the ice were to melt into the sea. This is because the ice sheets hold several quadrillion tons of land ice. One billion tons is called 1 gigaton (Gt), and 361 Gt of land ice melting into the ocean would cause 1 millimeter (mm) of global sea level change. One gigaton of water is the annual supply of water for a city the size of Los Angeles and its 8 million inhabitants. We use gigatons to quantify the mass loss of ice sheets.

The average precipitation on Earth is 1 meter per year. Greenland (average precipitation 24 centimeters per year) and Antarctica (average precipitation 17 centimeters per year) qualify as deserts (the Sahara gets 10 centimeters per year). Yet their combined annual cycle of precipitation is equivalent to a 7-millimeter fluctuation in global sea level because they cover such large areas, about 1.7 million square kilometers for Greenland and 14 million for Antarctica.

Ice accumulates in Greenland and Antarctica from snowfall, which slowly densifies into ice and deforms under its own weight to flow toward the ocean along rivers of ice called ice streams and glaciers. At the ocean boundary, ice melts in contact with ocean water or is released as icebergs that subsequently melt in the ocean. At the surface, snow/ ice mass is removed via wind transport, sublimation, evaporation, and surface melt. Surface melt that does not refreeze in place produces runoff, which reaches the bottom of the glacier, or bed, through cracks and holes (known as moulins). It emerges at the ice front (where ice meets ocean water; Figure 17.1) or at the grounding line (where ice detaches from the bed and becomes afloat in the ocean) as a buoyant plume of freshwater, often laden with sediments. Runoff in Greenland averages about 300 Gt per year, and the ice flux crossing the glacier fronts or grounding lines averages 400 Gt per year. In Antarctica, the ice flux crossing the grounding line is 2,200 Gt per year and runoff is nearly zero at present (Figure 17.2).

Glaciers and ice sheets move from a few centimeters per year near ice divides to a few kilometers per year at the front of the fastest moving glaciers. Around 13,000 years ago, sea level rose 4 meters per century for several centuries. This rapid sea level rise was associated with the collapse of the northern ice sheets, including parts of Greenland, northern Canada, and Scandinavia, but also parts of West Antarctica and, presumably, parts of East Antarctica yet to be identified. During the Holocene, sea level rose, in comparison, very slowly before rising 1.8 millimeters per year in the twentieth century and 3 millimeters per year at present. This rate is projected to increase as climate continues to warm up, land ice melts worldwide, and the oceans continue their thermal expansion. While the glaciers and ice sheets already control two-thirds of the global increase in sea level rise today, we expect greater rates of sea level rise in the future as the ice sheets in Greenland and Antarctica start melting faster. We face the distinct possibility that ice sheets will contribute multiple meters of sea level rise in the future.

Millions of people and trillions of dollars of infrastructure located along the coastlines of the world’s oceans will be threatened by as little as 1 meter (3 feet) of sea level rise, which is almost certain to happen by the end of the twenty-first century, and multiple meters of sea level rise are expected in the ensuing centuries. While adaptation may be possible in some places, people in many low-lying underdeveloped countries will have to move to higher ground or to other countries. Moving infrastructure such as industries, roads, housing, seaports, airports, and other critical facilities will cost trillions of dollars.

At present, predictions of sea level rise are affected by two major uncertainties: (1) the rate at which our emission of greenhouse gases into the atmosphere will continue to increase and warm the planet, and (2) the rate at which ice sheets and glaciers will respond to climate warming and collapse, which depends on physical processes that have not been fully elucidated and incorporated into physical climate models. Most current numerical ice sheet models use simplified and incomplete physical descriptions of ice sheet and ocean dynamics that systematically underestimate the risk of catastrophic melting. In the next sections, we will discuss the physical processes of ice melting, the history of melting, and what we can do about it.