Black water rising: the growing global threat of rising seas and bigger hurricanes.

• Author: Young, John

New Orleans is a sort of American Venice, built on a watery site ideal for commerce and surrounded by a huge delta assembled over millennia by the Mississippi River. Levees intended to contain the river's floods have coincidentally eliminated the annual silt deposits; as a result, along with the effects of oil and gas extraction, the area is subsiding. In New Orleans the problem has been compounded by groundwater use and the immediate pumping of rainwater after storms. Without rainwater's slow percolation into the soil, aquifers are never recharged, and the land compacts still further, especially under heavy structures.

The coastal parishes of Louisiana are sinking, on average, about 11 millimeters each year. At the same time, global sea level has been slowly rising over the last century, at 1.0-2.5 millimeters per year. The two trends have consigned half a million hectares of South Louisiana to the sea since 1932; the ongoing rate is about one and a half football fields every hour. Much of the lost land was coastal wetlands, which historically protected New Orleans by slowing and soaking up storm surges (the sudden rises in sea level that occur as hurricanes come ashore). The remaining wetlands are increasingly fragile, fragmented by canals built for navigation and access to the area's oil and gas.

All these factors make New Orleans particularly vulnerable to hurricanes, but within a few decades many more low-lying coastal areas around the world may be equally threatened. Global warming threatens to sharply accelerate the rate of global sea-level rise by melting glaciers and ice sheets in Greenland and Antarctica. Rising tropical sea surface temperatures also appear to be causing an increase in the average strength of hurricanes and tropical storms. A study by researchers at the Georgia Institute of Technology and the National Center for Atmospheric Research shows that the number of Category 4 and 5 hurricanes, the strongest storms, which are often accompanied by huge storm surges, has gone up by 80 percent in the last 30 years. Satellite measurements are increasingly revealing melting at both poles. Seismographs and laser and radar studies show the ice moving more quickly to the sea. In that ice are huge stores of fresh water, enough to raise global sea levels by catastrophic amounts if they are released. Driving the melt are warmer oceans and a warmer atmosphere, attributed by most scientists to human-caused emissions of greenhouse gases.

Climate, Oceans, and Ice

Sea level is almost completely determined by global temperatures, which control how much of the Earth's water is frozen on land as snow and ice. It has risen by about 120 meters since the peak of the last ice age 21,000 years ago. During the period of the most rapid melt of the giant ice sheets, about 14,000 years ago, sea level rose approximately 20 meters in only 400 years. After stabilizing 3-4,000 years ago, it began to increase again in the middle of the 19th century, rising since then between 1.0 and 2.5 millimeters per year.

In 2001, the Intergovernmental Panel on Climate Change (IPCC) estimated that sea levels would rise by .09-.88 meter by 2100. This estimate assumed that the slow expansion of warming seawater would be the major force driving the rise. Since 2001, however, evidence of strong warming at both poles has mounted, and a 2004 study argued that melting ice is much more important than thermal expansion, probably accounting for two-thirds to three-quarters of sea level rise in recent decades. The study implied a rise over the last century of two millimeters per year or more. A research report in late 2005 showed a slowing of the deep southerly flow of cold water in the North Atlantic thermohaline circulation, which many scientists expect would accompany a significant increase in fresh meltwater in the Arctic Ocean. Most disturbing, studies in early 2006 indicated that both the Greenland and Antarctic ice sheets may be losing mass.

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Other scientific findings raise the question of how soon elevated greenhouse gas levels will lock in unacceptable levels of future warming, and of sea level rise. Because of human activities such as burning fossil fuels, there is about two-thirds again as much carbon dioxide in the atmosphere now as the average over the last 400,000 years, and perhaps four times as much methane. The last time the atmosphere contained this much carbon dioxide was about 10 million years ago, when Greenland had no significant ice sheets, sea level was several meters higher, and temperatures were several degrees above today's. Today's higher levels mean the Earth is absorbing significantly more energy...