Rise in sea level can't be stopped, scientists say

LONDON, July 1 (Reuters) - Rising sea levels cannot be stopped over the next several hundred years, even if deep emissions cuts lower global average temperatures, but they can be slowed down, climate scientists said in a study on Sunday.

A lot of climate research shows that rising greenhouse gas emissions are responsible for increasing global average surface temperatures by about 0.17 degrees Celsius a decade from 1980-2010 and for a sea level rise of about 2.3mm a year from 2005-2010 as ice caps and glaciers melt.

Rising sea levels threaten about a tenth of the world's population who live in low-lying areas and islands which are at risk of flooding, including the Caribbean, Maldives and Asia-Pacific island groups.

More than 180 countries are negotiating a new global climate pact which will come into force by 2020 and force all nations to cut emissions to limit warming to below 2 degrees Celsius this century - a level scientists say is the minimum required to avert catastrophic effects.

But even if the most ambitious emissions cuts are made, it might not be enough to stop sea levels rising due to the thermal expansion of sea water, said scientists at the United States' National Centre for Atmospheric Research, U.S. research organisation Climate Central and Centre for Australian Weather and Climate Research in Melbourne.

"Even with aggressive mitigation measures that limit global warming to less than 2 degrees above pre-industrial values by 2100, and with decreases of global temperature in the 22nd and 23rd centuries ... sea level continues to rise after 2100," they said in the journal Nature Climate Change.

This is because as warmer temperatures penetrate deep into the sea, the water warms and expands as the heat mixes through different ocean regions.

Even if global average temperatures fall and the surface layer of the sea cools, heat would still be mixed down into the deeper layers of the ocean, causing continued rises in sea levels.

If global average temperatures continue to rise, the melting of ice sheets and glaciers would only add to the problem.

The scientists calculated that if the deepest emissions cuts were made and global temperatures cooled to 0.83 degrees in 2100 - forecast based on the 1986-2005 average - and 0.55 degrees by 2300, the sea level rise due to thermal expansion would continue to increase - from 14.2cm in 2100 to 24.2cm in 2300.

If the weakest emissions cuts were made, temperatures could rise to 3.91 degrees Celsius in 2100 and the sea level rise could increase to 32.3cm, increasing to 139.4cm by 2300.

"Though sea-level rise cannot be stopped for at least the next several hundred years, with aggressive mitigation it can be slowed down, and this would buy time for adaptation measures to be adopted," the scientists added.

The study is available at www.nature.com/nclimate (Reporting by Nina Chestney; Editing by Pravin Char)

- AlertNet

Sea Level Rise Accelerating in U.S. Atlantic Coast

Rates of sea level rise are increasing three-to-four times faster along portions of the U.S. Atlantic Coast than globally, according to a new U.S. Geological Survey report published in Nature Climate Change.

Since about 1990, sea-level rise in the 600-mile stretch of coastal zone from Cape Hatteras, N.C. to north of Boston, Mass. -- coined a "hotspot" by scientists -- has increased 2 - 3.7 millimeters per year; the global increase over the same period was 0.6 – 1.0 millimeter per year.

Based on data and analyses included in the report, if global temperatures continue to rise, rates of sea level rise in this area are expected to continue increasing.

The report shows that the sea-level rise hotspot is consistent with the slowing of Atlantic Ocean circulation. Models show this change in circulation may be tied to changes in water temperature, salinity and density in the subpolar north Atlantic.

"Many people mistakenly think that the rate of sea level rise is the same everywhere as glaciers and ice caps melt, increasing the volume of ocean water, but other effects can be as large or larger than the so-called 'eustatic' rise," said USGS Director Marcia McNutt. "As demonstrated in this study, regional oceanographic contributions must be taken into account in planning for what happens to coastal property."

Though global sea level has been projected to rise roughly two-to-three feet or more by the end of the 21st century, it will not climb at the same rate at every location. Differences in land movements, strength of ocean currents, water temperatures, and salinity can cause regional and local highs and lows in sea level.

"Cities in the hotspot, like Norfolk, New York, and Boston already experience damaging floods during relatively low intensity storms," said Dr. Asbury (Abby) Sallenger, USGS oceanographer and project lead. "Ongoing accelerated sea level rise in the hotspot will make coastal cities and surrounding areas increasingly vulnerable to flooding by adding to the height that storm surge and breaking waves reach on the coast."

During the 21st century, the increases in sea level rise rate that have already occurred in the hotspot will yield increases in sea level of 8 to 11.4 inches by 2100. This regional sea level increase would be in addition to components of global sea level rise.

To determine accelerations of sea level, USGS scientists analyzed tide gauge data throughout much of North America in a way that removed long-term (linear) trends associated with vertical land movements. This allowed them to focus on recent changes in rates of sea-level rise caused, for example, by changes in ocean circulation.

The report, Hotspot of accelerated sea-level rise on the Atlantic coast of North America, was published in the journal Nature Climate Change.

- USGS

Global Mass Balance from GRACE

In the first comprehensive satellite study of its kind, a University of Colorado Boulder-led team used NASA data to calculate how much Earth's melting land ice is adding to global sea level rise.

Using satellite measurements from the NASA/German Aerospace Center Gravity Recovery and Climate Experiment (GRACE), the researchers measured ice loss in all of Earth's land ice between 2003 and 2010, with particular emphasis on glaciers and ice caps outside of Greenland and Antarctica. The total global ice mass lost from Greenland, Antarctica and all Earth's glaciers and ice caps over the period studied was about 4.3 trillion tons (1,000 cubic miles), adding about 12 millimeters (0.5 inches) to global sea level. That's enough ice to cover the United States 1.5 feet (0.5 meters) deep.

About a quarter of the average annual ice loss came from glaciers and ice caps outside of Greenland and Antarctica (about 148 billion tons, or 39 cubic miles), while ice loss from Greenland and Antarctica and their peripheral ice caps and glaciers averaged roughly 385 billion tons (100 cubic miles) a year. Results of the study are published online Feb. 8 in the journal Nature.

"Earth is losing a huge amount of ice to the ocean annually, and these new results will help us answer important questions in terms of both sea rise and how the planet's cold regions are responding to global change," said University of Colorado Boulder physics professor John Wahr, who helped lead the study.

"The strength of GRACE is it sees all the mass in the system, even though its resolution isn't high enough to allow us to determine separate contributions from each individual glacier," said Wahr, also a fellow at the University of Colorado-headquartered Cooperative Institute for Research in Environmental Sciences. Traditional estimates of Earth's ice caps and glaciers have been made using ground measurements from relatively few glaciers to infer what all the world's unmonitored glaciers were doing. Only a few hundred of the roughly 200,000 glaciers worldwide have been monitored for longer than a decade.

One unexpected study result from GRACE was that the estimated ice loss from high Asian mountain ranges like the Himalaya, the Pamir and the Tien Shan was only about 4 billion tons of ice annually. Some previous ground-based estimates of ice loss in these high Asian mountains have ranged up to 50 billion tons annually, Wahr said.

"The GRACE results in this region really were a surprise," said Wahr. "One possible explanation is that previous estimates were based on measurements taken primarily from some of the lower, more accessible glaciers in Asia and were extrapolated to infer the behavior of higher glaciers. But unlike the lower glaciers, most of the high glaciers are located in very cold environments, and require greater amounts of atmospheric warming before local temperatures rise enough to cause significant melting. This makes it difficult to use low-elevation, ground-based measurements to estimate results from the entire system."

"This study finds that the world's small glaciers and ice caps in places like Alaska, South America and the Himalayas contribute about 0.4 millimeters (.02 inches) per year to sea level rise," said Tom Wagner, cryosphere program scientist at NASA Headquarters in Washington. "While this is lower than previous estimates, it confirms that ice is being lost from around the globe, with just a few areas in precarious balance. The results sharpen our view of land ice melting, which poses the biggest, most threatening factor in future sea level rise."

Launched in 2002, the twin GRACE satellites track changes in Earth's gravity field by noting minute changes in gravitational pull caused by regional variations in Earth's mass, which for periods of months to years is typically due to movements of water on Earth's surface. It does this by measuring changes in the distance between its two identical spacecraft to one-hundredth the width of a human hair. The spacecraft, developed by NASA's Jet Propulsion Laboratory, Pasadena, Calif., are in the same orbit approximately 220 kilometers (137 miles) apart.

- NASA