[Tide gauges have been used to measure sea level for more than 130 years. Satellite measurements now complement the historical record. (NASA Earth Observatory image by Joshua Stevens, based on data from the Commonwealth Scientific and Industrial Research Organization and NOAA)]
For thousands of years, sea level has remained relatively stable and human communities have settled along the planet’s coastlines. But now Earth’s seas are rising. Globally, sea level has risen about eight inches since the beginning of the 20th century and more than two inches in the last 20 years alone. According to the federal National Aeronautics and Space Administration (NASA), “all signs suggest that this rise is accelerating”.
Seas around the world have risen an average of nearly 3 inches since 1992, with some locations rising more than 9 inches due to natural variation, according to the latest satellite measurements from NASA and its partners. An intensive research effort now underway, aided by NASA observations and analysis, points to “an unavoidable rise of several feet in the future”.
“Sea level rise is a natural [physical] consequence of the warming of our planet”, states NASA’s press release dated August 26, 2015. “We know this from basic physics. When water heats up, it expands. So when the ocean warms, sea level rises. When ice is exposed to heat, it melts. And when ice on land melts and water runs into the ocean [water from unprecedented ice and snow melt off Antarctica, Greenland, land areas north of permafrost region in Northern Hemisphere, mountainous glaciers receding, worldwide] sea level rises”.
As the ocean has warmed, polar ice has melted, and porous landmasses have subsided, global mean sea level has risen by 8 inches (20 centimeters) since 1870. The rate of sea level rise is faster now than at any time in the past 2,000 years, and that rate has doubled in the past two decades.
While NASA and other agencies continue to monitor the warming of the ocean and changes to the planet’s land masses, the biggest concern is what will happen to the ancient ice sheets covering Greenland and Antarctica, which continue to send out alerts that a warming planet is affecting their stability.
NASA has been recording the height of the ocean surface from space since 1992, recording about 2.9 inches (7.4 centimeters) of sea level rise averaged for all the oceans in that 23 year period.
In 2002, NASA and the German space agency launched the Gravity Recovery and Climate Experiment (GRACE) twin satellites, capable of measuring the movement of mass, hence gravity, around Earth at intervals of every 30 days. GRACE has found that earth’s land masses move very little in a month; however, earth’s water masses move through melting, evaporation, precipitation and other processes. GRACE records these movements of water around the planet, while a new NASA network of more than 3,000 floating ocean sensors spread across the entire open ocean supplement that ocean water level data.
Observations from the new NASA ocean level data collection systems have revolutionized scientists’ understanding of contemporary sea level rise and its causes. NASA’s newest release state that: “We know that today’s sea level rise is about one-third the result of the warming of existing ocean water, with the remainder coming from melting land ice”, adding that “currently, regional differences in sea level rise are dominated by the effects of ocean currents and well known cycles, such as the Pacific Ocean’s El Niño phenomenon and the Pacific Decadal Oscillation.” But as the ice sheets located closer to the poles of the planet (which receive less direct solar radiation due to the tilting of the planet as it orbits the Sun), and the once “permanent” ice located at higher elevations around the world continues to melt as a direct result of measurably warming temperatures occurring as a direct consequence of a stronger greenhouse effect that is already unnaturally high due to human activities such as fossil fuel burning, deforestation, and paving over the earth’s still green landscape. NASA scientists now predict that the increasing meltwater resulting from the stronger greenhouse effect will overtake the formerly natural causes of regional variations of the ocean water levels and be the most significant contributor to the overall rise in sea level.
The recent advances in observing the world’s frozen regions using satellite measurements from NASA and its participating organizations have allowed scientists to accurately estimate annual ice losses from Greenland and Antarctica. Not only can they now determine how much sea level around the world is changing – as measured by satellite for the past 23 years – but they can also determine how much of the sea level rise is being caused by our warming of the earth’s biosphere, which includes both the atmosphere at or near the surface, the oceans, the land surface, and the biota (plant and animal kingdoms) that all together comprise earth’s biosphere.
GRACE’s record, spanning over the last decade, shows that the ice loss around the planet is now accelerating in Greenland and West Antarctica. The record shows Greenland has shed on average 303 gigatons of ice every year since 2004, while Antarctica has lost on average 118 gigatons of ice per year. Much of Antarctica’s ice loss has been shown to come from West Antarctica’s ice loss. Greenland’s ice loss has accelerated by 31 gigatons of ice per year, every year since 2004, while West Antarctica’s ice loss has accelerated to 28 gigatons per year.
“Given what we know now about how the ocean expands as it warms and how ice sheets and glaciers are adding water to the seas, it’s pretty certain we are locked into at least 3 feet of sea level rise and probably more,” said Steve Nerem of the University of Colorado, Boulder, and lead of NASA’s new Sea Level Change Team.
The Greenland Ice Sheet, spanning 660,000 square miles (an area almost as big as Alaska), and with a thickness at its highest point of almost 2 miles, has the potential to raise the world’s oceans by more than 20 feet. Situated in the Arctic, which is warming at twice the rate of the rest of the planet, Greenland has been shedding more ice in the summer than it gains back in the winter since 1992.
“In Greenland, everything got warmer at the same time: the air, the ocean surface, the depths of the ocean,” said Ian Joughin, a glaciologist at University of Washington. “We don’t really understand which part of that warming is having the biggest effect on the glaciers.”
What scientists do know is that warming Arctic temperatures – and a darkening surface of the Greenland ice sheet – are causing so much summer melting that it is now the dominant factor in Greenland’s contribution to sea level rise.
NASA has found that Greenland’s summer melt season now lasts 70 days longer than it did in the early 1970s. Every summer, warmer air temperatures cause melt over about half of the surface of the ice sheet – although recently, 2012 saw an extreme event where 97 percent of the ice sheet experienced melt at its top layer.
Greenland’s massive glaciers have sped up, too. Though many of the glaciers in the southeast, west and northwest of the island – an area that experienced quick thinning from 2000 to 2006 – have now slowed down, the melting rate at other areas Greenland’s massive ice sheet has not slowed. A study last year showed that the northeast Greenland ice stream had increased its ice loss rate due to warmer air temperatures.
“The early 2000s was when some big things revealed themselves, such as when we saw the fastest glacier we knew of, the Jakobshavn ice stream in Greenland, double its speed,” said Waleed Abdalati, director of the Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, and former NASA chief scientist. “The subsequent surprise was that these changes could be sustained for a decade – Jakobshavn is still going fast”, Abdalati said.
The Antarctic Ice Sheet covers nearly 5.4 million square miles, and area larger than the United States and India combined, and contains enough ice to raise the ocean level by about 190 feet. The Transantarctic Mountains split Antarctica in two major regions: West Antarctica and the much larger East Antarctica.
Though Antarctica’s contribution to sea level rise is still at less than 0.02 inches (0.5 millimeters) per year, several events over the past decade and a half have prompted experts to start warning about the possibility of more rapid changes this century.
The mountainous horn of the continent, the Antarctic Peninsula, gave one of the earliest warnings on the impact of a changing climate in Antarctica when warm air and warmer ocean temperatures led to the dramatically fast breakup of the Larsen B ice shelf in 2002. In about a month, 1,250 square miles of floating ice that had been stable for over 10,000 years were gone. In the following years, other ice shelves in the Peninsula, including the last remainder of Larsen B, collapsed, speeding up in the flow of the land lying glaciers that they were buttressing against the warming ocean.
In 2014, two studies focusing on the acceleration of the glaciers in the Amundsen Sea sector of West Antarctica showed that its collapse is currently well underway. And while one of the studies speculated that the demise of the ice overlaying West Antarctica could take as long as 200 to 1,000 years, depending on how rapidly the ocean heats up, both studies concurred that its collapse is already unstoppable, and that when it does collapse, the melt water will add up to 12 feet of sea level rise to the oceans.
The wind is also a factor in determining the timing of West Antarctica’s collapse. The “westerlies”, the winds that spin the ocean waters around Antarctica, are known to have intensified during the last decade, pushing the cold top layer away from the land, and thus allowing the warmer, deeper waters to rise and spill over the border of the continental shelf, flowing all the way back to the base of many of the ice shelves jetting out from the continent. As the ice shelves weaken from underneath, the glaciers behind them are predicted to speed up.
East Antarctica’s massive ice sheet, as vast as the lower continental U.S., remains an unknown in projections of sea level rise. Though it appears to be stable, a recent study on Totten Glacier, East Antarctica’s largest and most rapidly thinning glacier, hints otherwise. Research found two deep troughs that could lead warm ocean water to the base of the glacier and melt it in a similar way to what’s happening to the glaciers in West Antarctica. Other sectors grounded below sea level, such as the Cook Ice Shelf, Ninnis, Mertz and Frost glaciers, have also been found to be losing mass.
For the West Antarctic Ice Sheet, which largely rests on a bed that lies below sea level, the main driver of ice loss is the ocean. The waters of the Southern Ocean are layered: on top and at the bottom, the temperatures are frigid, but the middle layer is warm. The westerlies, the winds that spin the ocean waters around Antarctica, have intensified during the last decade, pushing the cold top layer away from the land. This allows the warmer, deeper waters to rise and spill over the border of the continental shelf, flowing all the way back to the base of many ice shelves. As the ice shelves weaken from underneath, the glaciers behind them speed up.
East Antarctica’s massive ice sheet, as vast as the lower continental U.S., remains the main unknown in projections of sea level rise. Though it appears to be stable, a recent study on Totten Glacier, East Antarctica’s largest and most rapidly thinning glacier, hints otherwise. The research found two deep troughs that could lead warm ocean water to the base of the glacier and melt it in a similar way to what’s happening to the glaciers in West Antarctica. Other sectors grounded below sea level, such as the Cook Ice Shelf, Ninnis, Mertz and Frost glaciers, are also losing mass.