Continuous satellite monitoring of ice sheets needed to better predict sea-level rise

The findings, published in Nature Geoscience, underscore the need for continuous satellite monitoring of the ice sheets to better identify and predict melting and the corresponding sea-level rise.

The ice sheets covering Antarctica and Greenland contain about 99.5 per cent of the Earth’s glacier ice which would raise global sea level by some 63m if it were to melt completely. The ice sheets are the largest potential source of future sea level rise – and they also possess the largest uncertainty over their future behaviour. They present some unique challenges for predicting their future response using numerical modelling and, as a consequence, alternative approaches have been explored. One common approach is to extrapolate observed changes to estimate their contribution to sea level in the future.

Since 2002, the satellites of the Gravity Recovery and Climate Experiment (GRACE) detect tiny variations in Earth’s gravity field resulting from changes in mass distribution, including movement of ice into the oceans. Using these changes in gravity, the state of the ice sheets can be monitored at monthly intervals.

Dr Bert Wouters, currently a visiting researcher at the University of Colorado, said: “In the course of the mission, it has become apparent that ice sheets are losing substantial amounts of ice – about 300 billion tonnes each year – and that the rate at which these losses occurs is increasing. Compared to the first few years of the GRACE mission, the ice sheets’ contribution to sea level rise has almost doubled in recent years.”

Yet, there is no consensus among scientists about the cause of this recent increase in ice sheet mass loss observed by satellites. Beside anthropogenic warming, ice sheets are affected by many natural processes, such as multi-year fluctuations in the atmosphere (for example, shifting pressure systems in the North Atlantic, or El Niño and La Niña events) and slow changes in ocean currents.

“So, if observations span only a few years, such ‘ice sheet weather’ may show up as an apparent speed-up of ice loss which would cancel out once more observations become available,” Dr Wouters said.

The team of researchers compared nine years of satellite data from the GRACE mission with reconstructions of about 50 years of mass changes to the ice sheets. They found that the ability to accurately detect an accelerating trend in mass loss depends on the length of the record.

At the moment, the ice loss detected by the GRACE satellites is larger than what we would expect to see just from natural fluctuations, but the speed-up of ice loss over the last years is not.

The study suggests that although there may be almost enough satellite data to detect a speed-up in mass loss of the Antarctic ice sheet with a reasonable level of confidence, another ten years of satellite observations is needed to do so for Greenland. As a result, extrapolation of the current contribution to sea-level rise of the ice sheets to 2100 may be too high or low by as much as 35 cm. The study, therefore, urges caution in extrapolating current measurements to predict future sea-level rise.

Continuous satellite monitoring of ice sheets needed to better predict sea-level rise

The findings, published in Nature Geoscience, underscore the need for continuous satellite monitoring of the ice sheets to better identify and predict melting and the corresponding sea-level rise.

The ice sheets covering Antarctica and Greenland contain about 99.5 per cent of the Earth’s glacier ice which would raise global sea level by some 63m if it were to melt completely. The ice sheets are the largest potential source of future sea level rise – and they also possess the largest uncertainty over their future behaviour. They present some unique challenges for predicting their future response using numerical modelling and, as a consequence, alternative approaches have been explored. One common approach is to extrapolate observed changes to estimate their contribution to sea level in the future.

Since 2002, the satellites of the Gravity Recovery and Climate Experiment (GRACE) detect tiny variations in Earth’s gravity field resulting from changes in mass distribution, including movement of ice into the oceans. Using these changes in gravity, the state of the ice sheets can be monitored at monthly intervals.

Dr Bert Wouters, currently a visiting researcher at the University of Colorado, said: “In the course of the mission, it has become apparent that ice sheets are losing substantial amounts of ice – about 300 billion tonnes each year – and that the rate at which these losses occurs is increasing. Compared to the first few years of the GRACE mission, the ice sheets’ contribution to sea level rise has almost doubled in recent years.”

Yet, there is no consensus among scientists about the cause of this recent increase in ice sheet mass loss observed by satellites. Beside anthropogenic warming, ice sheets are affected by many natural processes, such as multi-year fluctuations in the atmosphere (for example, shifting pressure systems in the North Atlantic, or El Niño and La Niña events) and slow changes in ocean currents.

“So, if observations span only a few years, such ‘ice sheet weather’ may show up as an apparent speed-up of ice loss which would cancel out once more observations become available,” Dr Wouters said.

The team of researchers compared nine years of satellite data from the GRACE mission with reconstructions of about 50 years of mass changes to the ice sheets. They found that the ability to accurately detect an accelerating trend in mass loss depends on the length of the record.

At the moment, the ice loss detected by the GRACE satellites is larger than what we would expect to see just from natural fluctuations, but the speed-up of ice loss over the last years is not.

The study suggests that although there may be almost enough satellite data to detect a speed-up in mass loss of the Antarctic ice sheet with a reasonable level of confidence, another ten years of satellite observations is needed to do so for Greenland. As a result, extrapolation of the current contribution to sea-level rise of the ice sheets to 2100 may be too high or low by as much as 35 cm. The study, therefore, urges caution in extrapolating current measurements to predict future sea-level rise.

Deep oceans may mask global warming for years at a time

Earth’s deep oceans may absorb enough heat at times to flatten the rate of global warming for periods of as long as a decade–even in the midst of longer-term warming. This according to a new analysis led by scientists at the National Center for Atmospheric Research (NCAR).

The study, based on computer simulations of global climate, points to ocean layers deeper than 1,000 feet as the main location of the “missing heat” during periods such as the past decade when global air temperatures showed little trend.

The findings also suggest that several more intervals like this can be expected over the next century, even as the trend toward overall warming continues.

“We will see global warming go through hiatus periods in the future,” says NCAR’s Gerald Meehl, lead author of the study.

“However, these periods would likely last only about a decade or so, and warming would then resume. This study illustrates one reason why global temperatures do not simply rise in a straight line.”

The research, by scientists at NCAR and the Bureau of Meteorology in Australia, was published online Sunday in Nature Climate Change.

Funding for the study came from the National Science Foundation (NSF), NCAR’s sponsor.

“The research shows that the natural variability of the climate system can produce periods of a decade or more in which Earth’s temperature does not rise, despite an increase in greenhouse gas concentrations,” says Eric DeWeaver, program director in NSF’s Division of Atmospheric and Geospace Sciences.

“These scientists make a compelling case that the excess energy entering the climate system due to greenhouse gas increases may not be immediately realized as warmer surface temperatures, as it can go into the deep ocean instead.”

The 2000s were Earth’s warmest decade in more than a century of weather records.

However, the single-year mark for warmest global temperature, which had been set in 1998, remained unmatched until 2010.

Yet emissions of greenhouse gases continued to climb during this period, and satellite measurements showed that the discrepancy between incoming sunshine and outgoing radiation from Earth actually increased.

This implied that heat was building up somewhere on Earth, according to a 2010 study by NCAR researchers Kevin Trenberth and John Fasullo.

The two scientists, who are both co-authors on the new study, suggested that the oceans might be storing some of the heat that would otherwise go toward other processes, such as warming the atmosphere or land, or melting more ice and snow.

Observations from a global network of buoys showed some warming in the upper ocean, but not enough to account for the global build-up of heat.

Although scientists suspected the deep oceans were playing a role, few measurements were available to confirm that hypothesis.

To track where the heat was going, Meehl and colleagues used a powerful software tool known as the Community Climate System Model, which was developed by scientists at NCAR and the Department of Energy with colleagues at other organizations.

Using the model’s ability to portray complex interactions between the atmosphere, land, oceans and sea ice, they performed five simulations of global temperatures.

The simulations, which were based on projections of future greenhouse gas emissions from human activities, indicated that temperatures would rise by several degrees during this century.

But each simulation also showed periods in which temperatures would stabilize for about a decade before climbing again.

For example, one simulation showed the global average rising by about 2.5 degrees Fahrenheit (1.4 degrees Celsius) between 2000 and 2100, but with two decade-long hiatus periods during the century.

During these hiatus periods, simulations showed that extra energy entered the oceans, with deeper layers absorbing a disproportionate amount of heat due to changes in oceanic circulation.

The vast area of ocean below about 1,000 feet (300 meters) warmed by 18 percent to 19 percent more during hiatus periods than at other times.

In contrast, the shallower global ocean above 1,000 feet warmed by 60 percent less than during non-hiatus periods in the simulation.

“This study suggests the missing energy has indeed been buried in the ocean,” Trenberth says. “The heat has not disappeared and so it cannot be ignored. It must have consequences.”

The simulations also indicated that the oceanic warming during hiatus periods has a regional signature.

During a hiatus, average sea-surface temperatures decrease across the tropical Pacific, while they tend to increase at higher latitudes, especially in the Atlantic, where surface waters converge to push heat into deeper oceanic layers.

These patterns are similar to those observed during a La Niña event, according to Meehl.

He adds that El Niño and La Niña events can be overlaid on top of a hiatus-related pattern.

Global temperatures tend to drop slightly during La Niña, as cooler waters reach the surface of the tropical Pacific, and they rise slightly during El Niño, when those waters are warmer.

“The main hiatus in observed warming has corresponded with La Niña conditions, which is consistent with the simulations,” Trenberth says.

La Niña Conditions Strengthen, Expected To Continue





The ongoing La Niña event started in the third quarter of 2007 and has already influenced climate patterns during the last six months across many parts of the globe, including in the Equatorial Pacific, across the Indian Ocean, Asia, Africa and the Americas. (Credit: NOAA/National Weather Service)
The ongoing La Niña event started in the third quarter of 2007 and has already influenced climate patterns during the last six months across many parts of the globe, including in the Equatorial Pacific, across the Indian Ocean, Asia, Africa and the Americas. (Credit: NOAA/National Weather Service)

The current La Niña event, characterized by a cooling of the sea surface in the central and eastern Equatorial Pacific, has strengthened slightly in recent months and is expected to continue through the first quarter of 2008, with a likelihood of persisting through to the middle of the year.



The ongoing La Niña event started in the third quarter of 2007 and has already influenced climate patterns during the last six months across many parts of the globe, including in the Equatorial Pacific, across the Indian Ocean, Asia, Africa and the Americas.



During the last three months, La Niña conditions have become slightly stronger. Sea surface temperatures are now about 1.5 to 2 degrees Celsius colder than average over large parts of the central and eastern Equatorial Pacific. This La Niña is in the mid range of past historically recorded events, but the slight further cooling in recent months will likely place it on the stronger side of the middle range.



During a La Niña event, sea surface temperatures in the central and eastern Equatorial Pacific become cooler than normal. Such cooling has important effects on the global weather, particularly rainfall. While sea surface temperatures cool in the central and eastern Equatorial Pacific, those in the west remain warmer. This is associated with increases in the frequency of heavy rain and thunderstorms in surrounding regions.


In contrast to La Niña, the El Niño phenomenon is characterized by substantially warmer than average sea surface temperatures in the central and eastern Equatorial Pacific.



These temperature changes in the Equatorial Pacific related to La Niña and El Niño are strongly linked to major global climate fluctuations and, once initiated, can last for 12 months or more.



Most interpretations of existing climatological data suggest the likelihood of La Niña conditions remaining heightened through the second quarter of 2008 and, at a lower level of confidence, into the first part of the third quarter.



Longer seasonal forecasts beyond the third quarter of 2008 are not considered to contain useful information at this stage on the continuation of La Niña or the rise of El Niño.



It is rare for a La Niña event to persist for two years or more, such as occurred from early 1998 to early 2000. The likelihood of the current La Niña continuing for such a period will remain unclear for some months, but will be closely monitored. Long-term statistics suggest it is more likely that in the latter part of 2008, neutral conditions will prevail, i.e., neither La Niña nor El Niño with no significant cooling or warming of Equatorial Pacific sea surface temperatures.

NASA Observes La Nina: This ‘Little Girl’ Makes A Big Impression





The blue area throughout the center of this image shows the cool sea surface temperature along the equator in the Pacific Ocean during this La Niña episode. (Credit: NASA/Goddard's Scientific Visualization Studio)
The blue area throughout the center of this image shows the cool sea surface temperature along the equator in the Pacific Ocean during this La Niña episode. (Credit: NASA/Goddard’s Scientific Visualization Studio)

Cool, wet conditions in the Northwest, frigid weather on the Plains, and record dry conditions in the Southeast, all signs that La Niña is in full swing.



With winter gearing up, a moderate La Niña is hitting its peak. And we are just beginning to see the full effects of this oceanographic phenomenon, as La Niña episodes are typically strongest in January.



A La Niña event occurs when cooler than normal sea surface temperatures form along the equator in the Pacific Ocean, specifically in the eastern to central Pacific. The La Niña we are experiencing now has a significant presence in the eastern part of the ocean.



The cooler water temperatures associated with La Niña are caused by an increase in easterly sea surface winds. Under normal conditions these winds force cooler water from below up to the surface of the ocean. When the winds increase in speed, more cold water from below is forced up, cooling the ocean surface.



“With this La Niña, the sea-surface temperatures are about two degrees colder than normal in the eastern Pacific and that’s a pretty significant difference,” says David Adamec of NASA’s Goddard Space Flight Center, Greenbelt, Md. “I know it doesn’t sound like much, but remember this is water that probably covers an area the size of the United States. It’s like you put this big air conditioner out there — and the atmosphere is going to feel it.”



While this “air conditioner” may be located in the equatorial Pacific Ocean, it has a great influence on the weather here in the United States and across the globe.



The cool water temperatures of a La Niña slow down cloud growth overhead, causing changes to the rainfall patterns from South American to Indonesia. These changes in rainfall affect the strength and location of the jet stream — the strong winds that guide weather patterns over the United States. Since the jet stream regulates weather patterns, any changes to it will have a great impact on the United States.



Those changes can be felt throughout the country. The Northwest generally experiences cooler, wetter weather during a La Niña. On the Great Plains, residents normally see a colder than normal winter and southeastern states traditionally experience below average rainfall.



The cooler waters of a La Niña event also increase the growth of living organisms in this part of the ocean. La Niñas amplify the normal conditions in the Pacific. These typically cool and abundant waters experience an increase in phytoplankton growth when the water temperature drops even further.


The increased circulation that brings up cold water from below also brings up with it nutrients from the deeper waters. These nutrients feed the organisms at the bottom of the food chain, starting a reaction that increases life in the ocean. NASA’s SeaWiFS satellite documented this increase in phytoplankton during the last La Niña period in 1998.



La Niña and El Niño episodes tend to occur every three to five years. La Niñas are often preceded by an El Niño, however this cycle is not guaranteed.



The lengths of La Niña events vary as well. “We need to watch to see if this La Niña diminishes, because they can last for multiple years. And if it does last for multiple years, the southern tier of the United States, especially the Southeast, can expect dryer weather. That is not a good situation. If this La Niña behaves like a normal event, we should see signs that it is beginning to weaken by February,” says Adamec.



So far this La Niña is behaving like a textbook case: following the predicted weather patterns, strengthening throughout the winter, and peaking toward January. According to NOAA’s Climate Prediction Center, this La Niña episode is expected to continue until the spring of 2008, with a gradual weakening starting in February.



NASA will continue to monitor this phenomenon with several of its key Earth observing satellites.



Instruments on NASA’s Terra and Aqua satellites measure sea surface temperature and observe changes to life in the ocean, changes of great importance to the fishing industry. The MODIS instruments on these satellites detected the temperature drop that signaled this La Niña period, and SeaWiFS continues to monitor ocean life.



Scientists also look at sea surface height to understand La Niña. The cooler ocean water associated with a La Niña contracts, lowering sea-surface heights. Over the past year, NASA’s Jason satellite has observed a lower than normal sea level along the equatorial Pacific where this current La Niña episode is taking place.



NASA also looks at changes in wind and rain patterns to study La Niña. The QuikSCAT satellite measures changes in oceanic surface winds, while the Tropical Rainfall Measuring Mission satellite observes changes in rainfall. These observations add to a fuller understanding of this phenomenon.



The current La Niña episode has far many reaching effects. What some may see as just a small change in sea surface temperature has a much greater impact on our climate here in the U.S. and across the globe, as well as implications for the fishing industry and the global economy. With the help of NASA’s earth observing fleet, scientists are becoming better equipped to observe and understand this phenomenon.