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Melting Glaciers Muck Up Earth’s Gravitational Field

The views expressed are those of the author and are not necessarily those of Scientific American.

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Photographs never quite capture the sparkling blue tint of glacial ice, so when I visited the Perito Moreno glacier in Patagonia on a backpacking trip through South America some years ago, I was happy to get this camera angle: the blue of the Argentine flag gives you a sense of what the blue of the ice looks like in person. Perito Moreno is still rolling on, but the vast majority of the world’s rivers of ice are retreating. Forget the models, the abstractions, the rhetoric: visit a glacier at regular intervals and you will see climate change right before your eyes.

But how can we be sure that glacial melting is truly a global phenomena rather than a string of unrepresentative anecdotes? Individual glaciers have always receded and advanced as local and global conditions change. To see the regional and planetary situation, climate scientists used to have to extrapolate from field studies of individual glaciers. Only about a decade ago did they begin to monitor glaciers on a global scale by using measurements of Earth’s gravitational field.

I recently met one of the scientists mining this data, geophysicist Emma Hill, on a visit to the Earth Observatory of Singapore, located at Nanyang Technological University. She and her colleagues had a paper last month on the effects of glacial melting in Alaska, and they’re rather more intricate than I’d expected.

The data comes from the joint NASA-German GRACE satellite launched in 2002. It actually consists of two satellites that use a microwave link to monitor the distance between them for slight deviations. The satellites move together or spread apart as they pass over areas of stronger and weaker gravity, demonstrating that our planet is not a perfectly spherical ball of uniform density. Mathematically, scientists represent the gravitational field in terms of spherical harmonics, which quantify undulations on different angular scales. GRACE has a resolution of a fraction of a degree and the data set consists of millions of numerical coefficients.

What I found engrossing was the richness of the data. Glaciers have multiple effects on Earth’s gravitational field. First, they are large lumps of mass that exert a direct gravitational pull on the satellites. Second, they also tug on nearby ocean water, causing a localized rise in sea level, which in turn acts on the satellite, amplifying the direct effect of the ice. Third, glaciers weigh down the land, pushing down on Earth’s crust. On the timescales we’re talking about, the depression of the land surface is elastic, like bending a plank of wood, as opposed to a fluidlike flow of rock.

The latter two effects complicate what glacier melting does to the local sea level, the height of the sea relative to the shoreline at a given location. Although the global average sea level has been rising several centimeters per decade as glaciers melt and ocean water warms and expands, the local sea level in polar regions can decrease, as water is no longer attracted gravitationally to glaciers and land freed of its icy yoke pops back up.

In Alaska, Hill’s team estimates that the sea level has been decreasing by nearly 1 centimeter per year–which is good news for Alaska, but only worsens the sea level rise in the tropics. Elsewhere, the first GRACE results, announced five years ago, showed that Greenland is shedding ice much faster than expected. More recent findings suggest the melting both there and in Antarctica has been accelerating.

Considering that melting ice caps are probably the single biggest climate threat, you’d think that governments would put a high priority on monitoring them. Alas, like many other Earth-observing missions, GRACE may well die before its replacement is good to go. The onboard batteries are no longer able to hold a full charge, so the instruments have to be cycled on and off, causing loss of data. “The satellites are showing their age,” says GRACE’s science operations manager, Srinivas Bettadpur of the University of Texas.

Relief, in the form of the unimaginatively named GRACE Follow-On, won’t come for another five or six years, meaning that there will be no overlap of the missions to help with calibration of the new instruments. Other instruments can provide partial coverage, notably the European GOCE mission, but GOCE was designed for high spatial resolution rather than tracking changes over time. While we’re not looking, a lot of that beautiful blue ice will have turned to beautiful blue sea.

Photo by George Musser. Diagrams courtesy of Emma Hill, Earth Observatory of Singapore.

George Musser About the Author: is a contributing editor at Scientific American. He focuses on space science and fundamental physics, ranging from particles to planets to parallel universes. He is the author of The Complete Idiot's Guide to String Theory. Musser has won numerous awards in his career, including the 2011 American Institute of Physics's Science Writing Award. Follow on Twitter @gmusser.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. thevillagegeek 10:31 pm 12/22/2011

    How long before the first contrarian’s post that tries to spin the local ‘sea level drop’ caused by the melting of glaciers into denial of global climate shifts and sea level rise?

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  2. 2. jdey123 8:18 am 12/23/2011

    The inconvenient truth is that global warming which started around 1977 and ceased around 2001, since when temperatures have been flat. There are a number of systems that measure global mean temperature, but GISS is the one that has presented the most compelling case for global warming. The right hand column is the 5 year mean, representing the temperatures 2 years back and 2 years forward plus current year’s global mean temperature.

    Since 2001, the warmists have been desperately clinging on to a story which is their only means of income. Warmists claim that a 5 year mean is a short term trend, and if you look at the long term trend which they conveniently say is 30 years, the true picture emerges. Of course, dependent on how you select the period of time that you are plotting the statistics, you will get an entirely different view as to whether the planet is warming, at a constant temperature or cooling. Statistics can be all things to all men.

    Until this year, warmists could still comfort themselves that the sea level was rising. There has now been a 6mm drop according to Nasa. Warmists are claiming that this global measurement is down to a weak La Nina event which ended early 2011 and which happens every 2-8 years. There was no drop when a previously weak La Nina event occurred in 2008.

    Of course, even if the planet is warming, is it due to mankind? The climate is complex and there are a huge amount of interactions involved in it e.g. volcanic eruptions, formation of mountains, reflection of sunlight from ice, cloud cover, oceanic currents, levels of sunlight, all of the gases in the atmosphere etc. etc. However, the majority of warmists claim that you can discount all of this, and just look at CO2 (a trace gas in our atmosphere representing 0.039% of the air that we breathe). The only reason why they have decided to choose CO2, is because there has been a demonstrable increase in the levels of this gas since the industrial revolution. Paleoclimatology (the study of climate from prehistory until today) shows no correlation between CO2 and global mean temperature. Warmists choose to ignore all of the science obtained by paleoclimatology scientists. CO2 doesn’t remain in the air for long, as it dissolves in the rain and is transferred to the land and oceans. It is used by sea creatures to create their shells, and by plants to breathe. When they die, over thousands of years, they decompose in to carbon. Mankind then extracts the carbon in the form of coal (from plants) and oil (from creatures). This process is known as the carbon cycle. Warmists claim that the carbon cycle was in equilibrium until mankind extracted coal and oil. However, mother nature also used to do this in the form of volcanic eruptions, forest fires etc. Mankind’s contribution to the carbon cycle pales in to insignificance compared to nature’s.

    Warmists need to spread alarmist stories about how the earth is facing catastrophe in order to shore up their support amongst a skeptical general public. Rest assured, there is no science behind this. The earth may be warming, cooling or static. The science needs to become much more sophisticated before we know the truth. It’s certainly been much warmer than the warmists project it will become in the next 500 years during the period that human life has evolved on this planet, however.

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  3. 3. neilrued 8:22 am 12/23/2011

    I wonder if poor funding on this follow-up study has been politically motivated, through fossil fuel companies motivated by greed, lobbying politicians to delay for as long as they can, further potentially successful scientific research programmes, whose data provide undeniable proof that climate change is not only happening, but may be getting worse than previously thought.

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  4. 4. jtdwyer 9:12 am 12/23/2011

    If I understand correctly, the gravitational effect of rising sea levels near a massive coastal glacier should hold true for any shoreline composed of dense material.

    As I understand, the effect being suggested here is the local reduction in sea level near a melting coastal glacier as its frozen mass diminishes.

    I would expect that any such local sea level reduction would generally be accompanied by a global sea level increase commensurate with the total reduction near diminishing coastal glaciers. Generally, in these conditions I’d expect the total mass and volume of sea water to increase…

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  5. 5. Wayne Williamson 4:23 pm 12/23/2011

    Lets see…replacing an old sat with something modern should cost a fraction of the original(If they don’t try to add more stuff(scientific term;-)). For once just replace whats there instead of reinventing it and costing a fortune….

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