Anthonares

Late last week I ran across an article that finally spurred me to write a Published Research Synopsis after a break of nearly 6 months. What got me so excited is not just the very important conclusion (Greenland is melting, and faster than measured before), but also the means by which that conclusion was reached.

Because of the enormous difficulty in measuring the mass of a continental ice sheet, Greenland’s ice mass budget has always been calculated indirectly. Some of the more exciting methods have involved using satellite radar interferometry (discussed here in this previous PRS) to measure the speed of ice sheet flow, but this requires several assumptions that can introduce significant errors. Other methods rely on spotty measurements of ice sheet thickness, snow accumulation, and meltwater discharge that also require lots of assumptions and extrapolation.

This week we take a look at a new result that measures the mass of the Greenland ice sheet by, well, by measuring the mass. The Gravity Recovery and Climate Experiment (GRACE) is a pair of orbiting satellites that measures the static gravity field of the Earth by precisely tracking the locations of each craft in relation to each other–but more on that later. GRACE has already turned out some amazing results including measuring the changes in moisture of large river basins, monitoring ocean currents, and measuring the rate of mass loss on Antarctica. This new set of data present what is essentially now a complete global picture of ice sheet melting, and it’s one that should make us all pay closer attention.

Citation:

Velicogna, I, and J Wahr (2006). “Acceleration of Greenland ice mass loss in spring 2004.” Nature 443(7109), pp. 329-331. [online at CiteULike.org]

Synopsis:

The data returned from the GRACE mission, explained in detail in the next section, are what are called “gravity anomalies”. The “anomaly” means that the data deviate from some expected value, or in this case, a previous measurement by GRACE. Positive anomalies indicate that more mass is present than expected, and negative anomalies indicate just the opposite.

Presented with the raw anomaly data shown on the right, Velicogna and Wahr examined five possible contributions to the data:

1. Water stored in the crust outside the Greenland ice sheet
2. Water stored in the oceanic crust surrounding Greenland
3. Atmospheric mass
4. Changes in crust density due to post-glacial rebound (the flexing of the Earth’s crust after a massive object is removed)
5. Changes in mass stored in the Greenland ice sheet

To remove each of the four effects from their analysis, Velicogna and Wahr used a series of models combined with data on ocean bottom and atmospheric pressures. The figure on the right does not yet account for the first four effects.

As it turns out, those first four effects had a net positive effect on the gravity anomaly, and removing them increased the estimated rate of Greenland ice sheet mass loss to an average of 248+/-36 km³/year for April 2002-2006, which equates to a global sea level rise of 0.5+/-0.1 mm/yr. The satellite-derived measurement of mass loss for 2005 discussed in a previous PRS was 224 km³/yr, well within the range of uncertainty in these results. The figure on the left shows the best estimate of mass loss from both the North and South Greenland ice sheets (the data were smoothed in order to remove month-to-month variability that can obscure overall trends). This figure shows two very important trends:

1. South Greenland is melting a whole lot more quickly than North Greenland, and
2. South Greenland is melting much more quickly in early 2006 than it was in early 2003. The melting of Greenland is accelerating.

Context:

Though this study requires a few models to extract the signal of ice sheet mass loss from other factors contributing to the total gravity anomaly, it is what I consider to be a direct measurement. In other words, the data you want is what you measure. GRACE provides us a unique view of our planet. For the first time we can see the fluxes of water, rock, and magma within our dynamic Earth.

The very close agreement between the results of this and other studies (including ones I haven’t mentioned here) pretty much seals the deal: Greenland is melting, and that melt rate is accelerating. The current (2001) Intergonvermental Panel on Climate Change (IPCC) report–the official word on scientific findings related to climate change–forecasts Greenland’s contribution to global sea level change to be between -20 and 90 mm over this century. If the rate we see in late 2005 and early 2006 continues without increasing, then we should expect around 70 mm of rise from Greenland melting.

The figure on the right is from a similar article published in March in Science by the same authors that shows the rate of mass loss from the West Antarctic Ice Sheet (red) and the much larger East Antarctic Ice Sheet (green). Their best estimate of mass loss showed the East Antarctic Ice Sheet to be roughly in balance, +/-56 km³/yr, and the West Antarctic Ice Sheet to be losing approximately 152+/-21km³/yr. These results, unlike those from Greenland, are highly influenced by the model of post-glacial rebound (item 4 in the list above), which accounts for the very large error bars on the estimate. Either way, those results show that Antarctica is actually losing mass, something that the IPCC predicted would not happen during the next century.

In a few more years, thanks now in large part to GRACE, the picture on global ice sheet masses will be much more complete. Improvements in estimates of post-glacial rebound in Antarctica will reduce uncertainty in those estimates, and the lengthened data record will help to understand trends more clearly. Also, we should start seeing the first studies on global sea level changes from GRACE. The data is coming in very quickly now, let’s just hope that policy makers can somehow keep up!

General Explanations

The Gravity Recovery and Climate Experiment (GRACE)
GRACE relies on several positioning techniques to determine both the absolute and relative positions between the two satellites with great accuracy. Each vehicle is equipped with accelerometers, star cameras, and GPS receivers. Additionally, they measure the change in distance to about 10 microns between each other by monitoring the doppler shift of a K-band microwave signal. Also, laser corner-cube reflectors provide highly accurate distances to ground stations from each craft

These measurements then tell us about the Earth’s static gravity field in the following manner: as the two craft fly in the same orbit, one in front of the other, slight increases or decreases in the mass acting on each craft changes the distance between them. Consider this example, when orbiting near the Himalayas, the first craft in the formation will “feel” the massive gravity of the mountains first and it will be pulled toward the mountains, away from the second craft. Then, the second craft will catch up once it too “feels” the Himalayas. The opposite is true as well, the first craft can seem to be slowed by a less-massive portion of the crust. GRACE’s orbit allows for measurements of the global gravity anomaly about once each month.

Post-Glacial Rebound
You may have encountered this term before as isostatic rebound, or isostacy. Basically, because the Earth’s crust overlies a viscous fluid mantle, concentrations of mass on the crust because of large lakes or ice sheets actually pushes the crust down beneath those objects. Those objects tend to leave much more quickly than the crust can respond, so areas that were covered in continental glaciers over 12,000 years ago are still rebounding today. Both Greenland and Antarctica had larger ice sheets in the past, so they, too are still undergoing post-glacial rebound. This effect, along with a variety of others, also complicates global sea-level measurements as shorelines rise the ocean appears to fall when in reality its volume has not changed.