Greenland and Antarctica are two places that climate scientists are studying intensely because of the potential for significant sea level rise were they to melt. Over the past 20 years, scientists have used a variety of methods to track ice loss, and they have found that Greenland has been losing ice more rapidly over the past decade than it had in the 1990s.  In fact, since 2004, ice loss has accelerated to such a high level that Greenland is now losing about 270 billion tons of ice per year.  Greenland’s contribution to sea level rise has been about 0.13-0.74 mm/yr, or about 4-23% of global sea level rise observed from 1993-2005.

In this week’s issue^1,2 of Science (subscription required), Michiel van den Broeke and colleagues used a couple of methods to confirm that this acceleration of ice loss is real and to understand why it’s happening.

The extent of ice in a glacier is like a bank account, but instead of money, we’re keeping track of ice.  When inflows (precipitation = snow) exceed outflows (mostly due to melting and runoff), the ice sheet gets bigger, just like a bank account grows when deposits exceed withdrawals.  We say that there is a positive surface mass balance.  When outflows exceed inflows, then the ice sheet shrinks, and we say there is a negative surface mass balance.

They found that before 1996, Greenland’s ice sheet had a positive mass balance (getting bigger) because precipitation exceeded runoff.   Between 1996-2004, precipitation and runoff both increased, and since these roughly cancel out one another, the ice sheet didn’t change much.  However, after 2004, precipitation stopped increasing while runoff continued to rise exponentially.  Mass balance has been negative for about five years now, with a cumulative mass loss of almost one trillion tons of ice in that span.  Amazing.

The next big question, therefore, is what’s causing precipitation to change? Will it go back up, thereby reversing the ice loss, or will it remain the same or decrease, causing loss to continue accelerating?  Nobody knows at this point.

¹van den Broeke (2009) Partitioning recent Greenland mass loss. Science 326:984

²Bowdoin people can access the article here.
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Photo credit:  http://www.flickr.com/photos/chrissy575/ / CC BY-NC 2.0

Here’s the bad news:

As sea levels rise, the oceans are beginning to lift up the ice along the margin from underneath (because ice floats in water).  When this happens, these large glaciers become ungrounded, and they slide off the land masses into the ocean (what scientists call “accelerated flow”).  Back to our ice block:  Imagine lifting it from the carpet side…what happens?  It slides completely off the plate.

Some of these moving glaciers can lose between 100 m – 1 km of ice per year. With all of this flow out to the oceans, eventually, the mass of ice on the land mass begins to thin (what scientists call “dynamic thinning”).

The news coming out of Greenland and Antarctic confirms this is happening more than once thought.  In this week’s issue of Nature, Pritchard et al.¹ use laser altimeters abord NASA satellites to map changes in thickness of polar ice to more accurately assess the rate of dynamic thinning.

Bottom line:  In Greenland the glaciers studied thinned about 84 cm per year.  In Antarctica, the glaciers studied thinned at a whopping 4-9 m per year, and the thinning looks to be penetrating more than 100 km into the continental interiors.  The magnitude and extent of thinning is more than was previously known.

How this translates to instability of these ice sheets is still unknown.  The West Antarctic Ice Sheet and Greenland each have enough ice to cause sea levels to rise ~ 6 m.  Stay tuned…

¹Pritchard, H.D., et al (2009) Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets. Nature 461:971-975.

photo credit: http://www.flickr.com/photos/rietje/ / CC BY-NC 2.0