2011-02-22 17:59:32Various estimates of Greenland ice loss
John Cook

john@skepticalscience...
121.223.91.161

Note: if anyone wants to examine and critique my Figure 3, here's the Excel spreadsheet (my first attempt at plotting error bars in Excel). Would be nice to down the track find other estimates and generate a more comprehensive graph - particularly as I'd like to plot a curve of best fit to clearly show the accelerating trend).


Various estimates of Greenland ice loss

Over the last few weeks, three different papers have been published that all examine ice loss from the Greenland ice sheet. What's interesting is all three papers use entirely different methods to measure the rate of ice loss. Even more interesting is that these three different methods paint a consistent picture of what's happening to Greenland.

Schrama et al 2011 uses gravity measurements from the GRACE satellites to determine any change in mass of the Greenland ice sheet (there's a great article The Riddle of the ice about Ernst Schrama's work). They find from March 2003 to February 2010, Greenland lost ice loss at a rate of 252 gigatonnes per year. A key result from their paper was to confirm that ice loss had spread to the north west of Greenland, a signal of warming progressing further into the Arctic.

 

Figure 1: Ice mass anomaly of the Greenland ice sheet as measured by GRACE gravity satellites (Schrama et al 2011).

Another paper Zwally et al 2011 uses satellite altimetry to determine the thickness of the Greenland ice sheet. They calculate that over 2003 to 2007, the ice sheet was losing ice at a rate of 171 gigatonnes per year. They then compare this to radar and airborne altimetry data from the 1990s. From 1992 to 2002, Greenland was only losing 7 gigatonnes per year.

Lastly, Rignot 2011 uses the Mass Balance Method to construct a 19 year record of ice loss from Greenland. This involves calculating the amount of snowfall on the surface, the amount of ice mass lost to wind and melt and the amount of ice lost calculated from glacier velocity and ice thickness. Putting all these pieces together gives the total amount of ice lost or gained over the ice sheet.

Over this nearly two decade period, Rignot finds a clear signal of accelerating ice loss. He then compares his results from the Mass Balance Method to results from GRACE data. Both show consistent rates of mass loss. Just as significantly, both are accelerating at similar rates.

Figure 2: Total ice sheet mass balance in Greenland from the Mass Balance Method (black) and GRACE gravity measurements (red). The acceleration is given in gigatonnes per year squared (Rignot 2011).

Out of curiosity, I thought I'd plot the results from all three papers on a single graph to see how the results compared. I also included Velicogna 2009. Each paper covers different time periods so I've indicated the time period (horizontal uncertainty bars) as well as the ice loss uncertainty (when provided).Note how the GRACE data shows a significantly larger uncertainty (the vertical bars) than the Altimetry uncertainty bars (which are barely visible).

Figure 3: Various estimates of Greenland ice loss.

While there are a range of estimates on Greenland ice loss, independent lines of evidence all paint a similar picture - Greenland is losing hundreds of billions of tonnes of ice per year and the rate is increasing as warming spreads into the Arctic.

 

2011-02-22 20:19:04
Ari Jokimäki

arijmaki@yahoo...
192.100.112.211

"They find from March 2003 to February 2010, Greenland lost ice loss at a rate of 252 gigatonnes per year."

Losing ice loss means that Greenland is gaining ice, right? ;)

"A key result from their paper was to confirm that ice loss had spread to the north west of Greenland, a signal of warming progressing further into the Arctic."

Is this your own interpretation or conclusion in the paper you're referring to? I think that ice loss spreading to north-west Greenland doesn't necessarily mean that warming is progressing further into the whole of Arctic, as it could be just a local phenomenon.

"While there are a range of estimates on Greenland ice loss, independent lines of evidence all paint a similar picture - Greenland is losing hundreds of billions of tonnes of ice per year and the rate is increasing as warming spreads into the Arctic."

Here the "warming spreads into the Arctic" seems strange. Isn't the warming already there? I mean that we have been told that the Arctic is warming faster than other parts of the Earth, so warming can't be spreading there if it has already been happening.

2011-02-22 20:21:53
Ari Jokimäki

arijmaki@yahoo...
192.100.112.211

Here are some more estimates (older ones) for you:

http://agwobserver.wordpress.com/2009/11/24/papers-on-polar-ice-sheets/

2011-02-23 15:24:37Thanks Ari
John Cook

john@skepticalscience...
121.223.91.161

The spreading to the Arctic is something Schrama mentioned to me in his email but I'll just leave it out for simplicity's sake. Also made a few other corrections (the blog post is Various estimates of Greenland ice loss and will go live on Friday) and added a whole bunch more estimates so Figure 3 is looking a lot more cramped now:

I added in Wu, the pink one that "corrects" for GIA to show what an outlier it is.

 

2011-02-24 04:53:54Comment
Robert Way

robert_way19@hotmail...
134.153.163.105
John I think you have to be careful for putting the Rignot et al (2011) one in there. It says that the mass balance of the two ice sheets is 450 GT year losing in 2006 and has accelerated by 36 a year over the 18 year period. I think you have it plotted as if it was not saying this. I would also exclude Zwally 2005. The Zwally 2011 paper says they used the wrong firn compaction rate in 2005 and that's why they got a positive value.

How about this.

 Greenland lost 250 GT in 2006 with an error bar of 40 GT.  Add the 21 a year acceleration to get the values updated to 2010 and subtract the values prior to that point to extend back to 1992. Once you have the yearly mass balance do an average over the whole period. I'll do this for you. The line for Rignot should be at 145 for the period 1992-2010 with vertical error bars of 185 for the upper and 105 for the lower.

PS how did you make this graph?


Update:
For your figure you should note that Rignot et al (2011) uses both GRACE and the MASS Balance + Discharge method. Also it might be useful to link to my post on how we measure ice sheet changes.
2011-02-24 14:34:43Rignot 2011
John Cook

john@skepticalscience...
121.223.91.161

How I worked out Rignot 2011 was I just looked at Figure 2a of his paper which showed X amount of ice loss over Y years - I just used that to calculate the rate of yearly loss over the 18 year period (something like 20 Gtonnes per year). So all I'm showing with Rignot 2011 is the average ice loss over 18 years, with no info on acceleration.

Will remove Zwally 2005.

I'll email you the Excel file.

2011-02-25 02:33:50New Information?
John Hartz
John Hartz
john.hartz@hotmail...
98.122.68.19

"The role of albedo and accumulation in the 2010 melting record in Greenland" by M Tedesco, et al, Environmetal Research Letters #6 (January-March 2011)

http://iopscience.iop.org/1748-9326/6/1/014005/fulltext