Sub-bullet a: Greenland

Sub-bullet a: "A clear warming trend is evidenced by the Greenland ice sheet"
Hierarchy: Part II:1:A:iv:a

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Introduction:
Greenland holds nearly 10% of all the ice in the world. Given its relatively southerly location, Greenland's ice is balanced precariously between its current state and cataclysmic melting, kept stable only by virtue of a microclimate controlled by the ice itself and by friendly and cold arctic ocean currents. The ice is so thick that the crust of Greenland is depressed in the interior from the weight of it. Warming should be relatively easy to spot here; many scientists think that Greenland is close to a sharp equilibrium point and that warming will cause a chain of " positive feedbacks"--some warming will trigger factors which compound the warming and trigger other factors, etc. Thus the changes in the Greenland ice sheet might happen quicker than expected given the large thermal inertia that such a huge block of ice has.

Greenland's climate is an anomaly when placed in the context of other land masses at its latitude. When my family traveled to Prudhoe Bay, AK this last summer, we found it warm and ice-free. Prudhoe Bay is at the northern edge of Alaska, at a latitude of 70 N [for comparison, Anchorage is at 61 N]. Greenland's permanent ice extends down to about 60 N, so far south that almost all of Alaska and the northern territories of Canada would be under ice if Greenland's ice sheet was typical. Why is Greenland covered in ice when most of the latitudes it occupies are free of ice elsewhere?

The glib answer is: because Greenland is colder than other areas at the same latitude. A little background is needed before we can answer why.

Greenland's ice sheet was formed during one of the many colder periods in the earth's history. These glacial periods have caused the latitude marking the ice sheet boundary to move southward, at times reaching down into the continental United States. About 10,000 years ago the last major ice age ended and temperatures returned to roughly what they are now. The equilibrium latitude shifted much farther north in a short period of time. The bulk of the ice sheets were now below the equilibrium latitude, and thus started slowly retreating northward. The retreat was so slow that it is probable that the remaining continental glaciers are still retreating to this day. For some reason, though, Greenland never got the message; some particular climate conditions caused the local equilibrium latitude around Greenland to be stuck farther south than it is for the rest of the Northern Hemisphere. It turns out that this effect is due to two main things: the East Greenland Current, an ocean current that cools Greenland's climate through shipping cold water right along the east coast Greenland from the Arctic; and the high altitude of the ice sheet in Greenland, which keeps the sheet colder than its latitude would dictate.

Warming would effect Greenland in very complex ways, and since the climate is dictated by an ocean current, understanding the subtleties of the current's response to warming would normally be critical to understanding the effect on Greenland's ice sheets. It would be impossible to delve into those subtleties here--albedo, salinity, feedback--but we can still draw some conclusions if the effects are clear enough. This requires an explicit logic that needs explaining: given the "well, duh" aspect of the statement "ice will melt more if temperatures increase" and also given that regional models predict a certain pattern of melting occurring as the result of the warming, IF expected melting patterns are conclusively observed, THEN the region is almost certainly warming. The burden of proof to the contrary will rest on those who wish to prove that the melting proceeds by some other mechanism.

I am not trying to cover up a logical flaw. Rather, I'm using the strongest logic that is possible in any kind of analysis of complex systems. A researcher can never be 100% confident in a causality solution in a system this complex, but it is not wrong to draw conclusions based on the best available data, assuming that as much rigor as is possible is dedicated to trying to understand the underlying causes and effects. Such a case this is. What this means practically is that we will look at the data, conclude confidently based on this data that it is TRUE or FALSE that the behavior of Greenland proves regional warming. The bullet value will only stay TRUE if sufficient evidence is not found for mechanisms other than warming causing the observed effects.
[Refer to bullet iv for further introductory material]

Study 1
Greenland mass balance from GRACE
Isabella Velicogna and John Wahr, GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L18505

Satellite gravity measurements confirm accelerated melting of Greenland ice sheet
J. L. Chen, et al., Science 313, 1958 (2006)

Recent Greenland ice mass loss by drainage system from sattelite gravity observations
S. B. Luthcke, et al., Science 314, 1286 (2006)

Basic: These papers utilize data from the GRACE experiment, with newer data in the second and third papers. This is the type of experiment every researcher would love to be part of. GRACE is a fantastic idea: two orbiting satellites working in tandem to measure the gravitational field above the earth. This is an exquisitely delicate experiment, but it was pulled off and is very successful. The idea is that ice will "pull" on the spacecraft through gravitational acceleration. This pull is measured each time the satellite passes over and compared across time. If the pull changes, that means that ice is accumulating or being lost on the ice sheet. The beauty of this approach is that it is independent of ice transport mechanism and is the first experiment that is directly measuring the quantity of ice present as opposed to indirect studies of depth or flow which use generous extrapolations and interpolations. The first study includes data from 2002-2004 and the second and third studies adds 2005 to that. The studies concluded that the mass balance of Greenland in the time period was large, negative, and accelerating, consistent with regional warming.

Details: Velicogna and Wahr: "We recover a decrease in total ice mass of 82 ± 28 km3 of ice per year." I am unable to access the rest of the paper, so I would appreciate anybody with an AGU membership to help me on this one. Again, although static or increasing mass balance is inconclusive, negative mass balance, especially one this large, is very strong evidence of warming. The image below, taken from the NASA site supporting this research, shows a very strong net mass loss around the periphery of Greenland and a small mass gain in the interior.

This figure, from Velicogna and Wehr, shows the total Greenland ice sheet mass observed by GRACE during the study:

The second study confirms this data and adds a crucial fact: melting accelerated even within the limited time scale of the study. This acceleration is evident in the following plots from Chen:
The second study revises the yearly net mass loss upwards and confirms the acceleration of mass lost at least on this small time scale.

The third study confirms the main results of the other studies but lowers the mass loss estimate from the second study.

GRACE tells us that the Greenland ice sheet is melting and by how much. But even more significant is that this melting is accelerating on a seasonal timescale. This is a crucial piece of the puzzle; receding glaciers can be attributed to "ice age rebound"--the still on-going process of ice retreat from the last ice age, when the ice sheets were much more expansive. But acceleration of melting can only be due to warming: rebound predicts a gradual deceleration of ice loss as the equilibrium point is approached.

Conclusion: GRACE data provides very strong evidence of north polar warming.

Study 2
Greenland ice sheet: high-altitude balance and peripheral thinning
W. Krabill, et. al., Science 21 July 2000:Vol. 289. no. 5478, pp. 428 - 430

Basic: Greenland ice sheet: high-altitude balance and peripheral thinning. What more to say?

Details: This study was conducted using laser altimetry from aircraft; this is how it was done before GRACE. Laser altimitry can only tell you the altitude of the surface of the ice so this approach is most useful for mass balance studies. The study specifically addressed the mass balance of Greenland. Above 2000m of altitude, there was a mass balance: net accumulation in the northern latitudes counteracted net loss in the southern latitudes. However, at all latitudes around the periphery there was widespread net loss. An included figure best describes the findings [flight tracks shown; also note that this is an elevation chart, not a mass chart like for GRACE]:

Although less comprehensive than the GRACE survey, it is clear that a large negative mass balance at the periphery of Greenland overwhelms the accumulation in the interior.

In sum: "Interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet, sufficient to raise global sea level by 0.13 millimeter per year--approximately 7% of the observed rise."

Discussion: This study found strong evidence for behavior which points to warming: ice thickening or balance in the interior and aggressive thinning at the periphery. However, the authors admit that the peripheral thinning cannot be explained by the temperature record alone, and that the mechanism for the mass loss is still a mystery. Study 3 below is a partial response to this study and attempts to solve this problem with glacial dynamics studies.

Conclusion: This study gives some evidence for north polar warming.

Study 3
Changes in the velocity structure of the Greenland ice sheet
Eric Rignot and Pannir Kanagaratnam , Science 17 February 2006 311: 986-990

Basic: Using data from satellite radar interferometry [bouncing electromagnetic waves off the surface of ice from space], Rignot and Kanagaratnam have measured the velocity of the glaciers of Greenland over the last decade. Note how this differs from the previous study: the altimetry data presented in Study 2 likewise bounced electromagnetic waves [in that case, lasers] off the surface of the ice sheet, but there it was for the purpose of measuring the altitude of the ice surface and in this study the purpose is to measure the speed at which the ice flows down glaciers. These glaciers are the conveyor belts for ice transportation from the interior of Greenland to the sea, so glacial acceleration is a sign that more ice is leaving Greenland and entering the oceans. The study not only found widespread glacial acceleration, but found that the portion of Greenland experiencing the acceleration is increasing dramatically. The effect is also found to dominate any increase in due to warming, and is presented as an explanation for the mechanisms behind the results found in Study 2.

Detailed: The causes of glacial acceleration are not very well known, although all plausible explanations are products of warming [such as surface meltwater percolating down under the ice and lubricating the interface between ice and rock]. The degree of acceleration is also not a terribly good measure of the warming; all it can say is that warming is occurring. The study found that total mass loss from Greenland doubled in the period 1996-2005. The important point is that acceleration is observed, and this is not a feature of ice age rebound. The authors state: "We detected widespread glacier acceleration below 66 [degrees] north between 1996 and 2000, which rapidly expanded to 70 [degrees] north in 2005. Accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year."

The figure below shows the velocity plots of some of the glaciers from the study. The black lines correspond to the oldest data, blue the next, and red the most recent. Since these are velocity plots, the earmark of acceleration will be the red plots lying "above" the blue ones and the blue ones above the black ones.

Conclusion: This study constitutes strong evidence of north polar warming.

Other studies
Progressive increase in ice mass loss from Greenland
R. Thomas, et al., GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L10503

Basic: Similar results from the ICEsat experiment.

Quote: "Laser altimeter measurements over Greenland show increasing thickening rates bove 2000 m, reflecting increasing snowfall in a warming climate. But near-coastal thinning rates have increased substantially since the mid 1990s, and net mass loss more than doubled from an average of 4–50 Gt per year between 1993/4 and 1998/9 to 57–105 Gt per year between 1998/9 and 2004. This increasing trend is very similar to findings from independent massbudget studies, but differs widely from ERS radar altimeter results."

Runoff and mass balance of the Greenland ice sheet: 1958–2003
Edward Hanna et. al., JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, D13108

Quote: "Runoff losses from the ice sheet were 264 (±26) km3 yr−1 in 1961–1990 and 372 (±37) km3 yr−1 in 1998–2003. Significantly rising runoff since the 1990s has been partly offset by increased precipitation. Our best estimate of overall mass balance declined from 22 (±51) km3 yr−1 in 1961–1990 to −36 (±59) km3 yr−1 in 1998–2003, which is not statistically significant."

The Greenland ice sheet and global sea-level rise
Julian A. Dowdeswell

Abstract: "The flow of several large glaciers draining the Greenland Ice Sheet is accelerating. This change,combined with increased melting, suggests that existing estimates of future sea-level rise are too low."

Seasonality and increasing frequency of Greenland glacial earthquakes
Göran Ekström, et al., Science 311, 1756 (2006)

Abstract: "Some glaciers and ice streams periodically lurch forward with sufficient force to generate emissions of elastic waves that are recorded on seismometers worldwide. Such glacial earthquakes on Greenland show a strong seasonality as well as a doubling of their rate of occurrence over the past 5 years. These temporal patterns suggest a link to the hydrological cycle and are indicative of a dynamic glacial response to changing climate conditions."

[Further studies requested]

Discussion
There is no doubt that Greenland is losing ice. And fast. A valid argument against concluding that warming is the cause of the loss is still present, however: since Greenland's climate is so controlled by the East Greenland Current [EGC], it is possible that the current is changing in ways that is encouraging the loss of ice on Greenland. The GRACE mass deficit maps are particularly encouraging to this line of thinking since a large majority of the mass loss occurs along the eastern coast, where Greenland and the current interface. I have dug deep to find evidence that the EGC has changed in any way since before the mass balance turned negative. I found no such evidence. Additionally, any change in the EGC could very well be tied to warming since the Arctic has seen the most dramatic effects attributed to warming of any region on Earth. Until strong evidence emerges that some other factor is forcing these dramatic mass losses, the sub-bullet value is TRUE. However, the evidence strength is demoted from "conclusive" to "strong" due to these concerns.

Brief summary of the issues: A complete description of the behavior of the Greenland ice sheet has to include two behaviors: ice mass loss--the change in the distribution and quantity of ice--and glacial dynamics--the change in the behavior of the ice. The dynamics data [such as Rignot and Kanagaratnam and Ekström] are strongly complementary to the mass balance data since glacial dynamics explains at least some of the mass loss and simultaneously acts as strong evidence of warming. Strong positive signatures of warming were found from both glacial dynamics studies and mass balance studies.

Summary of mass balance data:

Conclusion
This data constitutes strong evidence of north polar warming.

Sub-bullet value: TRUE