This guest post from BillLaurelMD provides a window on change in the Arctic — a window that we might prefer not to look through, but look we must. This is the next in an occasional series of diaries on the state of Northern Hemisphere Arctic sea ice, written in memory of Johnny Rook, who passed away in early 2009. He was the author of the Climaticide Chronicles.
In November 2009, the National Oceanographic and Atmospheric Administration issued a report card on the condition of the Arctic in 2009. I diaried the sections on changes noted in the Arctic atmosphere and sea ice. Now for the section on Greenland, where we find the largest land-based ice sheet in the Northern Hemisphere.
I’ll start with a description of the Greenland Ice Sheet and the methods used to determine whether it’s gaining or losing ice. Then I’ll talk about the results of studies over the last few years regarding the stability and mass balance of the ice sheet. Last, I’ll put those research results into the context of the NOAA 2009 Arctic Report Card.
Description of the Greenland Ice Sheet
A summary:
- The Greenland ice sheet formed during the late Pliocene or early Pleistocene (between 2-3 million years before the present (BP)).
- The mean elevation of the ice sheet is about 2.1 km (~1.33 miles), with a maximum altitude of over 3.2 km (~2.0 miles) at the summit of the Northern Dome.
- Its total volume is approximately 2.85 million km3
- Total melt of the ice sheet would lead to a sea level rise (all else being equal) of 7.2 meters (23.6 feet)
- The rate of change in ice sheet height per year (in cm) over the full ice sheet can be seen here.
- In 2007, a record loss of ice mass was reported, using a regional climate model with high resolution, calibrated to observations of the ice sheet.
See the Wikipedia entry for details, including mass balance trends from 1958-2003. This seems to be a fair representation of what is known about the Greenland ice sheet.
The mass balance calculations discussed in the Wikipedia entry were extended through 2007 in this article.
A Quick Primer on Ice Sheet Mass Balance
How does the Greenland (and other) Ice Sheet gain or lose mass? I have a little cartoon showing the basic components of glacial mass balance below.
At the surface of the ice sheet, gains (accumulation) come from:
- Precipitation in the form of snow, ice crystals (at the coldest temperatures) and ice
- Frost and rime deposition
Losses (ablation) occur through:
- Snow being blown off the ice sheet out to sea
- Sublimation and melting
- Calving of land ice into icebergs
- Melting from below where the ice sheet is over water, if the water is warm enough
- Flow of ice away from thickest areas because of the sheer weight of the ice. Where melt water gets down to the base of the ice sheet from melting through crevasses in the ice, the resulting bottom lubrication of the ice sheet can result in relatively rapid movement.
Methods Used to Measure Ice Sheet Mass Balance
Before the advent of remote sensing (satellite) measurements, stakes would be put up in representative locations around the ice sheet to determine the net addition (or subtraction) from surface processes, and somehow or other the loss from calving and subsurface runoff would be determined to get some guess at the change in mass of the Greenland ice sheet. Today, remote sensing of elevation changes, ice movement and calving of icebergs, the use of sophisticated ice sheet models forced with observed meteorological variables, and other methods allow the researcher to get a much better handle on the current state (and possible future evolution) of ice sheets.
According to this article from Geophysical Research Letters published in October 2008, the Greenland ice sheet lost mass in the 1960s, was in near balance through the 70s and 80s, and then showed mass loss accelerating from 1996 through 2007 (97 ± 47 Gigatonnes (Gt)/yr in 1996, 267 ± 38 Gt/yr in 2007, the year that the Arctic sea ice extent minimum was 23% lower than any previous year in the satellite record).
A graphic of the time series of Greenland ice sheet mass balance based on the data from this article can be seen here. The graphic includes error bars for specific yearly measurements, and a curving line for the model-based mass balance.
The 2009 Report Card
So how did 2008 fare in the grand scheme of things? The winter 2008 surface air temperatures were well below normal in the southwest and well above normal in the northeast (note the +8.7°C (15.7°F) anomaly at the coastal station in east-central Greenland). The cold weather in the southwest resulted in increased amount and thickness of sea ice in the Davis Strait, between Greenland and the Baffin Island and the rest of the Canadian Archipelago.
However, summer 2008 surface air temperatures were above normal over a larger area; two coastal stations had their warmest and second warmest summers (Upernavik and Nuuk, respectively) on record back to 1873. A number of other coastal stations with shorter records of about 50 years also showed record warmth.
The 2008 effect on Greenland ice sheet mass balance
The particularly warm summer 2008 season in Greenland lead to a continuation of ice sheet mass loss. The trend since 2000 can be seen below. Basically, the rate of loss of ice sheet area has run 106 km2 over the 9 year span, with all but 2% of the variance in ice area explained by the linear trend.
The area (not volume) loss from 2008 to 2009 was 106.2 km2, right in line with the trend. Increased precipitation (more than normal in the form of rain) was more than offset by increased ice melt and runoff. The melt season generally lasted longer than normal over the northern and western part of the ice sheet. Highest altitudes of the ice sheet showed shorter than normal length of time. Remote sensing of the surface of the ice sheet is used to determine the number of melt days.
Future Projections for the Greenland Ice Sheet
As stated earlier, the loss of the full Greenland ice sheet would result in a sea level rise of over 7 meters (over 23 feet). This would result in the loss of a number of major cities across the world; The concern then obviously is:
- What is the likelihood of the loss of this ice sheet,
- What critical thresholds must be met to initiate such a loss, and
- What pace can we can expect such loss to take place?
This is a tough problem to solve without observations to determine the processes involved and how they might work to remove an ice sheet. The last ice sheet that melted for which we can get some kind of measurements was the Laurentide Ice Sheet, which last reached a maximum extent 20,000 years before present (BP) and disappeared about 6,500 BP. It turns out that research into how the Laurentide retreated indicates that there were two periods of rapid melt and discharge into the oceans:
- At 9,000 BP, when sea level increased by 7 meters (1.3-cm/year), and
- At 7,500 BP, when sea level increased another 5 meters (1-cm/year)
These each represent a change over about 550 and 500 years, respectively. This melt was driven by increased solar insolation, resulting from variations in the Earth’s orbit (so-called Milankovitch cycles). (Note: According to this theory, the areas where ice sheets would form (about 65°N) will be experiencing a gradual increase in solar insolation over the next 50,000-100,000 years, with little to no risk of an ice age from Milankovitch forcing, barring a significant decrease in solar irradiance from some other source, or some other forcing that would result in cooling.)
The current peer-reviewed research that impacts forecasts for the Greenland ice sheet is as follows:
- We are getting a better handle on the physics and dynamics of ice sheet movement by observing the erratic surges of ice out of the interior to the oceans. These have been theorized to be caused, at least in part, to melt water getting to the base of the ice sheet through moulins in the ice (what I’ve labeled in my diagram above as crevasses). Ice movement has been observed to increase by as much as a factor of 4 just after the start of the melt season.
- Greenland has been losing ice mass at a greater rate than expected from past predictions, and global mean sea level has increased at a greater rate than predicted as well.
- From the IPCC report in 2007, seas could rise 59 centimeters (23.2 inches or almost 2 feet) by the end of this century — or more, if global temperatures rise faster than projected. A recent paper has suggested sea level rise may be as much as 1 meter (3.3 feet) by 2100.
- Non-linear processes may result in increases in the rate of ice sheet melt over time. One way this might happen is through surges of ice from the interior to the oceans, with subsequent calving into icebergs at the coastline.
More Information
For a good summary of the examination of the Laurentide ice sheet, I recommend starting with this source. For a discussion of the Greenland ice sheet and what might happen to it and sea level rise, I recommend this blog entry on Skeptical Science. In particular, see this comment in this blog entry for some additional journal citations, and summaries of them.