Rapid, accelerating glacier melt:
[Via InvestigateWest]
[Crossposted at Path to Sustainable]
Really bad news for North American glaciers today in a report in the Los Angeles Times. Global warming has melted glaciers in the United States at a rapid and accelerating rate over the last half-century, increasing drought risks and contributing to rising sea levels, the federal government will report today based on data from a 50-year study of glaciers in Alaska and the Pacific Northwest, reporter Jim Tankersley writes. The study focused on three benchmark glaciers, the Wolverine and Gulkana in Alaska and the South Cascade Glacier in Washington, which are representative of thousands of other glaciers across the continent.
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Glaciers and their runoff have been a relatively stable source of water, providing a necessary buffer against the fickleness of rainfall. But this buffer is rapidly disappearing. The South Cascade Glacier has lost 25% of its mass since the 50s.
You can read the report online with the somewhat boring title Fifty-Year Record of Glacier Change Reveals Shifting Climate in the Pacific Northwest and Alaska, USA.
Look at this series of the South Cascade Glacier:
The USGS has been measuring the net accumulation of snow and the net loss of ice. Of interest are the two coastal glaciers, the Wolverine and the South Cascade. Both require high amount of precipitation to grow because their relatively low elevations opens them up to summer heating. Interestingly because of their locations in Alaska and Washington respectively, they tend to have negatively correlated accumulations.
That is, the weather patterns that bring lots of precipitation to one also usually prevent precipitation at the other. This can be seen below:
In the 70′s when the South Cascade was increasing, the Wolverine was decreasing. This reversed in the early 80s. This is probably due to the Pacific Decadal Oscillation (PDO). Not as well know as its relative, the El Nino Southern Oscillation (ENSO), but very important up here.
What is worrisome i that this negative correlation disappeared starting the the 90s. This is the same time period during which the Gulkana glacier, which by being an inland glacier should have no connection to the wether patterns driving the 2 coastal glaciers, also showed rapid loss of mass. All three glaciers now show a tremendous positive correlation in their loss of mass.
Examination of the seasonal change in mass balance can reveal whether the loss of mass is due to decreased precipitation in the winter or increased heating in the summer. Their data indicates that since about 1990 most of the mass has been lost due to increased heating in the summer.
The authors state:
One interpretation of the trend towards increasing summer melt at the Benchmark Glaciers is that climate changes that are larger in scale and intensity than periodic shifts in ocean conditions (PDO) are underway.
Another aspect of higher global temperatures would be an increased hydrologic cycle. That is, the higher temperatures result in water entering the atmosphere more rapidly and then generally precipitating out as snow and water faster. Thus the cycle of rain, evaporation, rain takes less time.
The USGS examined the cycle seen in these three glaciers. As low altitude, maritime glaciers, the Wolverine and South Cascade should have a relative faster cycle. They have large amounts of snow in the winter because of their nearness to water but melt a lot in the summer, because of their low altitude. The Gulkana, on the other hand, is much cooler but gets less precipitation. It should have a lower rate of turnover.
The data through the 80s are as expected. The two maritime glaciers have a much higher turnover rate than the interior glacier. But starting in the late 80s, all three glaciers have a definite trend upwards in their turnover, demonstrating an across the board increase in precipitation and increase in meting, with the latter beginning to be much more overwhelming, resulting in the decreased size of the glaciers.
Their conclusions are stark
The net mass balance of a glacier is the difference between the mass gain through accumulation of snow and the mass loss through ablation. A glacier that is in balance with the current climate will have zero net balance and a steady size. A glacier that is out of balance with the current climate will grow (advance) or shrink (retreat) depending on how the climate is changing (colder or wetter climate will cause advance, warmer or drier climate will cause retreat).The USGS has been monitoring the mass balance of three glaciers in the Pacific Northwest and in Alaska for nearly 50 years. This mass balance record is now sufficient to interpret glacier responses to short- and long-term climate changes in the Pacific Northwest, the Alaska Coast, and the Alaska interior. What have we learned so far?
* All three glaciers have lost mass since USGS monitoring began more than four decades ago.
* Mass loss has accelerated during the last 15 years, coincident with the highest melt years on record.
* Mass balance of the coastal South Cascade and Wolverine Glaciers correlate well with the PDO during the first few decades of the period of record, showing the effects of ocean-condition periodicity on glacier health.
* This correlation has weakened during the last two decades, as global average temperature has increased.
* Mass turnover has increased throughout monitoring and the trend of increase has become stronger during the last two decades.The accelerating loss of mass, the weakening correlation with the PDO, and increasing mass turnover likely are the result of changes to warmer and (or) drier climate conditions that are affecting all three regions. The climate changes could be overwhelming the previously observed responses of the maritime glaciers to periodic shifts in ocean conditions, such as are represented by the PDO.
If the PDO shifted to a cool phase, it might be able to ameliorate some of these trends. But it would take many, many years of altered weather conditions to undo the damage already done. Not just a cessation in temperature related melting but an outright reversal of precipitation levels and a complete change in the hydrologic cycle.
At this point, not too likely.
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