Melting Ice Sheets Can Cause Earthquakes, Study Finds

admin@mysteriesunsealed.com — Mon, 17 Mar 2008 23:57:44 GMT

"As ice sheets melt, they can release pent-up energy and trigger massive earthquakes, according to new study.

Global warming may already be triggering such earthquakes and may cause more in the future as ice continues to melt worldwide, the researchers say.

A series of large earthquakes shook Scandinavia around 10,000 years ago, along faults that are now quiet, the scientists point out.

The timing of each earthquake roughly coincided with the melting of thick ice sheets from the last ice age in those same places.

Researchers had suspected that the melting had triggered these earthquakes by releasing pressure that had built up in Earth's crust.

Now a new study, the first to use sophisticated computer models to simulate how ice sheets would affect the crust in the region, bolsters this scenario.

The study showed that earthquakes are "suppressed in presence of the ice and promoted during melting of the ice," said study leader Andrea Hampel of the Ruhr University Bochum in Germany.

Hampel and a colleague had earlier found evidence that the shrinkage of a huge lake at the end of the last ice age had triggered a series of large earthquakes in Utah.

The new study shows this can happen even along faults that are normally quiet and are not prone to slip.

The new research will be published soon in the journal Earth and Planetary Science Letters.

Ancient Quakes Rocked Scandinavia

The ancient earthquakes marched northward through Scandinavia as ice sheets retreated.

They began in the south of what is now Sweden about 12,000 years ago, then hit south-central Sweden near modern-day Stockholm around 10,500 years ago (see Sweden map).

Finally the earthquakes hit Lapland, in northern Scandinavia, about 9,000 years ago.

Based on the amount that the faults slipped, it seems these ancient earthquakes were massive, registering about magnitude 8—bigger than the quake that devastated Kashmir in 2005.

(Related news photos: "Earthquake Devastation in Kashmir" [October 11, 2005].)

Today those Scandinavian faults rarely cause quakes, and when they do, the temblors are small, usually less than magnitude 5.

"With our new modeling technique we can model faults themselves and directly compare the slip on the model fault to the slip on natural faults," Hampel said.

The models showed that thick ice could weigh down the land, preventing a fault from slipping and thereby causing it to store up that energy.

The thicker the simulated ice sheets—from 325 to 6,500 feet (100 to 2,000 meters) thick—the more they suppressed earthquakes, and the bigger the earthquakes were after the ice sheets melted.

Since the amount of movement on the fault in the model matched the actual amount of slippage measured in the field, this supports the idea that the melting of ice sheets had triggered the earthquakes, Hampel said.

Global Warming Causing Quakes?

Such melt-induced earthquakes are not just a thing of the past and could be happening today, since global warming is melting ice worldwide, the team says.

"The frequency of earthquakes should increase in the future if the ice continues to melt," Hampel and colleagues write in their study.

"The current low level of seismicity in Greenland and Antarctica may be caused by the presence of the large ice sheets."

Jeanne Sauber of NASA's Goddard Space Flight Center in Greenbelt, Maryland, has led research showing a recent increase in earthquakes in Alaska when the ice was melting the most.

"All of sudden, between 2002 and 2006, we had warmer temperatures and much more rapid ice wastage," Sauber said.

Even though ice thickness shrank 10 percent or less, this was apparently enough to trigger small earthquakes in the summers when the ice was melting, the study showed.

"It's harder to see if there's an influence on large earthquakes, because they don't happen as often," Sauber added.

"We expect that in Greenland and Antarctica, if they start rapidly losing lots of ice, you would expect at least some little earthquakes." "

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Have we Passed the Global Warming Tipping Point?

admin@mysteriesunsealed.com — Mon, 11 Feb 2008 20:26:20 GMT

"Anthropogenic forcing could push the Earth’s climate system past critical thresholds, so that important components may “tip” into qualitatively different modes of operation. In the renowned magazine “Proceedings of the National Academy of Sciences” (PNAS) an international team of researchers describes, where small changes can have large long-term consequences on human and ecological systems.

“Society may be lulled into a false sense of security by smooth projections of global change,“ the researchers around Timothy Lenton from the British University of East Anglia in Norwich and Hans Joachim Schellnhuber from the Potsdam Institute for Climate Impact Research report. Global change may appear to be a slow and gradual process on human scales. However, in some regions anthropogenic forcing on the climate system could kick start abrupt and potentially irreversible changes. For these sub-systems of the Earth system the researchers introduce the term “tipping element”.

Drawing on a workshop of 36 leading climate scientists in October 2005 at the British Embassy, Berlin, Germany, a further elicitation of 52 experts in the field, and a review of the pertinent literature, the authors compiled a short-list of nine potential tipping elements. These tipping elements are ranked as the most policy-relevant and require consideration in international climate politics.

Arctic sea-ice and the Greenland Ice Sheet are regarded as the most sensitive tipping elements with the smallest uncertainty. Scientists expect ice cover to dwindle due to global warming. The West Antarctic Ice Sheet is probably less sensitive as a tipping element, but projections of its future behavior have large uncertainty. This also applies to the Amazon rainforest and Boreal forests, the El Niño phenomenon, and the West African monsoon. “These tipping elements are candidates for surprising society by exhibiting a nearby tipping point,” the authors state in the article that is published in PNAS Online Early Edition. The archetypal example of a tipping element, the Atlantic thermohaline circulation, could undergo a large abrupt transition with up to ten percent probability within this century, according to the UN climate report from 2007.

Given the scale of potentially dramatic impacts from tipping elements the researchers anticipate stronger mitigation. Concepts for adaptation that go beyond current incremental approaches are also necessary. In addition, “a rigorous study of potential tipping elements in human socio-economic systems would also be welcome,” the researchers write. Some models suggest there are tipping points to be passed for the transition to a low carbon society.

Highly sensitive tipping elements, smallest uncertainty:

Greenland Ice Sheet - Warming over the ice sheet accelerates ice loss from outlet glaciers and lowers ice altitude at the periphery, which further increases surface temperature and ablation. The exact tipping point for disintegration of the ice sheet is unknown, since current models cannot capture the observed dynamic deglaciation processes accurately. But in a worst case scenario local warming of more than three degrees Celsius could cause the ice sheet to disappear within 300 years. This would result in a rise of sea level of up to seven meters.

Arctic sea-ice - As sea-ice melts, it exposes a much darker ocean surface, which absorbs more radiation than white sea-ice so that the warming is amplified. This causes more rapid melting in summer and decreases ice formation in winter. Over the last 16 years ice cover during summer declined markedly. The critical threshold global mean warming may be between 0.5 to 2 degrees Celsius, but could already have been passed. One model shows a nonlinear transition to a potential new stable state with no arctic sea-ice during summer within a few decades.

Intermediately sensitive tipping elements, large uncertainty:

West Antarctic Ice Sheet - Recent gravity measurements suggest that the ice sheet is losing mass. Since most of the ice sheet is grounded below sea level the intrusion of ocean water could destabilize it. The tipping point could be reached with a local warming of five to eight degrees Celsius in summer. A worst case scenario shows the ice sheet could collapse within 300 years, possibly raising sea level by as much as five meters.

Boreal forest - The northern forests exhibit a complex interplay between tree physiology, permafrost and fire. A global mean warming of three to five degrees Celsius could lead to large-scale dieback of the boreal forests within 50 years. Under climate change the trees would be exposed to increasing water stress and peak summer heat and would be more vulnerable to diseases. Temperate tree species will remain excluded due to frost damage in still very cold winters.

Amazon rainforest - Global warming and deforestation will probably reduce rainfall in the region by up to 30 percent. Lengthening of the dry season, and increases in summer temperatures would make it difficult for the forest to re-establish. Models project dieback of the Amazon rainforest to occur under three to four degrees Celsius global warming within fifty years. Even land-use change alone could potentially bring forest cover to a critical threshold.

El Niño Southern Oscillation (ENSO) – The variability of this ocean-atmosphere mode is controlled by the layering of water of different temperatures in the Pacific Ocean and the temperature gradient across the equator. During the globally three degrees Celsius warmer early Pliocene ENSO may have been suppressed in favor of persistent El Niño or La Niña conditions. In response to a warmer stabilized climate, the most realistic models simulate increased El Niño amplitude with no clear change in frequency.

Sahara/Sahel- and West African monsoon - The amount of rainfall is closely related to vegetation climate feedback and sea surface temperatures of the Atlantic Ocean. Greenhouse gas forcing is expected to increase Sahel rainfall. But a global mean warming of three to five degrees Celsius could cause a collapse of the West African monsoon. This could lead either to drying of the Sahel or to wetting due to increased inflow from the West. A third scenario shows a possible doubling of anomalously dry years by the end of the century.

Indian summer monsoon - The monsoon circulation is driven by a land-to-ocean pressure gradient. Greenhouse warming tends to strengthen the monsoon since warmer air can carry more water. Air pollution and land-use that increases the reflection of sunlight tend to weaken it. The Indian summer monsoon could become erratic and in the worst case start to chaotically change between an active and a weak phase within a few years.

Lowly sensitive tipping elements, intermediate uncertainty:

Atlantic thermohaline circulation - The circulation of sea currents in the Atlantic Ocean is driven by seawater that flows to the North Atlantic, cools and sinks at high latitudes. If the inflow of freshwater increases, e.g. from rivers or melting glaciers, or the seawater is warmed, its density would decrease. A global mean warming of three to five degrees Celsius could push the element past the tipping point so that deep water formation stops. Under these conditions the North Atlantic current would be disrupted, sea level in the North Atlantic region would rise and the tropical rain belt would be shifted."

Article: Lenton, T. M., Held, H., Kriegler, E., Hall, J. W., Lucht, W., Rahmstorf, S. and Schellnhuber, H. J. (2008). Tipping elements in the Earth's climate system. Proceedings of the National Academy of Sciences, Online Early Edition

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