(Nov. 9) -- Massive volcanic eruptions can trigger a cooling effect on the Earth's climate, but the ones occurring now on Indonesia's Mount Merapi are unlikely to counterbalance years of uninterrupted warming.

While volcanic plumes can lead to lower global temperatures, the phenomenon, though sometimes dramatic, is usually short-lived. In the long run, some scientists say, greenhouse gases released by eruptions counteract the cooling.

The altitude of the ash from the current Merapi blast is 55,000 feet, according to Australia's Volcanic Ash Centre. This is comparable to the Mount Eyjafjallajokull eruption in Iceland in April but considerably lower than the 1991 Mount Pinatubo blast in the Philippines, which sent ash 78,740 feet into the atmosphere.

(Image courtesy of NASA)

Global cooling results from volcanic ash blocking incoming solar radiation high in the atmosphere. The degree of cooling depends on the amount of ash deposited, how high the ash rises and in what part of the globe the eruption occurs.

The amount of ash deposited high into the atmosphere cannot yet be determined for Mount Merapi since the eruption is ongoing, but it's not as great as the Pinatubo blast, at least so far. The Pinatubo blast is believed to have temporarily cooled the Earth.

An eruption strong enough to push ash into the stratosphere, which is the layer of the atmosphere above where weather occurs, can produce some cooling globally since, lacking cleansing precipitation, the ash tends to linger. An eruption that pushes ash into the stratosphere in a tropical region, such as occurred with Pinatubo, is more likely to produce global cooling than one in higher latitudes, such as Eyjafjallajokull.

Eruption of Mount Merapi could result in a temporary cooling affect on earth's climate

Bay Ismoyo, AFP / Getty Images

Indonesia's Mount Merapi spews thick ash into the sky as seen from Cangkringan in Sleman, Yogyakarta, on Monday.

Part of that interruption of warming in a portion of the globe that receives the greatest amount of radiation (near the equator) has a greater cooling effect than an interruption of warming in cooler parts of the globe (higher latitudes). But it's also due to global circulation of the stratosphere.

The stratospheric flow over the tropical regions, which starts at roughly 50,000 feet in altitude, tends to rise and spread in all directions, leading to a widespread dispersion of the ash, along with a more dramatic cooling effect.

In higher latitudes, even though it's easier for a volcano to push ash into the closer stratosphere (which starts at roughly 30,000 feet), the air flow tends to sink and move toward the poles. As a result, the ash does not spread as far and falls more quickly to the ground, resulting in a less dramatic cooling effect.

The cooling from the Mount Pinatubo eruption was relatively minor and short-lived, with global temperatures by 1995 matching pre-eruption levels. The cooling was not enough to break the trend of global warming that started before the eruption and has continued since. According to the National Oceanic and Atmospheric Administration, October 2010 marked the 307th consecutive month with temperatures above the 20th-century average.

Cooling from volcanoes can be much more extreme than the 1991 Pinatubo eruption. It is believed that the massive Mount Tambora eruption, in Indonesia in 1815, led to dramatic global cooling, including the so-called "Year Without a Summer" in 1816 and dramatic global effects for a couple of years to follow.

The cool-down hit the eastern United States, eastern Canada and northern Europe particularly hard, resulting in widespread famine and hundreds of thousands of deaths. The 1816 growing season in the eastern U.S. was shortened by early-summer snow and a late-summer frost, and the 1816-1817 winter was extremely cold and harsh.

Even if global cooling occurs as a result of a major volcanic eruption, the eruption releases carbon dioxide into the atmosphere, one of the main greenhouse gasses. Some climate scientists believe this will increase the amount of warming after the ash-induced cooling has ended.