Snowball Earth

There is one theory concerning the Earth being in a nearly frozen state 635 million years ago that climate scientists debate—an episode referred to as Snowball Earth. The term Snowball Earth describes the coldest state in which a planet can exist. In order for this to happen, the global mean temperature would have to be -74°F (-50°C). Most of the solar radiation would be reflected back into space by the high albedo of the snow and ice covering the planet.

There is evidence that supports this hypothesis during this later time period. Evidence exists in sedimentary rocks containing mixtures

Climate scientists believe that the Earth may have been frozen 635 million years ago. (NOAA, Ardo X. Meyer, photographer)

of coarse, unsorted boulders and cobbles mixed with fine silts and clays. These unsorted deposits are characteristic of ice and glacial deposition, and they are found on almost all the continents on Earth. Based on evidence found in sedimentary rocks, it has also been proposed that between 550 and 850 million years ago, two to four of these separate ice house incidents may have occurred.

The biggest debate by scientists on this subject concerns the geographic location of the continents. If some of the continents were located in the Earth's equatorial region (the Tropics), this supports the hypothesis that the Earth could have been entirely frozen. If, however, all the continents were located at high (polar) latitudes, they could all have been frozen, but the Tropics could still have remained unfrozen, meaning the entire Earth did not necessarily freeze, although large portions could have. Scientists have done much work to determine the geographic location of the continents.

Research conducted by Paul F. Hoffman (a field geologist) and Daniel P. Schrag (a geochemical oceanographer) of Harvard University has helped to answer many of the questions surrounding this notable climate event. One of the initial enigmas was the occurrence of glacial debris found near sea level in the Tropics. This evidence contradicted evidence seen today—glaciers near the equator now survive only at 16,404 feet (5,000 m) above sea level or higher. Even at the coldest segments of the last great ice age, glaciers did not form lower than 13,123 feet (4,000 m) in elevation. What Hoffman and Schrag found, however, was not only glacial debris near sea level but that it was mixed with unusual deposits of iron-rich rock. What made this odd was that those rocks should have been able to form only in an environment that had little or no oxygen in its atmosphere or oceans. According to scientific evidence, however, the Earth's atmosphere at that time should have closely resembled that of today. Even more puzzling was that there were also deposits of rocks that could have formed only in warm water found in the rock layers that formed just after the glaciers receded. This presented a puzzle: If the Earth were cold enough to ice over completely, how did it warm up again, especially to such extremely hot conditions? In addition to that conundrum, the carbon isotopic signature in the rocks hinted at a prolonged drop in biological production, leaving scientists to conclude that there had been a dramatic loss of life at that time in the Earth's history. Hoffman and Schrag make sense of these enigmas, however, in their field studies, as reported in an article they presented in the journal Science in January 2000.

Based on his discoveries and work in 1964 concerning the magnetic orientations of mineral grains in glacial rocks, W. Brian Harland of the University of Cambridge believed that the Earth's continents had all clustered together near the equator. Because he realized that glaciers must have covered the Tropics, Harland was the first geologist to propose the concept that the entire Earth had experienced a great ice age event. While Harland was busy with his research and trying to figure out just how glaciers could have survived the tropical heat, physicists were beginning to develop the first basic mathematical models of the Earth's climate system. In particular, Mikhail Budyko of the Leningrad

global warming trends

Geophysical Observatory discovered a way to explain this enigma. He developed a series of equations that described the way solar radiation interacts with the Earth's surface and atmosphere to control climate. As snow and ice accumulate on the Earth's surface, their high albedo cools the atmosphere and stabilizes and perpetuates their existence. What Budyko referred to as ice-albedo feedback is the same mechanism that helps modern polar ice sheets grow.

An interesting thing occurred with his experiments, however. His climate simulations found that the ice-albedo feedback can get out of control, which is what happened to cause Snowball Earth. When ice formed at latitudes lower than 30° north and south of the equator, the Earth's albedo began to rise at a faster rate because direct sunlight was striking a larger surface area of ice per degree of latitude. This caused the feedback to become so strong in his simulation that surface temperatures dropped severely, which quickly caused the entire Earth to freeze over.

At first Budyko was puzzled at his results, reasoning that if the entire Earth had frozen over, then it must have killed all life on Earth. Yet when scientists examined rocks that were 1 billion years old, they found microscopic algae that resembled modern forms, leading them to believe that life did not cease during this time. Budyko, along with other scientists, was also hesitant to take his model too seriously because he also thought that if the Earth had entered a runaway freeze, it would not be able to pull itself out of it.

The scientific attitude toward these questions began to change in the 1970s, however, when communities of organisms living in places once thought too harsh to allow life to survive were discovered. Seafloor hot springs today support microbes that thrive on chemicals instead of sunlight. This clued in Budyko and other climate scientists to the fact that during Snowball Earth, the volcanic activity that feeds hot springs would have continued to function and could readily have supported life.

The explanation for why the runaway freeze stopped was also answered with newly discovered evidence—it all hinges around CO2. In 1992, Joseph L. Kirschvink, a geobiologist at the California Institute of Technology, determined that during Snowball Earth, the planet's shifting tectonic plates continued to produce volcanoes above subduction zones, releasing CO2 to the atmosphere. While the CO2 was collecting in the atmosphere, there was no rainfall to erode rocks and bury carbon (because the water was frozen), thereby allowing CO2 levels to become extremely high. CO2 also entered the oceans through subsea volcanoes and vents. Slowly over the years, atmospheric CO2 built up and increased the radiative forcing due to the greenhouse effect. Eventually, temperatures at the equator reached the melting point, and the dark surface melt waters caused more solar radiation to be absorbed, soon melting and exposing even larger areas of meltwater. These feedbacks started the process of melting back the ice holding the planet in its grip and conceivably took only a few thousand years to recover from Snowball Earth. Therefore, Snowball Earth was ended by a large-scale intensified greenhouse effect—a large-scale global warming event.

scientists have found several pieces of evidence that support the existence of snowball Earth, such as the following.

• global distributions of glacial deposits on all continents

• land areas that would have been near the earth's equator at the time have glacial deposits on them

• evidence of flooding and water flow exists where land would have been pushed down under the weight of glaciers then uplifted when the heavy ice melted and water flowed off of it

• glacial marine deposits occur in areas where the warmest surface parts of the ocean are evidence of snowball earth also brings up the issue of rapid climate change and its importance to life on earth today. As evidenced from this event, co2 plays a critical role in the earth's climate. As humans continue to affect the climate by heating the atmosphere with greenhouse gases, rapid climate change is a serious possibility—one that could have far-reaching ill effects on humanity and the environment.

0 0

Post a comment