8. More on the Ice Ages

We have covered sufficient material to return to a discussion of the ice ages. In Cometary Impacts and Ice-Ages (2001),[88] two internationally recognized astrophysicist/ mathematicians discuss the impact of not just comets, but all bolide objects, on ice age cycles. They note that there should be no way for the Earth to emerge from an ice age due to positive feedback. As more glaciers form, the Earth's albedo increases, which means that more heat from the sun is reflected back into space. This makes the Earth colder, which causes more glaciers to form. A major event would be needed to disrupt this process.

Hoyle and Wickramasinge contend that the presence of water vapor in the atmosphere is actually more important to the maintenance of a warm Earth than the amount of carbon dioxide. This is because, compared to carbon, water vapor absorbs a larger part of the spectrum of infrared radiation (heat in radiation form) that otherwise would escape back into space. An increased concentration of water vapor in the atmosphere would therefore increase the mean temperature at the Earth's surface. As Hoyle and Wickramasinghe put it, "The impact of a kilometer-sized object, (or a slightly smaller asteroid) into a major ocean appears essential to the ending of an ice-age." Such objects have undoubtedly impacted into the open ocean, leaving no trace yet kicking up enormous amounts of water vapor which would persist in the atmosphere for a period of months to years and cause a spike in the Earth's temperature. This increase in the temperature would be the trigger that ends the ice age.

Hoyle and Wickramasinghe calculate that the impact of comet or meteorite 1 kilometer in diameter would occur at a rate of one every 100 thousand years- the same as the ice age periodicity. They point to several incidents of very rapid temperature increases in the climate and fossil records. Ice cores from Greenland and Antarctica (dated to about 14,600 and 11,500 years ago) show a temperature rise of some 12-15 degrees C in only a few decades. Fossil insect records show that the mean summer temperature in Britain rose at least 10 degrees C during the same period, "an essentially decisive indication of a catastrophic event asits cause." This article presents a hypothesis that should stimulate field researchers to gather empirical data confirming or falsifying it.

The major weakness of the hypothesis is that the authors make no mention of volcanism. Hoffman and Schrag have outlined how CO₂ released from volcanoes is the primary factor in causing the Earth to emerge from the most severe, overwhelming ice age known as "snowball Earth."[89] This episode occurred approximately 600 million years ago. During the snowball Earth episode, volcanoes emitted CO₂ at their regular rate. Normally, this input of CO₂ is counteracted by the removal of CO₂ by the weathering of silicate rocks (75% of the Earth's crust is rocks of this type). Weathering of the rocks produces bicarbonate, which incorporates the carbon and oxygen atoms of CO₂. Trapped in bicarbonate, the CO₂ is washed into the ocean and becomes sediment. But during snowball Earth, most of the planet's H₂O was in solid form, so the weathering did not occur. As a result, CO₂ of volcanic origin builds up in the atmosphere to extremely high levels. This raises the temperature and brings the planet out of the ice age.

Even for the majority of ice ages, volcanism must play a greater role than Hoyle and Wickramasinghe concede. In these less severe glaciations, a great portion of the planet's H₂O would still be ice, so weathering of silicate rocks would be less and there would be some buildup of CO₂ from volcanoes. Impacts of bolides into the open ocean could be a primary trigger ending many ice ages, but not the only factor.