Environmental effects

During his first expedition to Tunguska in 1927, Kulik noticed quite rapid recovery of forest after the catastrophe. As we have seen, he wrote in his diary: 'The young twenty-year-old forest growth has moved forward furiously, seeking sunshine and life.' This accelerated growth of trees that survived the catastrophe has been studied by many Russian scientists. They have noticed that the effect does not coincide with the limits of the fire and destroyed forest, and is observed not only in surviving trees but also in younger trees germinated after the catastrophe.

It has been suggested that this accelerated growth is the result of genetic mutation caused by a nuclear explosion. Longo's team examined the abundance of carbon-14 in the 1903-16 tree rings, but found no traces of nuclear processes. This observation contradicted Alekseev's belief that radioactive processes were possible on the surface of the Tunguska body. But the Italian team suggested that the accelerated tree growth seemed to derive from such improved environmental conditions after the explosion as ash fertilisation by charred trees, decreased competition for light, and greater availability of minerals due to increased distance between trees.

A detailed analysis of the environmental effects of the Tunguska catastrophe was conducted by the American atmospheric scientist Richard Turco and his colleagues in the 1980s. Turco's analysis is based on the assumption that the Tunguska object was an icy comet nucleus, rich in water, ammonia, carbon dioxide and methane. As this object passed through the atmosphere, these substances contributed to the production of 30 million tonnes of nitrogen oxide. After comparing nitrogen oxide generated in nuclear bombs, Turco concluded that the Tunguska event might be compared approximately to 'a large-scale 6,000-megaton nuclear "war" in terms of nitrogen oxide deposited in the stratosphere'. In addition to this massive injection of nitric oxide in the stratosphere, the object also added about 1.5 million tonnes of water. This water helped in the formation of noctilucent clouds which caused bright nights. But the nitric oxide contributed to a longer and deadly effect. By a complex series of reactions, nitric oxide converts stratospheric ozone into oxygen. These reactions depleted the ozone layer that protects us from harmful ultraviolet rays.

The dust veil that hung over the stratosphere for years also contributed to climate changes. Turco said that about 1 million tonnes (Ganapathy's estimate: 7 million tonnes) of dust would be likely to decrease the average surface temperature by about 0.05 degrees Celsius. This resulted in an overall cooling of about 0.2 to 0.3 degrees Celsius in the Northern Hemisphere. Turco pointed out that the cooling trend might have been initiated by the 1907 volcanic eruption in Russia. Turco's team also studied weather records from the early 1900s and noted several other unusual weather conditions that appeared to begin around 1908 and lasted for several years: (1) an increase (above a decreasing trend) in the mean surface temperature over North America in both January and July beginning in 1909-10; (2) an increase in total Arctic ice between 1908 and 1911; and (3) a 50 per cent decrease from normal values in the number of tropical cyclones in the Atlantic and Caribbean Oceans.

Turco's team concluded its study by saying that 'this most impressive and consequential natural event' might have historical significance. Ozone depletion and climatic changes associated with large meteorites may have had a role in past events, such as the death of the dinosaurs 65 million years ago.

Sure, we now know about the environmental impacts of the Tunguska object and its remnants, but there remains the little matter of the identity of the object: was it a comet, an asteroid or something else?

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