A world without ice by H. N. Pollack

Author:H. N. Pollack [H. N. Pollack]
Language: eng
Format: epub
Tags: Sciences
ISBN: 9781583333570
Published: 2009-10-15T17:27:28+00:00

VOLCANIC DIMMING IN THE TWENTIETH CENTURY

The eruptions of Tambora in 1815 and Krakatoa in 1883 were the signature volcanic events of the nineteenth century. The twentieth century was barely under way when the Santa María volcano in Guatemala erupted in 1902. This eruption blasted away much of the 12,000-foot summit of the mountain, sending some 1.3 cubic miles of volcanic ash high into the stratosphere and from there around the world. Although somewhat smaller than Tambora and Krakatoa, Santa María was estimated by volcanologists as probably one of the five biggest eruptions of the past few centuries. It was followed a decade later by the even bigger eruption of Novarupta, on Mount Katmai, in Alaska, which delivered more than four cubic miles of ash to the atmosphere.

The eruptions of Krakatoa, Santa María, and Novarupta in short order kept the atmosphere murky and the climate cooler for the better part of three decades. But after the 1912 eruption of Novarupta, there were no significant explosive volcanic events for a half century, which allowed the atmosphere to clear. The absence of volcanic dust also contributed to the climb in the global average temperature during the period 1910-50. But in the last half of the twentieth century, explosive volcanism returned. The eruptions of Agung in Indonesia in 1963, El Chichón in Mexico in 1982, and Pinatubo in the Philippines in 1991 kept the atmosphere dustier and the Sun dimmer than usual.

If these natural factors were the only ones at work in the last half of the twentieth century, a quieting Sun trying to penetrate a murkier atmosphere would have led to a slight cooling of Earth’s surface. But in fact the temperature has continued to climb nearly one Fahrenheit degree since the mid-twentieth century, indicating that natural factors alone were not in control of Earth’s climate. Indeed, for the first time in the history of Earth, other factors affecting climate—human factors—were growing in importance and beginning to overshadow the natural mechanisms.

Climatologists make a useful (albeit somewhat artificial) separation of the factors that cause changes in the climate, into natural and anthropogenic. Natural causes are those that are independent of human activity, whereas anthropogenic causes arise from human activity. It is safe to say that for most of Earth’s history the causes of climate change were entirely natural, simply because there were no humans present on the planet. Our human predecessors, various species of the genus Homo, first appeared on Earth some three million years ago. As their numbers grew and their technology improved, their impact on Earth and the climate has become increasingly apparent.

In their 2007 Fourth Assessment Report, the IPCC scientists concluded with 90 percent certainty that the rise in temperature in the latter several decades of the twentieth century was attributable mainly to human activities. This ascendancy of the anthropogenic component of climate change, surpassing the natural drivers, was a subtle and unheralded tipping point in the history of our planet.

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