Ice Ages

ENORMOUS ICE SHEETS hav, at times, spread across the globe, bringing with them a cold climate. One definition holds that the advance of glaciers to middle latitudes marks an ice age. Another, and perhaps complementary, definition holds that the spread of glaciers to encompass roughly one-third of the planet is the signature of an ice age. Glaciers have advanced and retreated from Earth s surface and from the ocean several times. The most exuberant estimate tallies 17 ice ages, A more conservative estimate is that Earth has endured the tumult of six ice ages. Scholars have debated not only the number of ice ages, but also their causes. The causes fall into three categories: astronomical, atmospheric, and terrestrial. The recognition of ice ages is recent. Only in the 1830s did Swiss zoologist Louis Agassiz, building on the work of predecessors, propose that parts of Europe, the Americas, and Asia had been covered by ice. As ice advanced and retreated, Agassiz asserted, it ground up rocks in its path and sometimes carried boulders a great distance, Agassiz thought in terms of a single ice age, but climatologists have increased this number in the 20th century.

The first ice age occurred, not thousands of years ago, but rather 2.6 billion years ago. The sun, com-paratively young, was not burning at full strength, though this fact does not explain why the ice age had not begun earlier, A more satisfactory explanation focuses on carbon dioxide. The amount of carbon dioxide in the atmosphere diminished, lowering the capacity of the atmosphere to trap heat and, thereby, warm the Earth, As the amount of carbon dioxide diminished, temperatures fell low enough for ice to spread over parts of the primeval continents. The first ice age, lasting 300 million years, ended 2,3 billion years ago.