The forests of today have evolved over millions of years
and have been profoundly shaped by swings between
warm and cold climates. Glacial periods usually lasted
80 000 to 100 000 years, interspersed with warmer
interglacial periods of 10 000 to 15 000 years. The last
great ice age ended about 10 000 years ago, leaving
forests on nearly 6 billion hectares, about 45 percent of
the earth’s land area. During the last 10 000 years,
cycles of changing climate and temperature have
continued to influence the world’s forests, while human
activity has also had an increasing impact.