A SHORT TOUR THROUGH 4.5 BILLION YEARS

The global warming debate is about what will happen in the next few hundred years. Our planet Earth is 4.5 billion years old, and over the planet's lifetime changes in temperature, greenhouse gas concentration, and sea level have occurred that dwarf any of the changes being discussed now. Life is thought to have begun roughly 3.5 billion years ago, perhaps earlier, with bacteria-like organisms whose fossils have been found and dated. They lived in the oceans in a world with only traces of or perhaps even no oxygen in its atmosphere. It was about 2.5 billion years ago that the first algae capable of photosynthesis started putting oxygen into the atmosphere, but to a level of only about 1% compared with the 20% of today. All the creatures of the time were small. This earliest period is largely a mystery that is still being unraveled. Recent work indicates that it was only about 540 million years ago that the oxygen concentration in the atmosphere rose to anything like today's values and larger plants and animals appeared.

From then to now saw the rise of many diversified life forms: the growth of giant plants and trees in the Carboniferous era 300 million years ago whose decay and burial gave us the supply of the fossil fuel we use today; a mass extinction about 250 million years ago whose cause is not understood; the rise and disappearance of the dinosaurs in another mass extinction about 65 million years ago, thought to have been caused by the collision of a giant meteor with the Earth.