De Menocal is investigating the possible cause of the drying that created the grasslands: long-term changes in tropical sea-surface temperatures in the faraway Pacific and Indian Oceans. De Menocal, who specializes in analyzing deep-sea sediment cores, sees the story in tiny plankton whose shells changed chemical composition depending on temperature; when they died, they settled to the bottom, leaving a layered record. 2 million years ago, the world was generally warmer than now, and tropical sea-surface temperatures were more or less the same across the oceans. Warmth generally produces rain, and it appears to have been wet in a huge east-west band from South America to east Africa.
But around that time, the globe began cooling, and sea-surface temperatures got differentiated. One core taken south of Indonesia shows that there, it actually warmed slightly. Meanwhile, another core, taken off the east coast of Africa, shows a pronounced cooling, and probable shifts in wind patterns in that region. De Menocal and colleagues including Yale University climate modelers Alexey Fedorov and Chris Brierly believe that this probably redistributed rains, spurring a 30 to 80 percent reduction over east Africa. The physics are fairly simple; in warmer areas, water evaporates, which makes rain possible. When water-laden air hits a bordering colder region, the water condenses, and the moisture drops out. Thus the now-cooler eastern Indian Ocean started intercepting rains that used to make their way to east Africa. Meanwhile, Indonesia became just about the rainiest place on earth. It remains so today.
Anthropologist René Bobe of George Washington University noted that grass-eating animals expanded greatly in abundance and diversity around this time in east Africa, and their fossils are easy to find. However, he said, fossils of early humans are much rarer, so their history is less well understood.
Whatever the meaning for early evolution, there could be relevance for today. Before the temperature-gradient switch was thrown, the atmosphere contained about 400 parts per million of carbon dioxide—possibly the reason the pre-human world was so warm. Then, carbon dioxide declined, and things cooled off. Now, with growing human emissions, we may reach 400ppm again by the year 2070–the first time in 2 million years. Will we end up reorganizing sea-surface temperatures and creating radically different weather systems than the ones the human race grew up with? “These results seem to indicate that if you dial into the future, the world might look quite different,” said de Menocal. “But the time scales of past changes were so much greater. What happened millions of years ago may not be what we see in 60 years. At this point, we just don’t know.”