The last time the Earth enjoyed greenhouse gas levels like those of today was roughly 4 million years ago, during an era known as the Pliocene. The extra heat of average temperatures as much as 4 degrees Celsius warmer turned the tropical oceans into a nice warm pool of bathwater, as noted by new research published in Nature on April 4.

By analyzing the ratio of magnesium and calcium in the shells of microscopic animals found in long cores of mud from the deep ocean, the researchers confirmed this massive oceanic warm pool. At about 28 degrees C, the surface sea temperatures were not much warmer than today's tropical oceans, but these warm waters covered much more of the global ocean surface.

Although such warm water might sound nice, such warm pools of water have profound weather effects; think of El Nino events in the present and the torrential rains this climate pattern creates in some areas. And a Pliocene-like reduction in temperature differences between polar and mid-latitude regions would have similarly profound effects on everything from the number of tropical cyclones in the Pacific Ocean to which areas are covered by desert in Africa, Australia and North America.

The warm water conditions of the Pliocene held steady for more than one million years. In fact, scientists do not understand completely what caused these conditions to change, though a decrease in atmospheric greenhouse gas concentrations of roughly 100 parts-per-million, to around 300 ppm probably helped.

In the last few hundred years, that's about how much humanity has raised greenhouse gas concentrations through the burning of fossil fuels, the cutting down of forests and other activities. Today’s concentration is about 394 ppm.

Although we cannot be sure that a similarly sized decrease in greenhouse gas concentrations ushered in the present era of ice ages singlehandedly, we can be sure that greenhouse gases change climate. And, unless we want to go back to the Pliocene when our distant ancestors split from chimpanzees, we might want to temper climate change by emitting less CO2, methane and other greenhouse gases.