Scientists may be closer to figuring out why the tropical forests that once covered the American Midwest more than 30 million years ago have now turned into cornfields.

And it all has to do with the temperature of the oceans.

According to a new study, the Antarctic Circumpolar Current (ACC) is believed to be responsible for these shifts in the world’s climate, and for longer than was originally believed.

Oceans and global temperatures are connected as warm waters create warmer temperatures and vice versa.

As water temperatures cool, so did the climate around the world, resulting in changes to landscapes.

The ACC is often considered the “mixmaster” of the oceans, as it redirects warmer waters back to the northern Atlantic Ocean. This redirection of water over time is responsible for the four-layer ocean current and the heat distribution system.

"What we have found is that the evolution of the ACC influenced global ocean circulation much earlier than previous studies have shown," said Rensselaer Polytechnic Institute scientist Miriam Katz, who led the study.

The debate among scientists has centered on when the global climate shifted and the extent of the role the ACC had in that.

"This finding is particularly significant because it places the impact of initial shallow ACC circulation in the same interval when the climate began its long-term shift to cooler temperatures," said Katz.

By looking at isotopes in fossil remains from the ocean, Katz and her team reconstructed environmental conditions and analyzed the amounts of carbon and oxygen, along with the ratio of magnesium versus calcium.

These analyses displayed evidence of the redirected ACC waters from more than 30 million years ago, a significantly longer period of time than earlier studies that put the development of the ACC at around 23-25 million years ago.

"By reconstructing the climates of the past, we can explore Earth system responses to current climate change," Katz said.

The study was funded by the National Science Foundation, and appears in the May 27 edition of Science.