These shorter studies have been based on methods that are very different from the Harvard-OSU research, but in the 2,000 years that they overlap, the results have been basically the same.

"Our data shows that ... they didn't miss anything," Marcott said. And the parallel results corroborate the precision of the new research as well, he said.

Humanity in the last 11,500 years

The scientists chose the period of time known as the "Holocene" for their research, because it is the most recent natural warm phase in Earth's history. It began at the end of the last Ice Age about 11,500 years ago, and we are still in it.

The Holocene has also been the epoch of human achievement, the beginning of civilization. Stable weather patterns helped people do more of everything they wanted to, partly because they no longer had to fight the cold of an ice age.

They began farming, which extended their own life spans and increased population on Earth. They built cities and roads, made art, developed languages and laws. They formed empires and nations.

Eventually, they invented machines, landing themselves in the industrialized age, driven by engines and turbines, which are powered by combustible fuel.

Thus began man-made greenhouse gases.

The world tomorrow

The main culprit is carbon dioxide, and its levels have jumped in the last 100 years, Marcott said. In the 11,000 years prior, it only changed "very slowly," he said.

Marcott is concerned about people's ability to adapt to a perhaps drastically changed climate.

"As civilization has grown, we're kind of set up for things not to change too much," he said.

The last time Earth has been as warm as it is projected to be by 2100 was before the last Ice Age started -- over 130,000 years ago. That's too long ago to gather reliable data on, he said.

He didn't want to speculate on what the world will look like, if global warming continues.

"I certainly hope we can pull ourselves out of it," he said.