Variations in how the Earth is tilted and its orbit around the sun make for a pattern of planetary warming phases followed by cooling phases across the millennia.
The team's research shows the Earth's overall temperature curve dipping down over about the past 4,000 years, but the downward plod comes to an abrupt halt in modern times.
"If you were to predict -- based on where we are relative to the position of the sun and how we are tilted -- you would predict that we would be still cooling, but we're not," Marcott said.
Instead, the planet is warming up. It hasn't been quite this warm in thousands of years. And it's getting hotter.
By 2100, the Earth will be warmer than ever before, Marcott said. If emissions continue as currently predicted until then, global temperatures will rise "well above anything we've ever seen in the last 11,000 years."
That could be a rise of 2 to 11.5 degrees Fahrenheit, according to the NSF.
What a long-range study means
To get a view on global temperatures that long ago, the researchers studied 73 sediment and polar ice samples, taken from all over the globe. Chemicals found in fossils deep down in the samples span the ages and are good indicators of historic temperatures on Earth, Marcott said.
The scientists did the study to put the global temperature trends into a long-range perspective, Marcott said. Critics of climate change research, which has generally covered the last 1,500 to 2,000 years, have complained that it has been too short-sighted.
They argue that the shorter studies have not taken into account that the warming Earth is seeing today could have happened before naturally -- thousands of years ago.