Carbon dioxide – the gas scientists say is most responsible for global warming – has reached levels in our atmosphere not seen in 3 million years, scientists announced this week in a new study.

At that time, sea levels were as much as 65 feet higher than they are now, Greenland was mostly green and Antarctica had trees.

“It seems we’re now pushing our home planet beyond any climatic conditions experienced during the entire current geological period, the Quaternary,” said study lead author Matteo Willeit of the Potsdam Institute for Climate Impact Research in Germany. “A period that started almost 3 million years ago and saw human civilization beginning only 11,000 years ago. So, the modern climate change we see is big, really big; even by standards of Earth history.”

© Provided by USA TODAY, a division of Gannett Satellite Information Network, Inc.Emissions of carbon dioxide (CO2) from human activities are the primary reason for climate change.

Willeit and his colleagues used sophisticated computer simulations of Earth’s past climate to reach their conclusion. He said his team compared their results with hard data from the deep sea, which matched what the computers said.

Today, CO2 levels measure over 410 parts per million. While that may not sound like a huge amount, scientists have known for decades that even trace amounts in the atmosphere can raise temperatures around the world.

The burning of fossil fuels such as coal, oil and gas releases greenhouse gases such as carbon dioxide and methane into Earth’s atmosphere and oceans.

The extra CO2 caused temperatures to rise to levels that cannot be explained by natural factors, scientists report. In the past 20 years, the world’s temperature has risen about two-thirds of a degree Fahrenheit, the National Oceanic and Atmospheric Administration said.

Carbon dioxide is called a greenhouse gas for its ability to trap solar radiation and keep it confined to the atmosphere.

It is invisible, odorless and colorless yet is responsible for 63% of the warming attributable to all greenhouse gases, according to NOAA’s Earth System Research Laboratory in Boulder, Colo.

All the nations of the world – except for the U.S. – are part of the Paris climate agreement, which aims to reduce humanity’s emissions of carbon dioxide and other greenhouse gases, in order to prevent rising global temperatures.

The study appeared in the peer-reviewed journal Science Advances, a publication of the American Association for the Advancement of Science.

This article originally appeared on USA TODAY: Earth’s carbon dioxide levels highest in 3 million years, study says