By the time our great-grandchildren have children of their own, we humans will likely have broken a climate record that has stood unchallenged for 56 million years.

New research has found that humans are pumping nearly 10 times more carbon dioxide into the atmosphere than what was emitted during Earth's last major warming event, called the Palaeocene-Eocene Thermal Maximum (PETM).

If carbon emissions continue to rise in the future, mathematical models predict that within the next few hundred years, we could be facing another PETM-like event. 

In other words, in the near future, Earth could resemble its distant past: a time when the Arctic was free of ice, inhabited by crocodiles and dotted by palm trees.

"You and I won't be here in 2159, but that's only about four generations away," warns palaeoclimate researcher Philip Gingerich from the University of Michigan.

"When you start to think about your children and your grandchildren, and your great-grandchildren, you're about there."

The PETM is often used as a benchmark for current global warming. During this time, rapid climate changes saw landscapes transformed, oceans acidified, and widespread extinctions triggered.

It took more than 150,000 years for the world to recover, but what happened then has nothing on what is happening now.

Global temperatures during the PETM peaked at about 7 degrees Celsius (13 degrees Fahrenheit) higher than today's average, and we are quickly catching up to those heights.

The new study suggests that if nothing changes, within 140 years, humans could pump out the same amount of greenhouse gases released during the entire PETM.

"The fact that we could reach warming equivalent to the PETM very quickly, within the next few hundred years, is terrifying," says Larisa DeSantis, a palaeontologist at Vanderbilt University, who was not connected to the new study.

The reason it's terrifying is because we are headed off the road map. Today, climate scientists use the PETM as a case study for what global warming might do to our planet and when those changes might be expected.

But as useful as this has been, today, we live in a different world. While the PETM is thought to have occurred from a comet or a volcano, our current climate catastrophe is being fuelled primarily by humans, at a rate unseen in Earth's climate record.

It's also happening in the middle of what should be a cooling trend, in a time when the world is full of different ecosystems and species.

With all of these variable factors, the new research suggests that using the PETM as a gauge for current warming may not be quite so useful in the future.

"Given a business-as-usual assumption for the future, the rates of carbon release that are happening today are really unprecedented, even in the context of an event like the PETM," says Gabriel Bowen, a geophysicist at the University of Utah, who was not connected to the new study.

"We don't have much in the way of geologic examples to draw from in understanding how the world responds to that kind of perturbation."

It looks like our descendants are on their own.

This study has been published in Paleoceanography and Paleoclimatology.