About three million years ago, horses and camels roamed on what today we know as the icy high Arctic region.

In 2013, an instrument in Hawai'i, United States, recorded a milestone we haven't seen since that period: the amount of carbon dioxide (CO2) in the atmosphere there exceeded 400 parts per million (ppm) for the first time since it began to be measured 55 years before.

Carbon dioxide is Earth's main greenhouse gas, which scientists say are causing global warming. These gases are released when we burn fossil fuels, like oil, natural gas and coal. The gases get trapped in the atmosphere, storing heat and making the world hotter.

Three million years ago, seas were at least 9.1 meters (30 feet) higher. Today, at that level, oceans would flood many major cities around the world.

A Warmer Planet

The planet was about 3.6 to 5.4 degrees Fahrenheit warmer back then. However, the Earth then was in the final stage of a prolonged greenhouse age, and CO2 concentrations were on their way down. This time, 400 ppm is a milestone on a far more rapid uphill climb toward an uncertain climate future.

Two independent teams of scientists measure CO2 on top of a mountain in Mauna Loa, Hawai'i. One team is from the U.S. National Oceanic and Atmospheric Administration (NOAA), the other from the Scripps Institution of Oceanography. Scripps is part of University of California at San Diego.

The NOAA posted that the daily average for May 9, 2013, was 400.03 ppm. The Scripps team later confirmed the milestone had been crossed.

The late Charles David Keeling started the Mauna Loa measurements in 1958. His "Keeling curve" is a measurement used to mark the steady climb in CO2 levels caused primarily by burning fossil fuels. This measurement has become a key marker of climate change.

When Keeling started at Mauna Loa, the CO2 level was at 315 ppm. When he died in June 2005, it was at 382. Why did he keep working for 47 years?

Keeling had a "faith that the world could be made better by devotion to just causes," said his son, Ralph. Now, the Keeling curve shows just how much humans are changing the world—for better or worse.

Plant Life And CO2

CO2 peaks in May every year. By June the level begins falling, as spring begins in the Northern Hemisphere. This is where most of the planet's land is concentrated.

Plants draw CO2 out of the atmosphere to make food and fuel new growth. By November, the CO2 level gets around 5 or 6 ppm lower than in May.

Then the curve will turn upward again: In the winter, plants stop making food. Still, they continue to burn energy, releasing CO2 back into the atmosphere.

Think of it as the breath of northern forests. It is the natural part of the Keeling curve. The human-made part is the breath's steady upward climb from one year to the next.

In 2016, Earth's whole atmosphere averaged 400 ppm for the whole year. We are now likely permanently above this number. What difference will that make?

Back To The Pliocene Period?

In a way, 400 ppm is just a number, like a .400 batting average in baseball. Still, no one has had a .400 average — 4 hits for every 10 at-bats — since Ted Williams in 1941. A .400 average is not much different than .399 or .401. A .400 average is just a marker.

Still, this says something important about baseball. The same goes for CO2 in Earth's atmosphere.

Lawmakers worldwide have tried to reach a global agreement on reducing fossil fuel emissions. However, their efforts have not succeeded as countries haven't been able to agree.

Many scientists argue that the CO2 concentration must be stopped at 450 ppm to avoid the worst effects of climate change. Some activists argue for a tougher goal of 350 ppm. NOAA has not recorded an average monthly CO2 reading below 350 ppm at Mauna Loa since October 1988.

The last time the concentration of CO2 was as high as 400 ppm was probably in the Pliocene Epoch, a time period between 2.6 and 5.3 million years ago. Until the 1900s, it certainly hadn't exceeded 300 ppm, let alone 400 ppm, for at least 800,000 years. That's how far back scientists have been able to measure CO2. They did so by measuring bubbles of ancient air trapped in Antarctic ice.

However, tens of millions of years ago, CO2 must have been much higher than it is now. There's no other way to explain how warm the Earth was then.

Hardly Any Green On Greenland Today

In the Eocene Epoch, some 50 million years ago, there were alligators and tapirs, a pig-like creature, living off northern Greenland. Today, Greenland is mostly covered in ice. But back then, these creatures were living in swampy forests, like those in the southeastern United States today. CO2 may have been anywhere from two to 10 times higher in the Eocene than it is today.