When Americans celebrated the first Earth Day on April 22, 1970, the planet's atmosphere was markedly different than it is today. Nearly 50 years ago, scientists measured Earth's levels of carbon dioxide — the planet's most important greenhouse gas — at around 325 parts per million, or ppm.

Now, almost five decades later, that number has shot up to around 412 ppm, nearly 90 ppm higher. It's a change atmospheric researchers, geologists, and climate scientists call unparalleled in at least 800,000 years, though it's likely carbon dioxide levels haven't been this high in millions of years.

"The rate of CO2 increase since the first Earth Day is unprecedented in the geologic record," said Dan Breecker, a paleoclimatologist at The University of Texas at Austin.

"No matter how you look at this it’s totally unprecedented," agreed Kris Karnauskas, an associate professor in the Department of Atmospheric and Oceanic Sciences at the University of Colorado Boulder.

"It’s totally unprecedented"

"The last time CO2 levels were this high, the sea level was many feet higher than it is today," added Matthew Lachniet, a climate scientist at the University of Nevada at Las Vegas. This was a warmer geologic period on Earth called the Pliocene, spanning some 2.5 to 5 million years ago. Earth's oceans were some 30 feet higher then, noted Lachniet, after the planet's ice sheets melted into the sea.

UPDATE: April 22, 2019, 9:05 a.m. EDT: As of 4/20/2019, the Scripps Institution of Oceanography reports that CO2 ppm is now over 414 ppm. Earlier in the month updates were postponed due to an equipment failure.

Just how unprecedented are today's CO2 levels?

Over the last million years, Earth's CO2 levels have certainly fluctuated, but they've naturally wavered between 180 and 280 ppm, explained Jason Briner, a paleoclimatologist at the University of Buffalo.

CO2 levels over the last 800,000 years. Image: Scripps Institution of oceanography

But on Earth Day in 2019 we've "now exceeded" even the highest ceiling of natural CO2 swings by some 130 ppm. In short, it's not normal. Especially over the last 49 years, since the first Earth Day.

"Dang," said Briner. "87 ppm in 49 years."

The CO2 rate isn't just really high — it's picking up steam

In the 1970s, after the first Earth Day, CO2 levels were going up by about 1 ppm per year. But in recent years the rate has increased to, on average, over 2 ppm, said Karnauskas. That rate is unheard of over the last 800,000 years (Scientists have direct proof of Earth's CO2 levels from as far as 800,000 years ago from air bubbles trapped in ancient ice.)

Previous rises in carbon dioxide levels have simply been more gradual events. "Past climate changes pale in comparison," said Karnauskas.

Earth can't keep up with these changes

We're pumping colossal amounts of CO2 into the planet's skies.

Normally, Earth can deal with this excess carbon. Over longer periods of time the planet absorbs the carbon into the oceans and the rocky ground. But today these changes are simply happening too rapidly. The planet just can't consume the CO2 deluge.

When the rate of CO2 release is fast, like it is now, this carbon is gulped up by the oceans, explained Breecker. Today, about 31 percent of human-generated CO2 is absorbed into the seas. But at such a fast rate (especially since the first Earth Day), the ocean surface can only soak up so much carbon dioxide at once, while the rest stays in the air and heats the planet.

When Earth has more time to deal with CO2 increases — say on the order of hundreds of thousands of years — this carbon is also stored away in rocks, in a well-understood process called "silicate weathering."

Rising CO2 ppm since around 2005. Image: nasa / noaa

But today, there's no time for these slow-moving natural processes to deal with historically high greenhouse gas emissions. C02 emissions are just increasing too quickly.

"The rate of CO2 emissions is very important," said Breecker. "It affects how much of the CO2 that is emitted stays in the atmosphere and thus contributes to warming."

Where are we headed?

Without significant and ambitious efforts to slash carbon emissions this century, we are going to blow through 500 ppm, no question.

In a "business-as-usual" scenario, which means emissions continue unchecked, humanity will see some outrageous carbon numbers, which portends significant future warming. "If we maintain business-as-usual, we can conceivably hit 700 ppm in the future relatively easily," said Lachniet.

Under this high carbon emissions scenario, global average temperatures are expected to rise this century to between 4.7 and 8.6 degrees Fahrenheit — depending on how sensitive Earth is to these unprecedented levels of CO2.

The red line shows a high carbon emissions scenario. Image: BOB KOPP / ECONOMIC RISKS OF CLIMATE CHANGE: AN AMERICAN PROSPECTUS

Even if society makes some emissions cuts — though not on the order of the ambitious emissions cuts called for by the historic Paris Agreement — we're on track for some 625 ppm by the century's end, noted Karnauskas. That's a 190 percent increase from CO2 levels on the very first Earth Day.

Although political leadership in the U.S. is still actively fostering misinformation about climate science, the United Nations (UN) has made clear that society must radically decarbonize to spare the future from the worst consequences of climate change. "The next few years are probably the most important in our history,” Debra Roberts, an environmental scientist and a lead author of the UN's latest climate report, said in a statement.

The most important thing to know about climate sensitivity is that it's not zero https://t.co/5c0VgTIN2X — Kate Marvel (@DrKateMarvel) March 21, 2019

Yet with 412 ppm and counting, we're already locked in for significant future warming. "The Earth will continue to warm for centuries in the future," said Lachniet. "It takes the planet a while to catch up."

"Scary times ahead."

More heat promises more severe drought and extreme, pummeling weather. But limiting the planet's carbon load — say, to under 500 ppm — will be a boon to children today, and to humanity beyond.

"The decisions we make or don’t make today will have an influence on climate 1,000 years from now," said Lachniet.

As things now look on Earth Day 2019, the trends and magnitude of the CO2 increase since 1970 doesn't bode well.

"Scary times ahead," said Briner.