Climate hits 400ppm of CO2 for first time in 3 million years

mauna loa observatory CO2 ppm chart
via UC San Diego

It is a sign of our rapidly changing world that we can get daily updates on the rising levels of carbon dioxide in the atmosphere via Twitter. @Keeling_Curve is the Twitter account of the Scripps Institution of Oceanography and tweets daily readings from the Mauna Loa Observatory in Hawaii, which has been measuring how much carbon dioxide is in the air since 1958. In what is a symbolically significant milestone, on May 9th NOAA reported CO2 levels of 400.03 parts per million (ppm), which is a level unseen for three million years.

This milestone is, undoubtably, bad news. However, the newsworthiness of this moment also serves as an opportunity to educate the public about what this number means for the climate and our future.

What does this number, 400 ppm, mean?

A post at the Scripps Institution of Oceanography puts the milestone in historic context:

Prior to the Industrial Revolution, natural climate variations caused atmospheric CO2 to vary between about 200 ppm during ice ages and 300 ppm during the warmer periods between ice ages. At the dawn of the Industrial Revolution, around the year 1780, the CO2 concentration was about 280 ppm, so CO2 had already risen by around 40 ppm before Keeling began his measurements. Anyone who has breathed air with less than 300 ppm CO2 is now over 100 years old!

What does 400 ppm look like?

Robert Monroe explains what a world with 400 ppm CO2 looks like:

Recent estimates suggest CO2 levels reached as much as 415 parts per million (ppm) during the Pliocene. With that came global average temperatures that eventually reached 3 or 4 degrees C (5.4-7.2 degrees F) higher than today’s and as much as 10 degrees C (18 degrees F) warmer at the poles. Sea level ranged between five and 40 meters (16 to 131 feet) higher than today.

In his great talk on why climate change is simple, David Roberts explained what scientists predict the world will look like at various levels of warming. With warming of 2º Celsius now appearing to be all but inevitable, Roberts focused on what we can expect with warming of 4º C.

Hell on Earth is how he described it.

climate change warming 4 degree celsius David Roberts/Screen capture

Watch the full speech for more.

Here are some reactions to the news.

Justin Gillis at The New York Times puts the warming in perspective:

For the entire period of human civilization, roughly 8,000 years, the carbon dioxide level was relatively stable near that upper bound. But the burning of fossil fuels has caused a 41 percent increase in the heat-trapping gas since the Industrial Revolution, a mere geological instant, and scientists say the climate is beginning to react, though they expect far larger changes in the future.

Damian Carrington at The Guardian notes that "the last time so much greenhouse gas was in the air was several million years ago, when the Arctic was ice-free, savannah spread across the Sahara desert and sea level was up to 40 metres higher than today."

"We are creating a prehistoric climate in which human societies will face huge and potentially catastrophic risks," said Bob Ward, policy director at the Grantham Research Institute on Climate Change at the London School of Economics. "Only by urgently reducing global emissions will we be able to avoid the full consequences of turning back the climate clock by 3 million years."

David Roberts points out why CO2 is a such a huge concern:

Remember, once CO2 is released into the atmosphere, it stays there for around 500 years. For all intents and purposes, it is forever. That’s why, over the long term, it is the atmospheric concentration of CO2 (which, by the way, is now hovering around 400 ppm) that will determine the severity of climate change.

Bryan Walsh at TIME says the 400 ppm number isn't as worrisome as the rate of warming:

The fact that we’re going to cross 400 ppm doesn’t mean that much by itself. It’s not like the sound barrier—the difference in warming between 399 ppm and 400 ppm would likely be minute. But the sheer rate of increase over just the past 55 years shows how fast global warming could hit us in the future—and the present—and underscores how much we’ve failed as a planet to slow down carbon emissions.

Brad Plumer at Wonkblog wonders about when the warming will slow:

Humanity is all but certain to zoom past 400 parts per million. The big question is, how far past?
For a long time, many climate experts thought we should aim to stabilize atmospheric carbon to about 450 parts per million. That goal looks daunting now. At the current rate, the world will pass that mark within a few decades. Indeed, even the most optimistic analyses of current trends, like this one from the International Energy Agency, which predicts that natural gas will displace coal, see us hitting at least 650 parts per million without drastic changes.

Greg Laden puts the milestone in perspective to get ahead of a denier talking point:

So, right now, CO2 should be at a short term peak. The range of this variation is around 8 ppm, so if we hit, say, 401 ppm next week, expect that value to go back below 400 ppm in a few weeks. In other words, we can and should note that we are probably hitting the 400 ppm barrier, but then later when we drop slightly below, temporarily, 400ppm, the climate science denialists will be all over that claiming that there is no global warming. Cuz they’re morons.

Andrew Sullivan waxes philosophic:

This is a new era – in which humanity has the power to change the entire climate of the planet so that it is more clogged with carbon than at any time since homo sapiens took dominion. This is the mark of dystopian science fiction – except that it’s real, and apparently unstoppable. What right does one species have to change the world’s climate so structurally it will destroy countless other life-forms? has launched a slick new page -- -- about what 400 ppm means to the climate movement:

Bill McKibben:

"We're in new territory for human beings--it's been millions of years since there's been this much carbon in the atmosphere. The only question now is whether the relentless rise in carbon can be matched by a relentless rise in the activism necessary to stop it."

Climate hits 400ppm of CO2 for first time in 3 million years
It is a sign of our rapidly changing world that we can get daily updates on the growing levels of carbon dioxide in the atmosphere via Twitter.

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