There are some records we'd rather not see broken. Carbon-dioxide levels in the atmosphere surpassed 400 parts per million for the first time since the U.S. government began recording the concentration of the gas in the air, and likely the first time in millions of years. High levels of carbon dioxide traps heat in the atmosphere and increases global temperatures. The last time it reached 400 parts per million was in the epoch called the Pliocene at least 3 million years ago, when sea levels may have been as much as 60 to 80 feet higher than they are today. For all of human civilization, carbon-dioxide levels have hovered around 280 parts per million.

Already we're seeing the deadly effects of climate change in the form of rising seas, wildfires and extreme weather of all kinds, and passing 400 PPM is an ominous sign of what might come next.

The safe level of carbon dioxide in the atmostphere is 350 parts per million, but the only way to get there is to immediately transition the global economy away from fossil fuels and into into renewable energy, energy efficiency, and sustainable farming practices in all sectors (agriculture, transport, manufacturing, etc.).

While the level fluctuates seasonally and varies across different latitudes, this is yet another sign that our dependence on fossil fuels is out of control.

What This Means

Bill McKibben, Co-Founder, 350.org: "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."

Dr. James Hansen, Former NASA Climatologist: "If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced ... to at most 350 ppm."

Payal Parekh, Coordinator, Global Power Shift: "Crossing the 400 ppm threshold is a somber reminder that we haven't taken the action we need. Nevertheless there is good reason for hope -- activists all across the globe are fighting the fossil fuel industry and demanding clean, just and affordable solutions to our energy needs. At Global Power Shift, a convergence in Istanbul this June, 500 mostly young climate activists from 135 countries will come together to plan a global strategy for change. Upon returning home they'll escalate action and create the global power shift our world needs to push for 350 ppm."

Brad Johnson, Campaign Manager, Forecast the Facts: "May 9, 2013, is a historic day in the worst possible sense of the word. For the first time in the history of the human race, atmospheric carbon dioxide levels have risen above 400 parts per million, NOAA reports. During the ice ages as homo sapiens ("wise man") evolved, levels were between 180 and 300 ppm. During the rise of human civilization, CO2 concentrations were only 280 ppm. Hundreds of billions of tons of fossil-fuel pollution have poisoned our climate, bringing to our world increasingly extreme floods, droughts, and wildfires. We must respond with urgent resolve to end this uncontrolled experiment on our only home.

"Yet the Republican Party maintains climate change denial as a central tenet of their party platform, and President Obama refuses to admit the threat projects like the Keystone XL tar-sands pipeline pose to our future survival. The scientific observations are not in doubt, and the facts are not negotiable. We are fed up, and we will continue to hold those who are pushing us off the climate cliff accountable."

Deirdre Smith, West Coast Fossil Free Organizer: "My grandmother said that we are always walking toward our goals or away from them. As we reach 400PPM I am thinking about her, and wondering which way the the Board of Trustees from any of the 40 fossil free campus campaigns I work with are walking. Perhaps, this will be the moment we decided to walk toward our common goals, to treat each moment as a chance to invest in our future. That's what my grandmother would hope, that's what I hope, that's what students are fighting for now."

Bryan Walsh at Time magazine writes:

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. As Ralph Keeling put it in a statement:

"I wish it weren’t true, but it looks like the world is going to blow through the 400-ppm level without losing a beat. At this pace we’ll hit 450 ppm within a few decades."

There’s no guarantee that we’d experience the same levels of warming in the future if CO2 levels stay that high, but it doesn’t look good. Nor will CO2 levels stop at 400 ppm—barring a virtually impossible immediate turn away from fossil fuels, CO2 emissions will keep growing globally, and CO2 concentrations will keep rising. The U.N.’s official goal is to keep CO2 levels below 450 ppm, and as Ralph Keeling indicated, we’re rapidly running out of time to make that happen. CO2 can stay in the atmosphere for centuries, which means that we’ve already baked in far more warming than we’ve yet experienced. But we will soon enough. The Keeling Curve tells us our past, but it’s also a roadmap for our future—a future that will almost certainly be hotter and wilder.

And from Damian Carrington at The Guardian:

For the first time in human history, the concentration of climate-warming carbon dioxide in the atmosphere has passed the milestone level of 400 parts per million (ppm). 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.

These conditions are expected to return in time, with devastating consequences for civilisation, unless emissions of CO2 from the burning of coal, gas and oil are rapidly curtailed. But despite increasingly severe warnings from scientists and a major economic recession, global emissions have continued to soar unchecked.

Our civilization developed and flourished in an era that never saw CO2 concentrations above 300 ppm. We are in new territory once again, and we have yet to show signs of slowing down, let alone stopping practices that are harmful to our world -- our Earth.