350 or Bust!

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The most important number in the world … that nobody's talking about.

Any talk about solving the climate crisis is necessarily peppered with all sorts of measurements: global temperatures, sea levels, emissions reductions, and so on. But, there's one metric out there that might be the most essential (and, sadly, least understood): the concentration of carbon dioxide in the earth's atmosphere,More so than any other statistic, it holds the fate of the planet in its balance. Researchers measure this concentration with the over-my-head, science-speak factor "parts per million" (ppm). As it goes up, our world warms-causing droughts to worsen, severe storms to increase, ice to melt, and sea levels to rise. The question isn't whether or not we need to slow its uptick-we do-but rather, "How high is too high?"The answer to that question, according to a report by ten of the world's top climate scientists (including NASA's James E. Hansen), is anything above 350 ppm. "If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted," the treatise says, then that's the ceiling.The problem with the 350 mark: We're already past it.Back before the Industrial Revolution, the concentration of carbon dioxide in the earth's atmosphere was around 275 ppm. For the past couple hundred years, this concentration climbed steadily as humans made a habit of digging up and burning fossil fuels and chopping down forests. Today it's up to 385 ppm and rising 2 ppm every year.Thus, Hansen et al.'s paper--titled Target Atmospheric CO₂: Where Should Humanity Aim?--is a tough diagnosis. But, it's not necessarily a fatal one. Longtime environmental writer and climate activist Bill McKibben compares it to a "patient who finds out from his doctor that his cholesterol is too high. We're out of the zone of safety and into the zone where heart attacks and strokes (ice shelf collapses? epic droughts?) become more likely." Along with a tireless team of young climate activists, McKibben started an organization called 350.org to "take the number 350 and beat it into every head and heart on planet Earth, to tattoo it into every brain." (Full disclosure: McKibben is a friend, longtime mentor, and reviewer of my book.)Fostering global awareness of such a wonky number is a tough task. In fact, it's still struggling to penetrate the research community (even with the support of Hansen, who's been ahead of the pack for more than 25 years). As climate science emerged as a field, researchers rather arbitrarily doubled the CO₂ concentration before the Industrial Revolution to arrive at the de facto ceiling of 550 ppm. Over the past five years, however, a whole slew of increasingly troubling studies revealed that the long term "negative feedback" loops in climatic systems-like thawing permafrost releasing trapped greenhouse gasses or heat-reflecting polar ice melting down to heat-absorbing open water-leading scientists to embrace a target of 450 ppm.Everyone agrees, of course, that immediate action is essential. Climate Progress blogger Joe Romm, an energy expert who exhaustively analyzes climate science and policy, makes a pretty bulletproof case that 350 ppm would be virtually impossible to reach, at least by the end of this century. "Whether the ultimate target is 350, 400, or 450," he notes, "you can't hit any of those targets without strong and relentless action starting January 20, 2009." To that end, Romm spent much of this past year comprehensively outlining what he calls the only politically feasible plan to pull up short of 450 ppm by 2100.The findings of the Target Atmospheric CO₂ group warn that resting on anything higher than 350 ppm would run the risk of returning the planet to a state when it was ice free, and sea levels were 70 meters (over 200 feet!) higher than they are today. "The stakes, for all life on the planet, surpass those of any previous crisis," the authors write in the paper's conclusion. "The greatest danger is continued ignorance and denial, which could make tragic consequences unavoidable."The "tipping level for catastrophic effects" is 350 ppm. We cannot afford to be ignorant of that any longer.(Photo courtesy of The Library of Congress)