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CO2 Hits Climate Milestone

If the folks at Guinness World Records kept tabs on climate change, they’d be taking note that the planet has hit a milestone: levels of heat-trapping carbon dioxide in the atmosphere have averaged more than 400 parts per million each day for the entire month of April. That’s the first time this has happened in at least 800,000 years — and maybe much longer than that.

The number doesn’t mean much by itself. Global warming isn’t measurably stronger at 400 ppm than it was at 399, or than it will be at 401. But it’s a strong reminder that this powerful, long-lived greenhouse gas is continuing to rise, pretty much unchecked. The increase started around 1800, when we started burning fossil fuels (mostly coal to start) in a big way at the start of the Industrial Revolution. It really took off in the 20th century as we began generating electricity and driving cars.

Before 1800, CO2 never went above 300 ppm as far back as we can measure. Scientists know that from studying air bubbles trapped in ancient ice in Greenland and Antarctica. They also know that when CO2 was relatively low, the planet went through a series of ice ages. When it was relatively high, Earth enjoyed relatively warm periods. But now we’ve got significantly more CO2 in the atmosphere than there was even during the warm periods, and climate scientists have some hints that we’re actually at the highest levels in perhaps 15 million years.

Carbon dioxide concentrations will probably drop below 400 ppm by midsummer, as the vast forests of the Northern Hemisphere leaf out fully and begin breathing it in. The sawtooth pattern in the graphic shows just that in careful CO2 measurements over the past half-century or so: it builds during winter when deciduous trees are leafless, then declines in summer when the trees and vegetation take in the CO2 before the process starts building again the next fall. Overall, though, the movement is steadily upward. We actually topped 400 ppm in May, 2013, but that was only for a few days. This year, we could stay there for a couple of months, starting in March and lasting possibly as long as July. Within a few years, we likely won’t drop below 400 ppm, even in fall.

We’re already seeing the consequences of rising CO2. There are more severe and longer-lasting heat waves, an increase in torrential downpours, rising sea levels, and ocean acidification. All of these effects and more are projected to get worse if carbon dioxide levels continue to rise.