Earth might be maxing out its best protection against a major pollutant

Isolated in the middle of the Pacific Ocean at 11,135 feet above sea level, two silent machines perched on the upper north face of Hawaii's Mauna Loa volcano print out a number every hour.

That figure - a measure of carbon dioxide concentrations in the atmosphere - has become the global standard for tracking how human activity is affecting the planet. It's been rising ferociously for decades - even as we've begun to pollute the planet less.

This worrisome phenomenon suggests that humans have essentially maxed out our planet's best protection against climate change. This protection comes in the form of so-called "carbon sinks" - patches of land and ocean that absorb large chunks of the carbon dioxide we pump into the atmosphere. For the past several decades, they've been helping to keep the planet from warming as quickly as it otherwise would. But now they may be at capacity, prompting the Earth to continue cooking even as we curb our emissions from oil and gas.

A recent New York Times article likened the phenomenon to "garbage workers going on strike, but on a grand scale."

If these sinks are full and they can no longer suck up a hefty portion of the emissions we're putting out, the future scenario is grim. Carbon dioxide levels would keep rising, and climate change would worsen.

Back in 1958, a scientist named Charles David Keeling was the first person to measure the CO2 in the air. His project at the Mauna Loa volcano, which was initially slated to run for only one year, turned into a 24/7 measurement of global CO2 levels. You've probably seen the rising line known by scientists as the "Keeling Curve" in a presentation or two about climate change.

Keeling's son Ralph now leads the carbon dioxide program at the Scripps Institute for Oceanography. In a recent post for Scripps, he explained that he reason CO2 levels didn't begin leveling off once humans started polluting less was "pretty simple."

Basically, Keeling explained, the sinks we have now can only suck up about half of the carbon dioxide we emit. The other half is still building up in the atmosphere. In order to start making that red line on the Keeling Curve level off, we'd have to slash our pollution levels by a whopping 50%.

That has not happened. Even in Europe, where greenhouse gas emissions have been decreasing pretty significantly, they only dropped 22% in the 25 years between 1990 and 2015.

Even if we begin to curb our emissions severely, we won't be out of the danger zone, Keeling wrote.

"Eventually, additional emissions cuts would be required because the sinks will slowly lose their efficiency as the land and ocean start to saturate."

Think of a dry sponge as an example. At first, that sponge is great at sucking up water. But as it gets more and more full, it can absorb less and less. And that, essentially, is what's happening to our carbon sinks.

That means that if we plan on stopping climate change, we're going to have to do a lot more than we're doing now.

"A permanent stabilization at current levels ... requires both an immediate 50% cut as well as a slow tapering thereafter," wrote Keeling, "eventually approaching zero emissions."