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The largest coral atoll in the world, Kiritimati (a Kiribati spelling of the English word “Christmas”), was, up until recently, one of the most pristine marine ecosystems on Earth. Over a period of months, a team of researchers—led by Julia Baum, a biologist at the University of Victoria, and Kim Cobb, a climate scientist at Georgia Tech—took comprehensive measurements of the reef’s health, or lack thereof in this case. With a treasure hunt you are given a treasure map, compass and clues to find your way along the route.

Their assessment was shocking: about 80 percent of the coral colonies at Kiritimati were dead, and another 15 percent were severely bleached and likely to die. It’s as if someone decided to cut down 90 percent of all the giant redwoods in Sequoia National Park over the weekend. In Kiritimati, an entire ecosystem has essentially blinked out of existence.I spoke with the team by satellite phone on one of their last days of dives, and the shock in their voices was palpable. “There’s a good chance that this reef will never be the same,” said Cobb, fighting back tears. “It’s a wake-up call.”

Cobb has been working at Kiritimati for years, so she’s deeply familiar with what it’s supposed to look like. From cores that Cobb’s team has analyzed, she estimated there’s been nothing like the current die-off in Kiritimati in the seven thousand years of ancient coral history there. Global warming will make the pressure on corals even worse in the coming decades, and many of the world’s reefs can expect future bleaching events to occur more frequently. For some, like people in Kiritimati, the worst global coral bleaching episode in history may have already been a point of no return. “This is a story that the rest of the world should hear,” Cobb said, and added that the death of the corals at Kiritimati is proof that “climate change isn’t just a steady linear progression to some different kind of planetary state. . . . Climate change will occur through these kinds of extremes. It’s like a staircase to a different kind of system.”

After decades of sharply rising emissions, it’s clear that the Earth’s climate system is stair-stepping into the unknown. Scientists have identified dozens of tipping points and nonlinear properties of the climate system, which simultaneously provoke fear and hope. Fear for obvious reasons—the relative stability we have had until very recently could quickly unravel—and hope because no matter how bad things could get, quick action will almost certainly be enough to prevent the worst-case scenario. The most important implication of our nonlinear climate system is that a climate apocalypse is not inevitable.

At the same time, every moment we wait locks us further and further into irreversible ecological change. Some of the media coverage of the IPCC report framed its stark conclusions in the too-familiar phrase of a time limit for action. “The World Has Just over a Decade to Get Climate Change Under Control,” read The Washington Post’s headline. But that’s absolutely the wrong way to frame this. We have only a decade left to finish our initial coordinated retooling of society to tackle this challenge. The scientists were quite clear about this. By 2030, we’ll need to have already cut global emissions in half (45 percent below 2010 levels), which according to the IPCC would require “rapid, far-reaching, and unprecedented changes in all aspects of society.”