For once, the news on the environmental front isn’t just good, it could be taken as a point of pride. This past week, scientists announced that the infamous ozone hole over Antarctica is starting to heal. In 1987, the world agreed to phase out chemicals that were destroying a layer of gas in the upper atmosphere that shields the planet from damaging ultraviolet light. Last week, in the journal Science, researchers said they’re finally starting to detect results. In September, the hole had shrunk by nearly 3.9 million sq. km from its peak in 2000.

There is a sobering side to this story, though: The chemicals responsible for the ozone problem break down in the atmosphere much more quickly than carbon dioxide connected to global warming does. That’s why the same MIT atmospheric chemist who announced the ozone improvement also argues that climate change caused by burning fossil fuels is essentially “irreversible.” For scientists, optimism and pessimism have to be tempered by the realities of chemistry and reaction rates.

That chemist, Susan Solomon, explained that while most pollutants break down quickly, atmospheric carbon dioxide is very stable — it doesn’t go anywhere for a long time. According to her calculations, it will take hundreds or even thousands of years for the excess created by coal plants and cars to be taken up by plants or absorbed into the oceans. And it will take even longer for the warmed atmosphere and oceans to cool back down.

That’s why so many scientists are keen to act now rather than waiting. Any worsening of the situation will get locked in for generations to come.

The ozone-layer story, she said, does inspire some hope that people can be proactive when it comes to global environmental problems. In the 1970s, scientists realized that common chemicals called chlorofluorocarbons would destroy ozone, and satellite measurements showed a loss of about 3 percent per decade. While this didn’t create any obvious day-to-day problems, it scared scientists because ozone in the upper atmosphere prevents Earth from getting a lethal dose of ultraviolet radiation. Without the ozone layer, there would be no life.

By the 1980s, scientists observed a much more disturbing trend — more than half of the stratospheric ozone had been lost over Antarctica. They started referring to this as an ozone hole. In some layers of the atmosphere, there was no ozone at all — “a total wipeout,” Solomon said.

Her investigations back then helped explain what was going on. She found that the chemicals responsible for ozone depletion — called chlorofluorocarbons — were destroying ozone much faster over Antarctica through a chemical reaction that takes place in very cold conditions on the surface of special clouds called polar stratospheric clouds. For that work, she was awarded the National Medal of Science. (The researchers who discovered the link to chlorofluorocarbons in the first place were awarded the Nobel Prize.)

The U.N.’s Montreal Protocol of 1987 banned the use of chlorofluorocarbons, which had been used as a propellant for hair spray and deodorant and a coolant for refrigerators and air conditioners. Some voiced concern that the treaty would deprive people of refrigeration. But substitutes were found, and life went on. (Even The Donald found another way to prop up his hair.) As a global call to action, she said, “this was unprecedented.”

Luckily for us, Solomon said, the ozone-depleting chemicals eventually break down into compounds that fall to the lower atmosphere where they dissolve in rain and get carried to the ground. She said scientists had expected the ozone hole to start improving by now, and were watching for changes using balloons and satellites. The hole is actually a seasonal event — it opens each Antarctic winter. The improvement, she said, is that the time period the hole stays open is shortening.

But when Solomon looked at carbon dioxide and global warming, she got a very different result: CO2 was going to hang around in the atmosphere for thousands of years. This wasn’t a big surprise to many scientists. Some had made similar kinds of calculations to reach roughly the same unsettling conclusion.

One of the first of these was done in the early 1990s by Penn State geoscience professor James Kasting. He looked at how quickly carbon dioxide released by burning fossil fuels would be taken up by plants or absorbed into the oceans. He also concluded that it would take centuries. Part of the problem, he said, is that the earth’s biosphere takes up and gives back carbon dioxide in equal measures. There’s a limit to how fast excess from human activity can be absorbed. The more carbon dioxide gets added to the system, the slower the normal carbon “sinks” can absorb it.

“We think the repair of the ozone hole is slow, but it’s fast compared to reversing the CO2 increase,” he said. It’s not that the climate will never recover, but that it will happen so slowly that on human timescales, it might as well be forever.

After Solomon’s paper on the irreversibility of climate change came out, she followed with another paper in Science, explaining that “irreversible” doesn’t mean “unstoppable.” Some degree of change is inevitable, and future centuries will be affected by our emissions, for sure. But for the most part scientists say it’s not too late to prevent the most severe consequences, which would include flooding of major cities and a new American dust bowl. The difference hinges on the decisions we make over the next few decades.

Faye Flam writes about science, mathematics and medicine.

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