The world was abuzz last week when China and the U.S. formally joined the Paris Agreement. As the world’s two biggest emitters, this was indeed good news and a positive step forward in confronting climate change. But it was just that: a “step.” The fact is that we have postponed our response to climate change for so long that mitigation is no longer sufficient—we will have to invest heavily in adaptation too. The planet itself will underscore that fact in the coming weeks when it does something truly remarkable—and with little fanfare.
It’ll happen on a volcano in Hawaii, where a monitoring station has been recording levels of carbon dioxide in our atmosphere continuously since the late 1950s. One of the most celebrated and storied datasets for earth scientists, the Mauna Loa observatory is little known outside climate circles. But the Keeling Curve data that it has produced has been extremely influential, having first quantified humanity’s role in increasing CO2 levels in our atmosphere. And, it first documented a remarkable thing: The Earth literally “breathing” as plants grow in spring and summer and take in CO2, and then slowly release CO2 as they are harvested or die off.
CO2 levels usually bottom out in late September or early October before the “exhale” of CO2 begins. Levels at this “exhale” point have always dropped below 400 parts per million (ppm)—the milestone at which scientists say it will be difficult to keep climate change in check once crossed. In the next few weeks, when this “exhale” point is upon us once again, we could very well cross that milestone and bid farewell to a sub-400 ppm world once and for all.
It’s worth taking a moment to consider this. In the past 50 years we have increased the CO2 in the atmosphere by 30%, at a rate that continues to increase, to a level that is higher than at any time in the past 15 million years. It is as astounding as it is concerning.
This underscores the inescapable problem with today’s climate mitigation policies: Any results will not come quickly enough to escape the impacts of climate change. The CO2 emitted today effectively persists in our atmosphere for decades or even centuries. So in the air that you are now breathing, a couple of the CO2 molecules could be coming from the first U.S. coal power plant built to provide power to New York City in 1882. We can see this history by looking at cumulative emissions, which show that the past emissions of the U.S. and Western Europe have gotten us where we are—not current big emitters like China.
Our policies do nothing to eliminate the global stock of CO2 in our atmosphere today. Instead, like the Paris Accord targets, they limit the flow of CO2 produced at a country level while the global stock continues to increase. And, in fact, some don’t even go that far, for various reasons that I’ve written about before. Countries like India are working to limit the carbon intensity of their economies. This will still allow emissions, the flow of CO2, to increase, just at a slower rate. Even the most optimistic policy scenario—where each country cooperates on aggressively mitigating CO2 emissions (the Paris Accord commitments are quite far off)—may only get us below 400 ppm again at the end of this century, as the figure shows.
The reality is, as countries slowly work to limit carbon, the parts per millions of CO2 in our atmosphere will continue to increase. With it, so will the clear signs each day that climate change is already upon us—from the massive flooding in Louisiana to California’s intense drought. Mitigation efforts—no matter how strong—will not help the millions of people here in the U.S. and around the world already being impacted by climate change. Strong mitigation to stem the tide of future change must be paired with policies to adapt to the changes we are already experiencing and will experience in the years ahead.
The truly unfortunate fact of this policy scenario where both mitigation and adaptation is needed is that it’s going to cost us a lot more than we would have needed to spend had we dealt with greenhouse gases long ago. Back in 1990, when the first scientific consensus report on climate change was released by the International Panel on Climate Change, it stated that the world only needs to reduce emissions by 2% a year to keep us well below a 400ppm world. We failed to act then, and now bear the double burden of climate costs—adapting to changes today and mitigating to prevent worse damages in the future.
What’s more, while mitigation is costly, adapting is even costlier. Calculating these costs is difficult and fraught with uncertainties (I’m part of a group of researchers working to improve these estimates in the coming years). But mitigating years ago would certainly have been the cheapest route. (See here for a simple summary of the current knowledge on the cost of mitigating versus adapting)
Breaking thresholds like 400 ppm reminds us that it takes a long time to see a difference in our environment. But they also show us how far into unknown territory our current path takes us. We’re about to say goodbye to a sub-400ppm world for at least the rest of our lives. All of the territory from now forward is unknown. Welcome to a new world. It’s time to start adapting.