I was an outsider to the climate negotiations in Paris, and I was astonished and delighted to hear of the 1.5 degrees Celsius target for peak warming that was agreed to in the last days of the negotiations, rather than the 2 C standing target for the last decade. It doesn’t sound like a huge difference, but we are already essentially at about 1 C, so 1.5 C represents half as much further warming. I always had the feeling that for a climate scientist to call 2 C a “danger limit” would be like an engineer offering a design for an unreliable bridge, on the grounds that the clients probably didn’t want to pay for a safe one. It is definitely necessary to do so, because the excess carbon accumulates in the carbon cycle—the interplay of carbon as it travels between atmosphere, ocean, and land surface—driving global temperature to plateau at a new normal that is just below the peak, so that we have a “forever” incentive to keep the peak warming as low as possible. A temperature target of 1.5 C at Paris, to me, meant that the leaders of the world in some sense get it about the scariness of the threat. The message from the scientific community was received!
What will it take, to make this new target a reality? There are complexities and uncertainties in the Earth’s climate system and carbon cycle, in which there is not a simple one-to-one connection between cause and effect; sometimes a relatively small input, working in combination with other factors, can have an end result that is far beyond its size, making it “nonlinear.” Fortunately for those of us who study it, there is also a simplifying effect, in which two nonlinearities effectively cancel each other out, much like an algebra problem. It turns out that the temperature responds nonlinearly to atmospheric carbon dioxide content, because of something called the band saturation effect (so that warming is proportional to the number of carbon dioxide doublings, rather than scaling directly to parts per million). Up to now, the counteracting nonlinearity has been the dissolution of carbon dioxide into the ocean (but that will decline as the acidity of the ocean increases and its ability to maintain a constant pH decreases).
As a result of these nonlinearities canceling each other, the peak temperature we expect turns out to depend simply and linearly on the total amount of carbon we ultimately release to the atmosphere. It doesn’t matter if it’s released quickly or slowly, what matters is ultimately how much is released. There is of course considerable uncertainty in this, due to how sensitive the climate is to changes in carbon levels and how much warming there ultimately is in relation to a rise in carbon dioxide. In addition, there is the possibility that once we humans release enough carbon dioxide into the atmosphere, it will trigger the release of maybe 50 percent more carbon from natural sources such as thawing peats and degrading soil.
So far, humankind has liberated about 0.5 trillion metric tons of carbon into the carbon cycle, from fossil fuels and deforestation. The warming that resulted from that is about a one-degree average Celsius temperature change globally. This means that we are already half-way toward an original goal of limiting the increase in global atmospheric average temperatures to 2 C, or 1 trillion tons of carbon. This would have been attainable by ramping down emissions, beginning the sooner the better. If we started today cutting emissions by about 3 percent per year, they would be down 80 percent by 2050 (a commonly cited metric among politicians), and the total would add up to enough to take us to 2 C.
The new goal of 1.5 C sets a limit of about 0.75 trillion tons. This target, which allows for 0.25 trillion tons net further emission, will probably require what some call “negative emissions,” which is another way of saying cleaning up our mess by removing carbon dioxidefrom the atmosphere. It would be expensive and energy intensive, and may come with the issue of what to do with the carbon dioxide, but it’s not impossible. Given that climate change from carbon dioxide is forever, it would be worth it, no matter how long it took. Of course, a much cheaper course would be to prevent the emissions in the first place.
The Paris agreement is, admittedly, toothless. There are no prescriptions of how much any nation should do in cutting carbon dioxide emissions and no penalties for failing to cut adequately. The only legal requirement is for each country to report its carbon dioxide emissions, make specific target pledges to decrease it, and to report on a regular basis whether it is successful or not.
That said, however, I think it can work, even if the enforcement mechanism is a national shame. Given the new target is 1.5 C instead of the old 2 C, it is clear that the message from the scientific community was received. The world’s leaders finally seem to get the scariness of the threat. And let us not forget the impact of public opinion; if the weather continues to get weirder and droughts more damaging, public and international pressure will ratchet up and intensify in a way that didn’t happen for other huge social transitions, such as the abolition of slavery (in most parts of the world). As recognition grows that carbon dioxide emission causes harm to others, the social structures that support it, such as fossil fuel subsidies and environmental laws, can change abruptly. Social systems seem to me even tippier than climate.
Atmospheric carbon dioxide is a global potential tragedy of the commons, and stabilizing Earth’s climate will require more collectively decided direction of human activity than we as a species have ever yet been forced to accomplish. But the Paris agreement gives me hope that we will rise to the occasion.
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