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.