1.5 vs 2.0 degree warming chart

I write this second report from my time at COP21 with a great deal of optimism. The energy here in Paris is amazing, with thousands of individuals from very diverse backgrounds working together to seriously address climate change. 

COP21 with Two Days to Go

December 9, 2015

1.5 vs 2.0 degree warming chart

I write this second report from my time at COP21 with a great deal of optimism. The energy here in Paris is amazing, with thousands of individuals from very diverse backgrounds working together to seriously address climate change. A near-final draft agreement was released earlier today, and overall it looks promising. On the critical issue of a limit to warming, this draft gives three options for nations to continue to debate: below 2° C, well below 2° C, and below 1.5° C.

Robert HowarthHowarth, the David R. Atkinson Professor of Ecology & Environmental Biology at Cornell University, is a member of Cornell's COP21 delegation (watch his COP21 video)

Coming into this COP, I think few expected an agreement any better than below 2° C. However, the science increasingly shows that is insufficient. I saw a great presentation yesterday by Pam Pearson of the International Cryosphere Climate Initiative. They released a report today on this work, which you can view at http://iccinet.org/thresholds. The message? The warming of the Earth already experienced (1° C as of this fall) has been enough to set off an irreversible loss of a major portion of the Antarctic ice sheet. No matter what society does now, that ice will continue to melt and continue to lead to large increases in sea-level rise. As we warm toward 1.5° C, the risk of other such irreversible changes multiplies: think loss of permafrost (with the massive stores of carbon and methane clathrates there!), irreversible loss of the Greenland ice sheet (which combined with the Antarctic loss would eventually lead to sea-level rises of 4 to 10 meters or more), and loss of most mountain glaciers (with huge ramifications for the water these provide to a large percentage of the people on this planet). Pam stressed in her talk, the International Crysophere Climate Initiative stressed in the report, and I stress again here: once we reach global temperatures that set these processes in motion, they will not be reversed on societal time scales. We are talking thousands of years.

The diplomats are hearing these science-based messages, at least in broad brush. And they seem likely to agree to a temperature limit of either less than 1.5° or well below 2°. Either is a huge step forward.

But keep in mind that the sum of the national commitments coming into COP21 will at best keep global warming below 2.7° C (some models suggest 3.5° or more, I am told in my informal conversations with other scientists here in Paris), even if the commitments are kept. And this is where another part of the likely agreement is essential: a formal reanalysis of progress in five years. I would prefer a shorter target – say, three years – but that is probably not practical. It will be a huge amount of work to track how we get to 1.5° or well below 2°. This is a sea-change in the climate-policy world, and one that will take a heavy infusion of new science.

Note that at the current trajectory of global warming, we will reach 1.5° in 12 to 15 years, and 2° in perhaps 35 years or less. And note that we cannot avoid this rate of warming by reducing carbon dioxide emissions. This is causing many of my colleagues to say the target is reachable, but they are wrong. The response of the climate to carbon dioxide emissions is slow, because there is a lot of carbon dioxide in the atmosphere, and it is exchanging slowly with huge pools of carbon in the world’s oceans and terrestrial ecosystems. But the climate responds quickly to changes in methane: there is less methane in the atmosphere, and it is not buffered by exchanges with huge global pools in the manner that carbon dioxide is. And methane is more than 100-fold more potent than carbon dioxide as a greenhouse gas, mass-for-mass, when both gases are in the atmosphere.

So if we are to meet a target of 1.5°, or even 2°, we need to focus on reducing methane emissions. The two largest sources of methane globally are animal agriculture and the oil and gas industry. In the U.S., the largest source is almost certainly the oil and gas industry, and there is little doubt that shale gas development from fracking has aggravated this.

A major science challenge moving forward is how to develop metrics that allow the policy community to see the long-term consequences of carbon dioxide emissions while also demonstrating the short-term consequences from short-lived climate pollutants (SLCPs, a new acronym I learned yesterday), of which methane is undoubtedly the most important climate driver. There is widespread agreement that the approach from Kyoto – comparing carbon dioxide and methane emissions only on an integrated 100-year time period – is simply not up to the job and must be replaced. But with what do we replace it? In my papers, I have urged comparing the two gases at an integrated 20-year time period as well (see howarthlab.org).

Another approach, and one that I sense may gain traction with policymakers, is simply to examine carbon dioxide and the SCLPs differently. We must reduce carbon dioxide emissions because those emissions will influence the global carbon cycle and have an effect on ocean acidity and climate for centuries to millennia to come. And we must reduce methane and other SCLPs because otherwise we cook the planet in the coming one to three decades in a way that has irreversible consequences on ice sheets, and seems likely to cause major ecological and societal damage even on this short time scale. The debate will go on, but I am very happy to be here in Paris and to be part of this event as one of the official observers from Cornell.