NASA’s James Hansen and Makiko Sato have a new draft paper that brings potential climate tipping points into more focus and the results are incredibly important. They examine some differences between two recent geologic times, the Eemian and the Pliocene, and today. During the Eemian, sea levels were 15-20 feet higher than today. During the Pliocene, sea levels were 82 feet higher than today. If we maintain our business-as-usual (BAU) greenhouse emissions path, the authors state that multi-meter sea level rise within this century becomes “almost dead certain” because of nonlinear responses to that forcing. To be clear, that means that future temperature increases will not be equal for the same amount of future emissions. Instead, future emissions will cause a radical and unreversable jump in global temperatures, which will lead to radical jumps in sea level rise. We won’t have a 1″ per year increase in sea level. The difference year after year will be greater and greater as the climate system attempts to find a new stable region.
I want to point out that Hansen and Sato use extremely strong language for climate scientists. Will it be strong enough to generate the political will necessary to take us off that path?
So what is the difference in global temperatures between today (during the Holocene epoch) and the Eemian period (130,000 years ago to 114,000 years ago), which was the second-to-last interglacial period? Only several tenths of a degree Celsius. Keep in mind that the relative stability of the climate system during the Holocene has been identified as one of the primary reasons our species was able to establish civilization. There is no rule stating that has to be the case in the future. If global temperatures rise back to that level, high-latitude (poleward) feedbacks that are large and amplifying start to kick in. These feedbacks are of a more desirable type: they are not runaway feedbacks. They amplify global warming forcing in the polar regions, but do not by themselves directly lead to catastrophic climate shifts.
The next question is this: what happens past “several tenths of a degree Celsius warming?”, or what happens if the peak temperature during the Holocene is exceeded by only 1°C? The large, amplifying feedbacks and additional forcing push us past a more critical tipping point: when ice sheets disintegrate and sea levels rise rapidly. That temperature rise is far, far below the 3-6°C rise projected by recent BAU emissions scenarios. These scenarios would, according to these new results, almost certainly led to tens of feet of sea level rise. Note that the 1°C threshold is even well below the European Union’s 2°C scenario. To date, the EU has been much more aggressive in phasing out fossil fuels than the Unites States or countries like China and India.
Historically, rise in global temperatures and sea level rise during natural climate forcing scenarios were “rapid” in relation to geologic time-scales. The transfer from a warmer globe to a cooler globe with ice sheets took many thousands of years. Humans have become the dominant forcing mechanism of the globe’s climate. Unless quite rapid and widespread action is taken, we face a large amount of sea level rise on an extremely short time frame. As the authors note: “It is difficult to predict a time scale for the sea level rise, but it would be dangerous and foolish to take such a global warming scenario as a goal.” Joe Romm points out that we will know whether the authors are correct in this assessment in less than 20 years. But we’re dangerously close to a critical tipping point that, if passed, would lead to locking our descendants into today’s worst-case climate projections.
Cross-posted at SquareState.