Climate change is likely to be the predominant scientific, economic, political and moral issue of the 21st century
Right now, we’re headed towards an ice-free planet. That takes us through the Eemian interglacial period of about 130,000 years ago when sea levels were 15 to 20 feet higher, when temperatures had been thought to be about 1°C warmer than today. Then we go back to the “early Pliocene, when sea level was about 25 m [82 feet] higher than today,” as NASA’s James Hansen and Makiko Sato explain in a new draft paper, “Paleoclimate Implications for Human-Made Climate Change.”
The question is how much warmer was it in the Eemian and early Pliocene than today — and how fast can the great ice sheets disintegrate?
We already know we’re at CO2 levels that risk catastrophe if they are sustained or exceeded for any extended period of time (see Science: CO2 levels haven’t been this high for 15 million years, when it was 5° to 10°F warmer and seas were 75 to 120 feet higher).
Hansen and Sato go further, saying we’re actually at or very near the highest temperatures of the current Holocene interglacial — the last 12,000 years of relatively stable climate that has made modern civilization possible.
They argue that the Eemian was warmer than the Holocene maximum by “at most by about 1°C, but probably by only several tenths of a degree Celsius.” Their make the remarkable finding, that sea level rise will be highly nonlinear this century on our current business-as-usual [BAU] emissions that:
BAU scenarios result in global warming of the order of 3-6°C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain.
While this conclusion takes them well outside of every other recent prediction of sea level rise (SLR), Hansen deserves to be listened to because he has been right longer than almost anyone else in the field (see “Right for three decades: 1981 Hansen study finds warming trend that could raise sea levels“). Also, at least one recent study that attempts to integrate a linear historically-based analysis with a rapid response term finds we are headed towards SLR of “as much as 1.9 metres (6ft 3in) by 2100″ if we stay on BAU (see “Sea levels may rise 3 times faster than IPCC estimated, could hit 6 feet by 2100“).
Hansen and Sato make their case for a strong nonlinear SLR based on a “phase change feedback mechanism,” that, as we’ll see, appears consistent with the recent scientific literature and observations1:
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