We have pushed atmospheric CO2 levels to 400 parts per million (ppm) for the first time in human existence.
At the same time, a truly remarkably set of paleoclimate data shows the climate is much more sensitive to CO2 than we thought. And that means returning as quickly as possible back to 350 ppm is a vastly more rational course of action for a non-suicidal civilization, than, say continuing our unrestrained march toward 600 ppm, then 800, and then 1000.
NOAA reported Friday that the daily mean concentration of CO2 in the air around Mauna Loa, Hawaii, surpassed 400 parts per million this week:
At the same time, a major new Science study of paleoclimate temperatures — based on ”the longest sediment core ever collected on land in the Arctic” – revealed what happened the last time we had similar CO2 levels:
“One of our major findings is that the Arctic was very warm in the Pliocene [~ 5.3 to 2.6 million years ago] when others have suggested atmospheric CO2 was very much like levels we see today. This could tell us where we are going in the near future. In other words, the Earth system response to small changes in carbon dioxide is bigger than suggested by earlier models,” the authors state.
Yes, contrary to one or two (misreported) models suggesting a climate sensitivity on the low side, this study joins the myriad analyses of data that find it is likely to prove on the high side. For instance, recent observations of relative humidity in the tropics and subtropics found that “Future warming likely to be on high side of climate projections,” according to a November paper in Science.
How sensitive is the climate to increases in CO2, according to this “absolutely new knowledge” of paleoclimate temperatures?
Another significant finding to emerge from this first continuous, high-resolution record of the Middle Pliocene is documentation of sustained warmth with summer temperatures of about 59 to 61 degrees F [15 to 16 degrees C], about 8 degrees C [14 F] warmer than today.
This period of Arctic warmth “coincides, in part with a long interval of 1.2 million years when the West Antarctic Ice sheet did not exist.” Indeed, sea levels during the mid-Pliocine were about 25 m [82 feet] higher than today!
It is worth noting that a 2009 analysis in Science found that when CO2 levels were this high 15 to 20 million years ago, it was 5° to 10°F warmer globally and seas were also 75 to 120 feet higher.
The risks of failing to sharply curtail carbon pollution are enormous if the climate sensitivity is on the low side (see “Memo To Media: ‘Climate Sensitivity’ Is NOT The Same As Projected Future Warming, World Faces 10°F Rise“). But the risks of inaction are beyond incalculable if climate sensitivity is in the middle end of the range, let alone the high end suggested by the paleoclimate data:
Science (1/11) study — On our current emissions path, CO2 levels in 2100 will hit levels last seen when the Earth was 29°F (16°C) hotter: Paleoclimate data suggests CO2 “may have at least twice the effect on global temperatures than currently projected by computer models”
As I explained in Nature online back in 2008 (here), once you factor in carbon-cycle feedbacks, even the uber-cautious Fourth Assessment report (AR4) of the IPCC makes clear we are headed toward 1000 ppm (the A1FI scenario). That conclusion has been supported by just about every major independent analysis, including a recent report by PricewaterhouseCoopers (see Study: We’re Headed To 11°F Warming And Even 7°F Requires “Nearly Quadrupling The Current Rate Of Decarbonisation“).
This new paper is just the latest to suggest the Arctic will warm much faster than the models have suggested. For instance, back in 2006, scientists analyzed deep marine sediments to understand the Paleocene Eocene thermal maximum, a brief period some 55 million years ago of “widespread, extreme climatic warming that was associated with massive atmospheric greenhouse gas input.” That Nature study (subs. req’d) found Arctic temperatures almost beyond imagination–above 23°C (74°F)–temperatures more than 18°F warmer than climate models had predicted when applied to this period. The three dozen authors conclude that existing climate models are missing crucial feedbacks that can significantly amplify polar warming.
Clearly our climate models don’t do a good job of explaining what’s happening in the Arctic right now:
Arctic sea ice is melting much, much faster than even the best climate models had projected (actual observations in red). The reason is most likely unmodeled amplifying feedbacks. The image (from Climate Crocks via Arctic Sea Ice Blog) comes from a 2007 GRL research paper by Stroeve et al.
And this underestimation of polar amplification in turn leads the authors of the new study — and many other scientists — to conclude that the climate’s overall sensitivity is on the high side. As the UK Guardian reports: