Physicist John Cook of Skeptical Science has a good piece on “The significance of the CO2 lag” that I’m reposting here, followed by a discussion of the best-studied feedbacks and their likely impact (with links to the literature).
When we examine past climate change using ice cores, we observe that CO2 lags temperature. In other words, a change in temperature causes changes in atmospheric CO2. This is due to various processes such as warmer temperatures causing the oceans to release CO2. This has lead some to argue that the CO2 lag disproves the warming effect of CO2. However, this line of thinking doesn’t take in the full body of evidence. We have many lines of empirical evidence that CO2 traps heat. Decades of lab experiments reveal how CO2 absorbs and scatters infrared radiation. Satellite measurements find CO2 trapping heat and surface measurements confirm more radiation at CO2 wavelengths returning to the Earth’s surface. So the full body of evidence gives us these two facts: warming causes more CO2 and more CO2 causes warming. The significance should by now be obvious. The CO2 lag is evidence of a climate positive feedback.
The magnitude of this positive feedback is calculated in Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change (Scheffer 2006). In this paper, they use reconstructions of past CO2 and temperature to empirically calculate the positive feedback between global warming and CO2. First, they look at pre-industrial CO2 variations during glacial cycles and the Little Ice Age. The relationship between CO2 and temperature is roughly linear.
Figure 1. Relationships between past atmospheric CO2 concentrations and reconstructed temperatures. (a) Reconstructed Northern Hemisphere temperatures of the period 1500-1600 plotted against CO2 levels 50 years later from the Law Dome record. (b) CO2 vs temperature for a 400.000 years period of glacial cycles reconstructed from the Vostok ice core.
Over these periods, changes in CO2 are assumed to be primarily driven by temperature. This is because mechanisms other than changing CO2 (such as changes in solar output) drove temperature over these periods. So looking at Figure 1, we can calculate the effect that temperature change has on CO2 levels. However, this is complicated by the fact that different carbon cycle processes operate on different time scales. On a time scale of years, warming has an effect of around 3 ppm of CO2 per degree Celsius. On a scale of centuries, the effect is much larger – around 20 ppm of CO2 per degree Celsius.
What we’re interested in is the expected global warming by the end of the 21st century so century time-scales are the focus. The most important period for estimating sensitivity of CO2 to temperature on century time-scales is the Little Ice Age. Figure 1a shows how CO2 levels dropped (with a time lag of 50 years) in response to the drop in temperature in this period. From this is calculated a positive feedback of between 15 to 78% on a century-scale.
The benefit of this study is it provides an independent, empirical method of calculating the positive feedback from the CO2 lag. These results are consistent with what’s been found in simulation studies. So when someone mentions to you that CO2 lags temperature, remind them they’re actually invoking evidence for a positive feedback that further increases global warming by an extra 15 to 78%.
— John Cook
JR: We continue to learn about the dangerous positive carbon-cycle feedbacks that threaten to amplify the impacts of human-caused GHGs:
- Science stunner: “Clouds Appear to Be Big, Bad Player in Global Warming” “” an amplifying feedback (sorry Lindzen and fellow deniers)
- So many amplifying methane feedbacks, so little time to stop them all
- Science: Global warming is killing U.S. trees, a dangerous carbon-cycle feedback
- Study: Water-vapor feedback is “strong and positive,” so we face “warming of several degrees Celsius”
Indeed, the best evidence is that the climate is now being driven by amplifying feedbacks — see, most notably:
- The drying of the Northern peatlands (bogs, moors, and mires).
- The destruction of the tropical wetlands
- Decelerating growth in tropical forest trees “” thanks to accelerating carbon dioxide
- Wildfires and Climate-Driven forest destruction by pests
- The saturation of the ocean carbon sink
- The desertification-global warming feedback
The most worrisome of the by amplifying feedbacks is the defrosting of the permafrost: Using the first “fully interactive climate system model” applied to study permafrost, the researchers found that if we tried to stabilize CO2 concentrations in the air at 550 ppm, permafrost would plummet from over 4 million square miles today to 1.5 million. If concentrations hit 690 ppm, permafrost would shrink to just 800,000 square miles. See also “Science shocker: Vast East Siberian Arctic Shelf methane stores destabilizing and venting — NSF issues world a wake-up call: “Release of even a fraction of the methane stored in the shelf could trigger abrupt climate warming.”
High emissions levels + positive feedbacks = climate catastrophe:
- M.I.T. doubles its 2095 warming projection to 10°F “” with 866 ppm and Arctic warming of 20°F
- Our hellish future: Definitive NOAA-led report on U.S. climate impacts warns of scorching 9 to 11°F warming over most of inland U.S. by 2090 with Kansas above 90°F some 120 days a year “” and that isn’t the worst case, it’s business as usual!“
- U.S. media largely ignores latest warning from climate scientists: “Recent observations confirm “¦ the worst-case IPCC scenario trajectories (or even worse) are being realised” “” 1000 ppm
- UK Met Office: Catastrophic climate change, 13-18°F over most of U.S. and 27°F in the Arctic, could happen in 50 years, but “we do have time to stop it if we cut greenhouse gas emissions soon.”
- “The Copenhagen Diagnosis” warns “Without significant mitigation, the report says global mean warming could reach as high as 7 degrees Celsius by 2100.”
- NOAA stunner: Climate change “largely irreversible for 1000 years,” with permanent Dust Bowls in Southwest and around the globe
The time to act was a long time ago, but now is much better than later — and infinitely better than too damn late.