The major media continue to sow confusion on one of the central questions of our time: How much warming will we subject our children and countless future generations to?
The answer to that question depends primarily on four factors:
- The so-called “equilibrium climate sensitivity” – the sensitivity of the climate to fast feedbacks like sea ice and water vapor. The ECS is how much warming you get if we suddenly adopt a super-aggressive effort to cut carbon pollution and only double CO2 emissions to 560 ppm — and there are no major “slow” feedbacks. We know the fast feedbacks, like water vapor, are strong by themselves (see Study: Water-vapor feedback is “strong and positive,” so we face “warming of several degrees Celsius” and Skeptical Science piece here).
- The actual CO2 concentration level we hit, which on our current emissions path is far, far beyond 550 ppm (see U.S. media largely ignores latest warning from climate scientists: “Recent observations confirm … the worst-case IPCC scenario trajectories are being realised” — 1000 ppm).
- The real-world slower (decade-scale) feedbacks, such as tundra melt (see “Carbon Feedback From Thawing Permafrost Will Likely Add 0.4°F – 1.5°F To Total Global Warming By 2100“).
- Where they live — since people who live in the mid-latitudes (like most Americans) are projected to warm considerably more than the global average.
The media, perhaps aided by some scientists who aren’t great at communications, tend to focus on just #1, a number the IPCC Fourth Assessment Report pegged as “likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.” While the majority of studies tend to be in the middle of the range, a couple have been near the low end, though some have been at the higher end.
In any case, focusing on the fast-feedback sensitivity perhaps made sense in the distant past when there was some reasonable chance of stabilizing at 560 parts per million atmospheric concentrations of carbon dioxide (double the preindustrial level) and some hope the slow feedbacks might not matter.
Indeed, the scientific community focused on a doubling I think in part because they didn’t believe humanity would be as self-destructive as brainless frogs and ignore the increasingly dire warnings for over two decades now.
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“). That means it doesn’t matter terribly much whether the ECS is 3C, or, say, only 2.5C.
It is worth noting that while the Thawing Permafrost Could Cause 2.5 Times the Warming of Deforestation (!) and add up to 1.5°F to warming in 2100 by itself, “Participating modeling teams have completed their climate projections in support of the [IPCC's] Fifth Assessment Report, but these projections do not include the permafrost carbon feedback.” D’oh!
Given that the Arctic is already losing ice decades faster than any AR4 model had projected, we should expect that the permafrost will go faster than the models suggest. Indeed a 2008 study by leading tundra experts found “Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss.” The study’s ominous conclusion:
We find that simulated western Arctic land warming trends during rapid sea ice loss are 3.5 times greater than secular 21st century climate-change trends. The accelerated warming signal penetrates up to 1500 km inland….
Anyone who tells you the recent literature suggests things will be better than we thought, hasn’t read the recent literature. In a 2010 AAAS presentation, the late William R. Freudenburg of UC Santa Barbara discussed his research on “the Asymmetry of Scientific Challenge“: New scientific findings since the 2007 IPCC report are found to be more than twenty times as likely to indicate that global climate disruption is “worse than previously expected,” rather than “not as bad as previously expected.”
“Projections of global warming relative to pre-industrial for the A1FI emissions scenario” — the one we’re currently on. “Dark shading shows the mean ±1 s.d. [standard deviation] for the tunings to 19 AR4 GCMs [IPCC Fourth Assessment General Circulation Models] and the light shading shows the change in the uncertainty range when … climate-carbon-cycle feedbacks … are included.”
Again, we are headed to 11F and just keeping to 7F will take a major effort. But warming beyond 7F is “incompatible with organized global community, is likely to be beyond ‘adaptation’, is devastating to the majority of ecosystems & has a high probability of not being stable (i.e. 4°C [7F] would be an interim temperature on the way to a much higher equilibrium level,” as climate expert Kevin Anderson explains here.
Everyone interested in what we face should should read the recent World Bank Climate Report, which concluded, “A 4°C [7°F] world can, and must, be avoided” to avert “devastating” impacts. Also worth reading is the Royal Society Special Issue on Global Warming, which details the “hellish vision” of 7°F (4°C) world (and is the source of the figure above). The concluding piece in the issue notes soberly: