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:
… a 4°C world would be facing enormous adaptation challenges in the agricultural sector, with large areas of cropland becoming unsuitable for cultivation, and declining agricultural yields. This world would also rapidly be losing its ecosystem services, owing to large losses in biodiversity, forests, coastal wetlands, mangroves and saltmarshes, and terrestrial carbon stores, supported by an acidified and potentially dysfunctional marine ecosystem. Drought and desertification would be widespread….
In such a 4°C world, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world.”
We have dawdled so long that even abundant, cheap natural gas can’t save us (see IEA’s “Golden Age of Gas Scenario” Leads to More Than 6°F Warming and Out-of-Control Climate Change). In fact, the IEA’s 6°F high-gas scenario — appropriately acronymed G.A.G. — super optimistically assumes that not only does oil consumption peak in 2020 and drop over 10% by 2035 — but so does coal! That won’t happen without very aggressive climate policies.
Steve Easterbrook’s post “A first glimpse at model results for the next IPCC assessment” shows that for the scenario where there is 9°F warming by 2100, you get another 7°F warming by 2300. Of course, folks that aren’t motivated to avoid the civilization-destroying 9°F by 2100 won’t be moved by whatever happens after that.
I will have more to say about “slow” carbon-cycle feedbacks in a subsequent post, but it is always worth noting that the paleoclimate record suggests the ultimate warming we are going to see is likely to be considerably higher than the fast-feedbacks sensitivity suggests:
- Science (2009): 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
- Scientists analyzed data from a major expedition to retrieve deep marine sediments beneath the Arctic 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.” This 2006 study, published in Nature (subs. req’d), found Artic temperatures almost beyond imagination–above 23°C (74°F)–temperatures more than 18°F warmer than current 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.
- A study published in Geophysical Research Letters (subs. req’d) looked at temperature and atmospheric changes during the Middle Ages. This 2006 study found that the effect of amplifying feedbacks in the climate system-where global warming boosts atmospheric CO2 levels-”will promote warming by an extra 15 percent to 78 percent on a century-scale” compared to typical estimates by the U.N.’s Intergovernmental Panel on Climate Change. The study notes these results may even be “conservative” because they ignore other greenhouse gases such as methane, whose levels will likely be boosted as temperatures warm.
- Another study published in Geophysical Research Letters, “Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming” (subs. req’d), looked at temperature and atmospheric changes during the past 400,000 years. This study found evidence for significant increases in both CO2 and methane (CH4) levels as temperatures rise. The conclusion: If our current climate models correctly accounted for such “missing feedbacks,” then “we would be predicting a significantly greater increase in global warming than is currently forecast over the next century and beyond”–as much as 1.5°C warmer this century alone.
- Science stunner (2011): 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.”