Science stunner: 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"
by Joseph Romm, Climate Progress, January 13, 2011
The disinformers claim that projections of dangerous future warming from greenhouse gas emissions are based on computer models. In fact, ClimateProgress readers know that the paleoclimate data is considerably more worrisome than the models (see Hansen: ‘Long-term’ climate sensitivity of 6 °C for doubled CO2). That’s mainly because the vast majority of the models largely ignore key amplifying carbon-cycle feedbacks, such as the methane emissions from melting tundra (see Are Scientists Underestimating Climate Change?).
Climate models are invaluable tools for understanding Earth’s climate system. But examination of the real world also provides insights into the role of greenhouse gases (carbon dioxide) in determining Earth’s climate. Not only can much be learned by looking at the observational evidence from Earth’s past, but such know ledge can provide context for future climate change.
The atmospheric CO2 concentration currently is 390 parts per million by volume (ppmv), and continuing on a business-as-usual path of energy use based on fossil fuels will raise it to ∼900–1100 ppmv by the end of this century (see the first figure) . When was the last time the atmosphere contained ∼1000 ppmv of CO2? Recent reconstructions [2–4] of atmospheric CO2 concentrations through history indicate that it has been ∼30–100 million years since this concentration existed in the atmosphere (the range in time is due to uncertainty in proxy values of CO2). The data also reveal that the reduction of CO2 from this high level to the lower levels of the recent past took tens of millions of years. Through the burning of fossil fuels, the atmosphere will return to this concentration in a matter of a century. Thus, the rate of increase in atmospheric CO2 is unprecedented in Earth’s history.
What was Earth’s climate like at the time of past elevated CO2? Consider one example when CO2 was ∼1000 ppmv at ∼35 million years ago (Ma) . Temperature data [5,6] for this time period indicate that tropical to subtropical sea surface temperatures were in the range of 35 to 40 °C (versus present-day temperatures of ∼30 °C) and that sea surface temperatures at polar latitudes in the South Pacific were 20-25 °C (versus modern temperatures of ∼5 °C). The paleogeography of this time was not radically different from present-day geography, so it is difficult to argue that this difference could explain these large differences in temperature. Also, solar physics findings show that the Sun was less luminous by ∼0.4% at that time . Thus, an increase of CO2from ∼300 ppmv to 1000 ppmv warmed the tropics by 5-10 °C and the polar regions by even more (i.e., 15-20 °C).
What can we learn from Earth’s past concerning the climate’s sensitivity to greenhouse gas increases? Accounting for the increase in CO2 and the reduction in solar irradiance, the net radiative forcing — the change in the difference between the incoming and outgoing radiation energy – of the climate system at 30-40 Ma was 6.5-10.0 W m−2 with an average of ∼8 W m−2. A similar magnitude of forcing existed for other past warm climate periods, such as the warm mid-Cretaceous of 100 Ma . Using the proxy temperature data and assuming, to first order, that latitudinal temperature can be fit with a cosine function in latitude , the global annual mean temperature at this time can be estimated to be ∼31 °C, versus 15 °C during pre-industrial times (around 1750) . Thus, Earth was ∼16 °C warmer at 30-40 Ma. The ratio of change in surface temperature to radiative forcing is called the climate feedback factor . The data for 30-40 Ma indicate that Earth’s climate feedback factor was ∼2 °C W−1 m−2. Estimates [1,11] of the climate feedback factor from climate model simulations for a doubling of CO2 from the present-day climate state are ∼0.5-1.0 °C W−1 m−2. The conclusion from this analysis — resting on data for CO2 levels, paleotemperatures, and radiative transfer knowledge — is that Earth’s sensitivity to CO2radiative forcing may be much greater than that obtained from climate models [12-14].
Elsewhere (Hansen et al. 2007a), we have described evidence that slower feedbacks, such as poleward expansion of forests, darkening and shrinking of ice sheets, and release of methane from melting tundra, are likely to be significant on decade-century time scales. This realization increases the urgency of estimating the level of climate change that would have dangerous consequences for humanity and other creatures on the planet, and the urgency of defining a realistic path that could avoid these dangerous consequence.
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 15-78% 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.
A second 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.
“We found that a warming of 12 degrees Fahrenheit would cause some areas of the world to surpass the wet-bulb temperature limit, and a 21-degree warming would put half of the world’s population in an uninhabitable environment,” Huber said.
“When it comes to evaluating the risk of carbon emissions, such worst-case scenarios need to be taken into account. It’s the difference between a game of roulette and playing Russian roulette with a pistol. Sometimes the stakes are too high, even if there is only a small chance of losing.”
The above arguments weave together a number of threads in the discussion of climate that have appeared over the past few years. They rest on observations and geochemical modeling studies. Of course, uncertainties still exist in deduced CO2and surface temperatures, but some basic conclusions can be drawn. Earth’s CO2concentration is rapidly rising to a level not seen in ∼30 to 100 million years, and Earth’s climate was extremely warm at these levels of CO2. If the world reaches such concentrations of atmospheric CO2, positive feedback processes can amplify global warming beyond current modeling estimates. The human species and global ecosystems will be placed in a climate state never before experienced in their evolutionary history and at an unprecedented rate. Note that these conclusions arise from observations from Earth’s past and not specifically from climate models. Will we, as a species, listen to these messages from the past in order to avoid repeating history?
- Royal Society special issue details ‘hellish vision’ of 7 °F (4 °C) world — which we may face in the 2060s!
- A stunning year in climate science reveals that human civilization is on the precipice
- 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 — “We have shown that this dramatic rise in sea level is associated with an increase in CO2 levels of about 100 ppm.”
- Nature Geoscience study: Oceans are acidifying 10 times faster today than 55 million years ago when a mass extinction of marine species occurred
- S. Solomon et al. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon et al., Eds. (Cambridge Univ. Press, Cambridge, UK, 2007).
- M. Pagani, J. C. Zachos, K. H. Freeman, B. Tipple, & S. Bohaty, Science 309, 600 (2005); 10.1126/science.1110063. doi: 10.1126/science.1110063 Abstract/FREE Full Text
- B. J. Fletcher, S. J. Brentnall, C. W. Anderson, R. A. Berner, & D. J. Beerling, Nat. Geosci. 1,43 (2008). CrossRefWeb of Science
- D. O. Breecker, Z. D. Sharp, & L. D. McFaddenn, Proc. Natl. Acad. Sci. U.S.A. 107, 576(2010). Abstract/FREE Full Text
- P. K. Bijl, S. Schouten, A. Sluijs, G. J. Reichart, J. C. Zachos, & H. Brinkhuis, Nature 461,776 (2009). CrossRefMedlineWeb of Science
- P. N. Pearson et al., Geology 35, 211 (2007). Abstract/FREE Full Text
- D. O. Gough, Sol. Phys. 74, 21 (1981). CrossRef
- D. L. Royer, Geochim. Cosmochim. Acta 70, 5665 (2006). CrossRefWeb of Science
- G. R. North, J. Atmos. Sci. 32, 2033 (1975). CrossRef
- The cosine temperature expression can be integrated analytically to obtain the global annual mean temperature. Paleotemperatures from (5) for a subtropical location and a high southern latitude location were used to determine the two coefficients in the analytical expression for global mean temperature.
- S. E. Schwartz, Clim. Change; 10.1007/s10584-010-9903-9 (2010).doi:10.1007/s10584-010-9903-9 CrossRef
- J. Hansen et al., Open Atmos. Sci. 2, 217 (2008).
- P. K. Bijl, A. J. Houben, S. Schouten, S. M. Bohaty, A. Sluijs, G. J. Reichart, J. S. Sinninghe Damsté, & H. Brinkhuis, Science 330, 819 (2010). Abstract/FREE Full Text
- D. J. Lunt et al., Nat. Geosci. 3, 60 (2010). CrossRefWeb of Science