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Model based conclusions?

Posted by mbcannell on 14 Dec 2013 at 00:59 GMT

The authors state: "Atmospheric CO2 and other GHGs have been well-measured
for the past half century, allowing accurate calculation of their climate forcing." However it is far from clear that "accurate calculations" are possible due to the serious uncertainties in the models (note that all models have failed independent tests of hindcasting).

The idea of coral destruction by a fall in pH due to an increase CO2 is also quite uncertain. That ancient limestone corals exist and were laid down when C02 levels were very much higher than 1000 ppmV (carboniferous period) immediately points to the uncertainty in such conclusions/predictions and some laboratory tests also show little effect of CO2 per se (Raynaud et al., 2003).

No competing interests declared.

RE: Model based conclusions?

jim-hansen replied to mbcannell on 20 Dec 2013 at 18:30 GMT

On the contrary, we can calculate climate forcing accurately. The calculation does not require a climate model. It is a radiation calculation, employing, e.g., the pre-industrial CO2 amount and today's CO2 amount. Required radiative (absorption) properties of CO2 are measured in the laboratory and independently calculated from basic quantum theory.
The commenter may be intending to question the calculated climate response, not the climate forcing. The climate response requires knowledge also of climate sensitivity, which the commenter seems to believe we obtain from a climate model. Actually we employ climate sensitivity (an equilibrium fast-feedback sensitivity of 3 degrees Celsius for doubled atmospheric CO2) obtained from analysis of paleoclimate data. It happens that climate models favor a similar climate sensitivity, but models by themselves have a wider range of uncertainty than that which we infer from paleoclimate information.
Yes, coral reefs have existed at prior times when atmospheric CO2 was at higher levels than today. However, the rate of change of ocean temperature and ocean chemistry today is much greater than during these previous changes (possibly for tens of millions of years; Honisch et al. 2012). Today's much higher rates of change dwarf the ability of organisms such as reef-building corals to genetically adapt to the new conditions. Despite the slower rates of change in the paleo record, many ‘reef gaps’ (periods in which carbonate reefs are absent during times of high CO2) exist in the record (Veron 2008). Rapid changes in temperature are almost certain to also cause well defined deleterious thresholds to be exceeded by mid to late century (Hoegh-Guldberg 1999). What we point out is that coral reefs are now threatened, and in observed decline, because of the combined effect of a range of factors including ocean warming and increasing ocean acidification. On time scales that humanity cares about, the prospects for healthy coral reefs are extremely poor if we do not halt the rapid growth of atmospheric CO2.
Furthermore, the suggestion that CO2 amounts were higher than 1000 ppm during the Carboniferous is in doubt, indeed, it is highly unlikely. Although direct CO2 measurements are not available, proxy CO2 measures (Royer 2014) and carbon cycle models (Berner 2008) suggest that 300-500 ppm is probably more representative of Carboniferous CO2 amount.
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2. Veron JEN: Mass extinctions and ocean acidification: biological constraints on geological dilemmas. Coral Reefs 2008, 27:459-472.
3. Hoegh-Guldberg O: Climate change, coral bleaching and the future of the world's coral reefs. Marine and Freshwater Research 1999, 50:839-866.
4. Royer DL: Atmospheric CO2 and O2 during the Phanerozoic: tools, patterns, and impacts. In Treatise on Geochemistry 2nd Edition, Elsevier, 2014
5. Berner RA: Phanerozoic atmospheric oxygen: New results using the GEOCARBSULF model. American Journal of Science 309: 603–606.

No competing interests declared.