Long Cao and Ken Caldeira from the Carnegie Institution at Stanford have a new paper in Geophysical Research Letters on atmospheric carbon dioxide (CO2) stabilization and ocean acidification, a critical topic for current marine science and public policy. Hoegh-Guldberg et al (2007) illustrated the essential chemistry at the heart of this problem as follows:

Essentially, as CO2 dissolves into the oceans it forms an acid leading to decreased coral calcification and growth through the inhibition of aragonite formation (the principal crystalline form of calcium carbonate deposited in coral skeletons). The increased acidity caused by increasing atmospheric CO2 is known as ocean acidification and it is a separate, though inter-related, phenomenon to increased temperatures caused by CO2 acting as a greenhouse gas.

Cao and Caldeira (2008) found “that even at a CO2 stabilization level as low as 450 ppm, parts of the Southern Ocean become undersaturated with respect to aragonite [and] therefore, preservation of existing marine ecosystems could require a CO2 stabilization level that is lower than what might be chosen based on climate considerations alone.”

These results are similar to Hoegh-Gulberg et al (2007), who concluded “… contemplating policies that result in [CO2]atm above 500 ppm appears extremely risky for coral reefs and the tens of millions of people who depend on them directly, even under the most optimistic circumstances.”

Hoegh-Guldberg et al (2007) illustrated the expected the conditions of coral reefs under different levels of atmospheric carbon dioxide and temperature increases as follows:

These findings are very significant for governments around the world and other policy-makers because much of the current policy debate on climate change focuses on stabilizing greenhouse gases, including carbon dioxide, between 450-550 parts per million carbon dioxide equivalents, thereby allowing a rise in mean global temperatures of around 2-3°C (e.g. Stern 2007; Garnaut 2008; Australian Treasury 2008).

One potential point of confusion for policy-makers in understanding the implications of the results of scientific papers such as Cao and Caldeira (2008) and Hoegh-Guldberg et al (2007) is the difference between stabilization targets based only on atmospheric carbon dioxide levels and targets based on all greenhouse gases, commonly termed carbon dioxide equivalents (CO2-e or CO2-eq). There is considerable confusion surrounding these terms in the scientific literature and the policy debate. Because of this confusion some authors avoid using carbon dioxide equivalents altogether when discussing targets for stabilizing anthropogenic climate change (e.g. Hansen et al 2008).

The major advantage of carbon dioxide only targets is that they are simple to understand and can be measured directly with precision; however, because carbon dioxide is not the only greenhouse gas, targets based on stabilizing the combined effect of all greenhouse gases are important.

The term “carbon dioxide equivalents” is used in two main ways for climate change stabilization targets:

• Some authors use “carbon dioxide equivalents” to refer to the concentration of carbon dioxide that would give the same warming effect as the collective effect of all of the greenhouse gases in the atmosphere. This approach excludes the cooling effect of aerosols (e.g. Stern 2007; Garnaut 2008).

• Some authors define carbon dioxide equivalent concentrations as the net forcing of all anthropogenic radiative forcing agents including greenhouse gases, tropospheric ozone, and aerosols but not natural forcings. This approach includes the cooling effects of aerosols (e.g. Meinshausen et al 2006).

Targets for stabilizing temperature rises between 450-550 ppm “carbon dioxide equivalents” generally refer to the term in the latter usage, thereby considering all components of the atmosphere affecting global temperature rises. However, Gavin Schmidt (2007) noted the confusion surrounding the term on the RealClimate blog in the context of comments by Tim Flannery that “we are already at 455 ppm carbon dioxide equivalents.”

The differences between the usages of these terms are very significant and care must be taken not to confuse them. The terms are very different quantitatively as is evident from the following points draw from IPCC (2007: 102):

•    Atmospheric carbon dioxide reached 379 ppm in 2005 and was increasing by around 2 ppm per year.

•    Including the effect of all long-lived greenhouse gases such as methane, the total concentration of atmospheric greenhouse gases was around 455 ppm carbon dioxide equivalents in 2005 (range 433-477 ppm).

•    However, the cooling effects of aerosols and landuse changes reduce radiative forcing so that the net forcing of human activities was about 375 ppm carbon dioxide equivalents for 2005 (range 311-435 ppm).

While net forcing of all greenhouse gases, aerosols and landuse changes is roughly equal to the effect of carbon dioxide alone at the present time, this may not remain the case in the future particularly if atmospheric pollution levels decrease (thereby reducing the cooling effects of aerosols). Stabilizing at 450-550 ppm carbon dioxide alone could mean significantly higher levels of stabilization when measured in carbon dioxide equivalents, leading to much higher levels of temperature rise than 2-3°C. For instance, if the cooling effect from aerosols was to approach zero in the future due to improved pollution controls, stabilization of atmospheric carbon dioxide alone at 500 ppm might lead to a net warming effect around 570 ppm carbon dioxide equivalents.

There is a real potential for governments and policy-makers to misunderstand and mis-apply information based on using either carbon dioxide only figures or carbon dioxide equivalents. In such circumstances, perhaps the best solution is to clearly flag the differences in the usage of these terms.

While strict word limits in journals might prevent it, authors such as Cao and Caldeira (2008) and Hoegh-Guldberg et al (2007) could avoid the potential for confusion by including in the introduction a statement similar to the following:

“Here we use atmospheric carbon dioxide concentrations to discuss ocean acidification. The use of carbon dioxide alone should not be confused with stabilization targets based on “carbon dioxide equivalents”, which includes the effects of all long-lived greenhouse gases and, in some contexts, the effects of other radiative forcing agents such as aerosols and landuse changes (IPCC 2007).”

Key points:
There are two key take-home points from this discussion:

1.    Carbon dioxide emissions from burning fossil fuels affect coral reefs and other marine life in two separate, though inter-related, phenomena: increased temperatures caused by carbon dioxide acting as a greenhouse gas and increased ocean acidity decreasing calcification and coral growth.

2.    Be careful when discussing stabilization levels for atmospheric carbon dioxide to explain that targets based on carbon dioxide alone differ from targets based on all greenhouse gases and other radiative forcing agents, commonly grouped in the term “carbon dioxide equivalents.”

References:

• Australian Treasury, Australia’s Low Pollution Future: The Economics of Climate Change Mitigation (Australian Treasury, Canberra), http://www.treasury.gov.au/lowpollutionfuture

• Cao L and Caldeira K (2008) “Atmospheric CO2 stabilization and ocean acidification” Geophys. Res. Lett., 35, L19609, doi:10.1029/2008GL035072

• Garnaut R (2008), Garnaut Climate Change Review Final Report (Cambridge University Press), http://www.garnautreview.org.au/index.htm

• Hansen J, et al (2008), “Target Atmospheric CO2 – Where Should Humanity Aim?” Open Atmospheric Sciences Journal, http://www.bentham.org/open/toascj/openaccess2.htm

• Hoegh-Guldberg O, et al (2007) “Coral Reefs Under Rapid Climate Change and Ocean Acidification” Science 318, 1737, DOI: 10.1126/science.1152509.

• IPCC (2007), Climate change 2007: Mitigation. Contribution of WGIII to the AR4 (Cambridge University Press), http://www.ipcc.ch/ipccreports/ar4-wg3.htm

• Meinshausen M, Hare B, Wigley TML, van Vuuren D, den Elsen MGJ, and Swart R, (2006) “Multi-gas emissions pathways to meet climate targets” 75 Climatic Change 151.

• Schmidt G (2007), “CO2 equivalents” RealClimate, http://www.realclimate.org/index.php?p=482

• Stern N (2007), Stern Review of the Economics of Climate Change (Cambridge University Press).