You’ve got questions, and we have answers. No question is too simple or complex for our panel of science advisors, who stand ready to field your questions about climate change. 

This week, Nadir Jevanjee answers a question we received about CO₂ emissions. Nadir is a Research Physical Scientist at NOAA’s Geophysical Fluid Dynamics Laboratory.

Read other Q&A here, and don’t be shy about asking your questions here.

Warmly,
The C-Change Conversations Team

Question: There is evidence that as the oceans warm they release CO₂. If CO₂ is a major cause of global warming then this represents a positive feedback cycle. If that is the case then why, in the past (over millions of years), have there been times of higher global temperatures and higher CO₂ levels that have not resulted in an ever increasing global temperature?  What turned off the ‘switch’ in the past?  If anthropogenic CO₂ is about 5% of all CO₂ emissions and if CO₂ is about 5% of all greenhouse gases (GHG) – water vapor being 95% – then anthropogenic CO₂ represents about 0.25% of total GHG.  How is it that elimination of such a small fraction of GHG will reverse global temperature changes.

Answer: Sounds like there are two separate questions here. The first is about a ‘carbon-climate feedback,’ in which increasing temperatures drive CO₂ out of the ocean, which should then contribute to further warming. This is indeed a positive feedback, as the questioner points out, but positive feedbacks do not always amplify forever; in most cases, actually, the amplification peters out. This means at some point in the future, ocean temperatures will stabilize at a new equilibrium, but at a much warmer point than today. Unfortunately, it will not happen soon enough to prevent temperature increases that are inhospitable to our species.

As for anthropogenic CO₂ being a small fraction of CO₂ emissions and all GHGs – that probably stems from a confusion between gross and net fluxes of carbon into the atmosphere. As can be seen in Fig. 3 here, there are vast exchanges of carbon between the atmosphere and both the oceans and land biosphere (the gross fluxes), but in a preindustrial climate these net out to zero (zero net flux). Put more simply, the greenhouse gases emitted by the land and ocean get reabsorbed, keeping earth’s natural reservoirs in balance.

This is important. Before humankind started using fossil fuels, GHG levels stayed relatively stable for 10,000 years and helped set and maintain the earth’s temperature at a level that enabled our species to thrive and grow. The GHG that we are adding by burning fossil fuels, while seeming small in the overall picture (gross fluxes), is enough to upset that balance. Small amounts of greenhouse gasses play an outsized role in impacting our climate – they comprise less than .04% of our atmosphere yet prevent our planet from being a frozen uninhabitable wasteland – so adding to them even a relatively small amount has already led to big changes in climate systems.

Also, it is important to remember that climate, like the human body, is impacted by many influences. For example, as the questioner points out, warmer oceans (all else being equal) hold less CO₂, just as warm soda goes flat much faster. Yet, the ocean CO₂ levels are rising, despite the ocean’s warmer temperatures. This is because the dominant influence on oceanic CO₂ levels is the CO₂ concentration in the atmosphere, which must be in balance with the CO₂ in the ocean. As atmospheric CO₂ levels rise, there is more CO₂ entering the ocean from the atmosphere than exiting it, so the oceanic CO₂ levels must rise until a new balance is achieved.

Good references for these topics are “The Global Carbon Cycle” by David Archer, and “The Physics of Climate Change” by Lawrence Krauss.