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.
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.