CO2 is a greenhouse gas that caused warming during past climate changes, notably as a feedback amplifying other factors

Key takeaway

Because of the physics of the transfer of electromagnetic radiation, CO₂ acts as a greenhouse gas, meaning that temperatures must increase as the concentration of CO2 increases. This is true whether natural factors initiate warming (as in many past climate changes) and the release of CO₂, or the warming is initiated by a release of CO₂ (as humans have done).

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Verdict:

Flawed reasoning

Claim:

Increases in atmospheric CO2 followed increases in temperature. Therefore, CO2 levels could not have forced temperatures to rise.

Verdict detail

Flawed Reasoning: A lag between the initiation of past warming due to other factors and rising CO2 does not mean mean that CO2 cannot have caused temperature to increase further.
Misrepresents a complex reality: CO2 has been shown to begin increasing before temperature in some instances of past climate change, but also acted as a feedback that amplified warming caused by other factors like cycles in Earth’s orbit.

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Full Claim

Historically, increases in atmospheric CO2 followed increases in temperature, they did not precede them. Therefore, CO2 levels could not have forced temperatures to rise.

Christopher Colose

Research Scientist, SciSpace LLC, NASA Goddard Institute for Space Studies

This is both false and irrelevant.

Claims that “CO₂ led temperature in the past, therefore cannot have caused it to rise” originated over a decade ago from a misrepresentation of ice core research (that itself has been subject to significant refinements in dating). It was based on the fallacy that since other factors influence climate (in this case, changes in the Earth-Sun geometry) and that the carbon cycle is affected by climate, the converse cannot be true. Of course, this is not logically coherent, and in practice is wrong since the radiative effect of CO₂ is well-established. Indeed, CO₂ would not be expected to fluctuate on its own 100,000 year timescale on its own, independent of the climate.

In fact, more recent research* shows that CO₂ still led global temperatures and the full deglacial process, unlike in older literature that examined only Antarctic sites.

CO₂ has also “led” global temperature on geologic timescales, and is largely responsible for how Earth’s temperature evolved over the last 50 million years. There are many ways to change the partitioning of carbon between the Earth and atmosphere, and how this happens is not relevant for the fact that if more CO₂ is in the atmosphere, the planet will get warmer. Today, however, the excess source of carbon to the atmosphere is from humans.

• Shakun et al (2012) Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation, Nature

Lauren Simkins

Assistant Professor, University of Virginia

This claim is flawed. Ice core records of past greenhouse gas and atmospheric temperature change¹, coupled with records of ocean temperature and circulation changes², indicate that there are complex feedbacks between earth-atmosphere-ocean changes that lead to naturally variable greenhouse gas changes. In some cases during past deglaciations, increases in CO₂ have lagged methane (CH₄) increases and associated atmospheric temperature rise, owing to natural processes that induce greenhouse gas release into the atmosphere. This is not the case for twentieth century and beyond human-induced atmospheric CO₂ and temperature increases.

Regardless of the source and cause of atmospheric CO₂ increase, it will have a warming effect. Basic science does not change; CO₂ is a greenhouse gas that is released into the atmosphere by burning of fossil fuels and leads to atmospheric warming.

• 1- Monnin et al (2001) Atmospheric CO2 Concentrations over the Last Glacial Termination, Nature
• 2- Skinner et al (2010) Ventilation of the Deep Southern Ocean and Deglacial CO2 Rise, Science

Jeremy Fyke

Postdoctoral researcher, Los Alamos National Laboratory

In the natural Earth system, CO₂ release acted as a feedback of naturally-forced change (e.g. due to millennial-scale, gradual, changes in the Earth’s orbit). Thus, CO₂ is clearly established as an important forcer of, for example, ice ages. This demonstrates it’s effectiveness as a radiative gas. Now, of course, the situation is flipped because humans are actively emitting CO₂. This is why it is now a “forcer” rather than a “feedback”. This change in no way impacts our century-old understanding of how CO₂ warms the climate.