Rising temperatures may cause a rise in carbon dioxide, but this does not refute human-caused climate change

Scientists have known that Earth’s climate has changed dramatically over its 4.5 billion-year history, from ice ages to warming periods. Several factors have caused these drastic changes in climate, including variations in the Sun’s intensity, variations in the orbit of Earth, and volcanic eruptions. Scientists have used ice core samples from Antarctica to reconstruct climatic records over hundreds of thousands of years. Based on Antarctic ice core data, changes in CO₂ follow changes in temperatures by about 800 years (although there are studies that suggest this lag is much smaller). The rise in temperature before a rise in CO₂ has led some to conclude that CO₂ simply cannot be responsible for current global warming. This is the conclusion of an article posted on the website “The Daily Skeptic.” The author Chris Morrison uses a study published in November 2020 in the journal Sci to reinforce the claim that temperatures have risen before a rise in carbon dioxide, thus “throwing into doubt the whole of the current theory of human-driven global warming.” This is not the first time this argument has been made by climate change skeptics. For example, Joe Barton, a congressman from Texas stated that “An article in Science magazine illustrated that a rise in carbon dioxide did not precede a rise in temperatures, but actually lagged behind temperature rises by 200 to 1000 years.  A rise in carbon dioxide levels could not have caused a rise in temperature if it followed the temperature.”

We find this claim to be misleading because it fails to tell the whole story. Increasing CO₂ levels can be the cause AND effect of further warming. 

Yes, it seems that increases in CO₂ follow increases in temperature by up to about 1,000 years during interglacial periods. After all, this natural increase in CO₂ helps bring the planet out of ice ages and moderates temperatures during interglacial periods. However, human activities have increased the concentration of carbon dioxide in our atmosphere, amplifying Earth’s natural greenhouse effect. Despite the global pandemic, the global average amount of carbon dioxide hit a new record high in 2020: 412.5 parts per million.¹

We know that CO₂ is a greenhouse gas because it absorbs and emits certain frequencies of infrared radiation. Basic physics tells us that gases with this property trap heat radiating from the Earth. If higher temperatures lead to more CO2 it does not negate the fact that CO₂ leads to higher temperatures. As ocean temperatures rise, oceans release CO₂ into the atmosphere,  In turn, this release amplifies the warming trend, leading to yet more CO₂ being released.  Rising CO₂ levels make the temperature higher than it otherwise would have been. 

Andrew Dessler, Professor of Atmospheric Sciences at Texas A&M University weighs in on the claim that the rise of CO₂ is not the cause of recent temperature increases.

This argument is nonsense.  The scientific community is 100% sure that the increase in CO2 we see in the atmosphere is from the combustion of fossil fuels (with a contribution from land-use changes).

This conclusion is supported by several independent lines of evidence.  First, for the past half-century, each year’s increase in carbon dioxide in the atmosphere has been on average 44 % of what humans released into the atmosphere in that same year. Thus, when humans were emitting smaller amounts of carbon dioxide in the 1960s, atmospheric carbon dioxide was increasing at a slower rate than when humans were dumping large amounts of carbon dioxide in the atmosphere, as we are today. If the source of carbon dioxide emissions were non-human, there is no reason that it would track human emissions so closely.

Second, the carbon dioxide can be chemically “fingerprinted” to show that it comes from fossil fuels. The method is based on isotopes of carbon. All carbon atoms have six protons, but carbon’s isotopes have different numbers of neutrons. The chemical properties of an atom are for the most part set by the number of protons and electrons, so isotopes tend to have similar chemical properties. The chemistry, though, is not identical. Because of this, fossil fuels have a particular isotopic signature (enhanced in carbon-12 and depleted of carbon-14). Scientists measuring the composition of the increasing CO2 in the atmosphere find that it matches the isotopic composition of fossil fuels.

Finally, a challenge for any alternative theory: we know humans are dumping 40 billion tons of CO2 into the atmosphere every year.  If this is NOT causing the increase, where is all of that CO2 going?  The alternative theories never provide an explanation.

As reported by New Scientist back in 2007…

The ice ages show that temperature can determine CO2 as well as CO2 driving temperature. Some sceptics – not scientists – have seized upon this idea and are claiming that the relation is one way, that temperature determines CO2 levels but CO2 levels do not affect temperature.

To repeat, the evidence that CO2 is a greenhouse gas depends mainly on physics, not on the correlation with past temperature, which tells us nothing about cause and effect. And while the rises in CO2 a few hundred years after the start of interglacials can only be explained by rising temperatures, the full extent of the temperature increases over the following 4000 years can only be explained by the rise in CO2 levels.

It is the general consensus of scientists that human emissions of CO₂ and other greenhouse gases push the Earth further out of the range of climate conditions that have characterized the past few million years.²

1. https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide

2. https://iopscience.iop.org/article/10.1088/1748-9326/ac2966