Misleading Carbon dioxide is the dominant driver of contemporary warming, which is happening very rapidly. However, other factors were also responsible for regulating Earth’s climate on longer timescales in the geological past, such as the distribution of continents, volcanic activity and the brightness of the sun.
Misleading While a few studies in the 1970s speculated about the possibility of global cooling, the majority of the climate research community indicated that future warming would occur.
CLAIM: “[Carbon dioxide] is about 0.0003 or 0.0004% of the atmosphere…it is not a climate heat driver…when the Earth had extraordinarily larger amounts of carbon dioxide in the atmosphere, we had an ice age and there were no human beings here…They [climate scientists] thought in the 1970s we were going to have a new ice age.”
A recent video published on CBN News’ Facebook page alleges that carbon dioxide is not responsible for driving contemporary global warming. Jerome Corsi, a former political scientist with a history of promoting unfounded conspiracy theories, claims that carbon dioxide is a “trace element” in Earth’s atmosphere and not a “climate heat driver.” Corsi goes on to argue that “when the Earth had extraordinarily larger amounts of carbon dioxide in the atmosphere, we had an ice age and there were no human beings here.”
According to Corsi, carbon dioxide has been unjustifiably “picked on” as a driver of global warming by environmental activists and political figures. He also claims that, in the 1970s, climate scientists were warning of “global cooling” rather than global warming.
It is true that carbon dioxide is a minor constituent of Earth’s atmosphere by percentage. However, it is just above 0.04% today, while Corsi falsely claims carbon dioxide represents 0.0003 to 0.0004% of the atmosphere. Importantly, though, the relatively low percentage weight of atmospheric carbon dioxide does not indicate that its role in regulating climate is negligible. As James Renwick, a climate scientist at Victoria University of Wellington, notes, “carbon dioxide is the major ‘climate heat driver,’ in terms of the radiation budget of the planet.”
Indeed, a consensus of scientific evidence[1,2] indicates that contemporary global warming is caused by human emissions of carbon dioxide and other greenhouse gases. This finding is also substantiated by various national and international climate reports such as the U.N. International Panel on Climate Change (IPCC) Sixth Assessment Report and U.S. Fourth National Climate Assessment.
Carbon dioxide is a greenhouse gas – a type of gas that prevents heat from leaving the atmosphere and warms the Earth. “There are other gases that are more effective than carbon dioxide at absorbing and re-radiating heat (e.g. methane), but carbon dioxide wins in the long term because of its long lifetime of hundreds to thousands of years,” says Renwick. In addition to its long atmospheric residence time, or the amount of time a carbon dioxide molecule remains in the air, it also constitutes a majority of total greenhouse gas emissions. As such, carbon dioxide is in fact responsible for most of the warming observed since the mid 20th century (Fig. 1).
Figure 1. Time series of effective radiative forcing (warming) caused by various mechanisms. Note that carbon dioxide (gray) contributes more to warming than all of the other well-mixed greenhouse gases (WMGHG). Note also that the majority of warming is caused by carbon dioxide and other greenhouse gases. Source: U.S. Fourth National Climate Assessment.
While 0.04% of the atmosphere may seem small, it is important to note that carbon dioxide comprised less than 0.03% of the atmosphere prior to the onset of human emissions during the industrial revolution (Fig. 2). This change represents a dramatic departure from natural changes in atmospheric carbon dioxide, which typically varied between approximately 0.02% (200 ppm) to 0.03% (300 ppm) during the last 800,000 years and changed over periods of tens of thousands of years.
Figure 2. Atmospheric carbon dioxide concentration over the last 800,000 years. Note that modern levels of >400 ppm (>0.04%), driven by anthropogenic emissions, are a rapid departure from natural variability. Source: NASA.
But what about temperature over this period? Corsi claims that we had an “ice age” under “larger amounts of carbon dioxide.” While he does not invoke a specific example in Earth history, paleoclimate research has found that Earth’s glacial-interglacial cycles, periods of relative cooling (ice growth) and warming (ice melt), have indeed occurred in concert with changes in atmospheric carbon dioxide (Fig. 3), at odds with Corsi’s claim.
Figure 3. Atmospheric carbon dioxide (top) and temperature change (bottom) over the last 800,000 years. Note that high levels of carbon dioxide are associated with high temperatures and low levels of carbon dioxide are associated with low temperatures. Source: Time Scavengers.
Carbon dioxide is, however, just one of many climate forcing mechanisms. For example, while glacial-interglacial changes do correspond to changes in carbon dioxide, they also correspond to changes in Earth’s orbit and tilt. As such, explanations for what drives glacial-interglacial variations typically invoke a synergy of different mechanisms, including changes in Earth’s orbit, ocean circulation and atmospheric greenhouse gases[5,6,7]. Looking further back in time, paeloclimate research also generally supports a relationship between atmospheric carbon dioxide concentration and Earth’s surface temperature over the last 66 million years (Fig. 4).
Figure 4: Reconstructed changes in Earth’s surface temperature (top) and atmospheric carbon dioxide (bottom) over the last 66 million years. Source.
Such associations do not necessarily indicate that carbon dioxide was always the main mechanism regulating changes in climate, particularly at periods in Earth’s more distant geological past. “If you go back far enough, over 100 million years ago, you can find periods where it looks as though there were high carbon dioxide levels and cool temperatures, though again the uncertainties are large,” says Renwick. “At those times though, many things were very different, such as the distribution of continents and ocean on the planet, the amount of volcanic activity (blocking out sunlight), and the brightness of the sun. Carbon dioxide is very important for the energy budget of the Earth, but it is not the only factor on very long time scales.”
It is, however, misleading to invoke drivers of geological-scale climate change in order to discredit the driving role of carbon dioxide in modern climate change. Climate scientists from different disciplines have evaluated the influence of all modern climate forcing mechanisms and have repeatedly and consistently concluded that greenhouse gases are the dominant cause.
This is not the only misleading argument made by Corsi. He goes on to claim that climate scientists in the 1970s were warning that “we were going to have a new ice age.” This claim is commonly repeated in climate contrarian circles in an effort to undermine the reliability of climate scientists. However, a survey of the climate literature between 1965 and 1979 found that only 7 studies projected future cooling. By contrast, 44 studies projected warming. Climate science has evolved dramatically since the 1970s and all of the available scientific evidence continues to support projected warming driven by human emissions of greenhouse gases.
James Renwick, Professor, Victoria University of Wellington:
Jerome Corsi is right that carbon dioxide is a minor element of the atmosphere, it currently makes up around 0.04% of the air (this is however more than he claims). However, it is crucially important for regulating earth’s energy budget and surface temperatures. But he is not right about much else. Carbon dioxide is the major ‘climate heat driver,’ in terms of the radiation budget of the planet. There are other gases that are more effective than CO2 at absorbing and re-radiating heat (e.g. methane), but carbon dioxide wins in the long term because of its long lifetime of hundreds to thousands of years. This has been understood for a long time – the role of CO2 in warming the earth was discovered in the mid-19th century. It has not been ‘picked on’ recently by climate activists. The warming of the globe from increasing carbon dioxide concentrations in the air was first calculated in 1896. A good source on the history of the science is Spencer Weart’s book ‘The Discovery of Global Warming.’
Through the past 2.6 million years, the period of recent ice ages, carbon dioxide has gone up and down in step with temperature, bottoming out at around 180 parts per million in the depths of a “glacial maximum” and peaking at around 280 parts per million in the warmer interglacial periods. Going back further, CO2 levels were certainly higher than present, but so were temperatures (Fig. 3).
As Corsi says, CO2 levels dropped from 500M or so years ago to the beginning of our ice age period 2.6M years ago. There were lots of ups and downs along the way, and the atmospheric concentration did not start at 7500 parts per million CO2, it was closer to 700ppm. It probably did go above 1000ppm over 100M years ago, but the uncertainties are large:
Atmospheric carbon dioxide concentration over the last 400 million years. Source: Earth.org.
In any case, in broad terms, yes, CO2 levels have dropped from the ‘hothouse conditions’ of 50 million years ago to the ‘icehouse conditions’ of the last few million years. In that period of time, temperatures have risen and fallen with CO2, as they have done through the recent ice ages (Fig. 3). With current emissions, we’ll be back at the “hothouse” level next century if we don’t take action. That would be catastrophic for humanity.
If you go back far enough, over 100 million years ago, you can find periods where it looks as though there were high CO2 levels and cool temperatures, though again the uncertainties are large. At those times though, many things were very different, such as the distribution of continents and ocean on the planet, the amount of volcanic activity (blocking out sunlight), and the brightness of the sun. CO2 is very important for the energy budget of the earth, but it is not the only factor on very long time scales.
The last point about the ‘ice age scare’ in the 1970s… yes, there were a few papers published in the 1970s wondering about this, after a couple of decades of slight global cooling. The media picked up on this and there were articles published in Time magazine and elsewhere talking about the possibility of an impending ice age. But, in the climate research community, the vast majority of papers published through the 1970s were about global warming, not global cooling. There’s a good discussion here.”
- 1 – Oreskes (2004) The scientific consensus on climate change. Science.
- 2 – Cook et al. (2013) Quantifying the consensus on anthropogenic global warming in the scientific literature. Environmental Research Letters.
- 3 – IPCC (2021) Sixth Assessment Report.
- 4 – U.S. Fourth National Climate Assessment, Climate Science Special Report.
- 5 – Denton et al. (2010) The Last Glacial Termination. Science
- 6 – McManus et al. (1999) A 0.5-Million-Year Record of Millennial-Scale Climate Variability in the North Atlantic. Science
- 7 – Raymo et al. (1997) The timing of major climate terminations. Paleoceanography
- 8 – Rae et al. (2021) Atmospheric CO2 over the Past 66 Million Years from Marine Archives. Annual Review of Earth and Planetary Sciences
- 9 – Peterson et al. (2008) The myth of the 1970s global cooling scientific consensus. Bulletin of the American Meteorological Society