Optimal atmospheric CO2 for dinosaurs and plants is harmful for humans; current concentration is higher than Homo sapiens have ever experienced

During Earth’s 4.5 billion year history, the atmosphere has undergone dramatic changes and only started to appear like it does today around 290 million years ago (mya). Today’s air is composed of several gasses with different concentrations, like nitrogen (78.08%), oxygen (20.95%), argon (0.93%), and carbon dioxide (0.04%). The air 600 mya, for example, had only about one-fifth of today’s oxygen level.

Plants first appeared as far back as 500 mya^[1]. Reptiles first appeared about 320 mya, and the dinosaurs only existed approximately from 243 mya until 66 mya. Mammals appeared about 225 million years ago, but anatomically modern humans, or homo sapiens, are only 260-350 thousand years old. Humans, dinosaurs, and plants all respire air, although plants do it very differently than mammals and reptiles.

Considering these different evolutionary timelines under different atmospheric compositions, can we assume that animals, reptiles, and plants thrive breathing the same air? Is the optimal level of carbon dioxide for dinosaurs and plants also optimal, or at least feasible, for humans? Does all this mean anything for climate change?

Youtuber Jimmy Corsetti, who’s channel Bright Insight has 1.61 million subscribers, suggested on X and Instagram that because dinosaurs and plants thrive under very high CO₂, increasing in CO₂ is good for life on Earth and not bad for humans. This means, according to Corsetti, “CO₂ is Not causing a Climate Crisis”. Here we explore the reasons why this claim is misleading because it lacks context and uses flawed reasoning.

Optimal carbon dioxide levels for dinosaurs and plants is toxic for humans over the long-term

The atmospheric concentration of CO₂ during the time of the dinosaurs was much higher than it is today, which currently stands at 422 parts per million (ppm). The article Corsetti screenshotted shows Dr. Paul Olsen’s response to the following question submitted by a reader: “How did plants and animals survive around 200 million years ago when the carbon dioxide concentration went up to 6,000 parts per million?”. Olsen, a geologist and paleontologist at Columbia Climate School’s Lamont-Doherty Earth Observatory, explains that the CO₂ concentrations during the time of the dinosaurs (the Mesozoic Era) was in the 2000 to 4000 ppm range and that humans could potentially survive, but only with the help of technological innovations and not because of physiological ability. To be clear, the atmospheric concentration of CO₂ during the time of the dinosaurs was never near 6000 ppm as Figure 1 shows^[2], and it has never come close to that in the 66 million years since the dinosaurs went extinct^[3].

Figure 1 – The rise and fall of atmospheric-CO₂ (red trendline) over hundreds of millions of years (top x-axis), based on paleoclimatological reconstructions. Note the short timespan where Homo sapiens have existed, which has only featured low atmospheric-CO₂ (right y-axis), especially compared to the Age of the Dinosaurs. Ice ages are indicated by blue shaded areas from the top x-axis. Based on reference [2], modified by Dr. Paul Olsen (source).

According to Jacobson et al.’s (2019) recent synthesis of scientific literature from different fields exploring the impacts on CO₂ air concentrations on humans, potential health risks can occur with exposure as low as 1000 ppm^[4]. While concentrations above 5000 ppm are known to be harmful in both the short and long-term, research indicates that even concentrations below 5000 ppm “poses direct risks to human health”, including inflammation, reduced higher-level cognitive abilities, bone demineralization, kidney calcification, oxidative stress and endothelial dysfunction. Another literature review found physiological changes occur at CO₂ exposures levels between 500 and 5000 ppm, effects on cognitive performance begin at 1000 ppm during short-term exposure, and respiratory symptoms are detected in children exposed to indoor CO₂ concentrations higher than 1000 ppm^[5]. Azuma et al. (2018) concluded that atmospheric CO₂ concentration needs to be urgently suppressed to be able to efficiently control indoor concentrations.

Most research on this topic is related to indoor environments, with humans under high exposure for limited time-frames corresponding to shift work in potentially hazardous workplace conditions. The Occupational Safety and Health Administration (United States Department of Labor) permissible exposure limit for CO₂ is 5000 ppm over 8 hours. Concentrations above 40 000 ppm are considered immediately dangerous to life or health. There has been no research exploring prolonged human environmental exposure (weeks, months, years, lifespan) to elevated CO₂ outdoors, especially for vulnerable demographics. Jacobson et al (2019) concluded that removing CO₂ from the atmosphere will be necessary if emissions continue at current levels, based on their confirmation that “prolonged exposures as low as 1000 ppm CO₂ affect human health and well-being”.

In addition, during the Cretaceous period (145-66 mya) which ended when the dinosaurs went extinct, the average temperature was about 5-10°C higher than today and sea levels were 100 meters higher or more^[6]. For comparison, the most extreme global warming future scenario (representative concentration pathway (RCP)) under consideration by the IPCC for the year 2100, called RCP8.5 or the “business as usual” scenario, implies a likely temperature increase of 3.2-5.4°C^[7]. Figure 2 shows that the fossil-fuel intensive shared socio-economic pathway (SSP), called the SSP5-8.5 scenario, will lead to atmospheric-CO₂ concentrations that match or even exceed Eocene or mid-Cretaceous levels^[8]. It also illustrates just how unprecedented even a return to even just 1000 ppm CO₂ would be, which has not been seen for tens of millions of years.

Figure 2 – Comparing paleoclimates (past) with future climate scenarios side-by-side from the year 2020, including both global mean temperature (left y-axis, °C, only for the past) and global atmospheric-CO₂ concentration (right y-axis, ppm, logarithmic scale, for the past and the future). Trendlines are smoothed to show long-term trends, and temperature colors are scaled related to pre-industrial levels. The small global maps on the right indicate different shared socio-economic pathway (SSP) scenarios (source).

As for plants, Science Feedback has previously addressed flawed claims that because CO₂ is used in greenhouses to promote plant growth, high CO₂ in the atmosphere is good for the world’s plants and therefore not a concern for humans. Plants need more than just CO₂ to photosynthesize and grow; they also need water, sunlight, and other plant nutrients like nitrogen and phosphorus. High CO₂ in the atmosphere is directly linked to negative impacts on ecosystems through global warming which will constrain plant growth and limit any benefits from high CO₂. For example, the increase in droughts in some regions or flooding in other regions are not beneficial for plants, regardless of high atmospheric CO₂ (called the “CO₂ fertilization effect”). Karin Kirk puts it simply with the title of her article for Yale Climate Connections: “More CO₂ in the atmosphere hurts key plants and crops more than it helps”.

But even if we ignore this (and all other climate related concerns of high CO₂), and we focus just on maximizing plant growth as we do in greenhouses as referenced by Corsetti, the optimal CO₂ levels for plants are still in conflict with human well-being. All plants respond differently to increased CO₂, but they all generally follow a downward parabola (upside down “U”), increasing yield with increasing CO₂ until reaching an optimal point (vertex) after which yield decreases with increasing CO₂ (Figure 3, right). Different plants have different optimums, but many greenhouse recommendations reference ideal concentrations at or surpassing 1000 ppm, assuming no other limiting factors exist. For example, Figure 3 (left) shows how increasing CO₂ concentrations helps three species of grasses grow, with their ideal concentrations for biomass yield at 915, 1178, and 1386 ppm^[9].

Figure 3 – Left: The impact of elevated CO₂ concentrations (x-axis) on above ground biomass (a), below ground biomass (b), and total biomass (c) of three species of perennial grasses as quantified in Zheng et al. (2018). Right: Generalized relationship between CO₂ level and plant growth rate. Note that a CO₂ concentration of 300 ppm was considered “Normal Air” at the time this illustration was first sketched by Roger H. Thayer (see here for an early version), which has been reproduced over the years despite the global average now being 422 ppm (e.g., like the figure above provided by the Oklahoma State University Extension).

As mentioned above, these elevated levels of CO₂ concentration in the atmosphere would be potentially harmful for humans when inhaled indefinitely without respite (every hour of every day), especially for vulnerable demographics. So not only does high CO₂ raise new challenges for plant growth (such as drought and flooding) and ecosystem stability, the optimum CO₂ level for plants is incompatible with human physiology over the long-term. The high CO₂ enjoyed by dinosaurs and plants is just not relevant for human well-being, making Corsetti’s suggestions misleading.

Because of human activities, carbon dioxide is now higher than our species have ever experienced

CO₂ is considered the most important greenhouse gas because it has such a long residence time in the atmosphere, it is by far the most abundant, and it contributes the most to global warming and climate change. The direct and indirect environmental impacts of high anthropogenic-CO₂ emissions are not just related to temperature increases, but also other changes across different components of the Earth system. Aside from air temperature increase, climate change has been observed with the oceans getting warmer, ice sheets shrinking, glaciers retreating, snow cover decreasing, sea level rising, arctic sea ice decreasing, and extreme weather events increasing in frequency (see here for evidence provided by NASA).

Relative to global concentrations in the year 1750, atmospheric CO₂ has increased by more than 47% because of human activities^[10]. As indicated in Figures 1 and 2, the current level of atmospheric-CO₂ (422 ppm) is now higher than the human species has ever experienced. Air trapped in ancient ice cores has demonstrated that CO₂ has not been this high for at least the past 800 000 thousand years (ice record limit). NOAA stated in 2022 that CO₂ levels are now comparable to the Pliocene Climatic Optimum, between 4.1 and 4.5 million years ago. As a reminder, Homo sapiens have only been around for 260-350 thousand years.

Rather than the steep reduction in CO₂ emissions needed to meet climate goals and limit global warming, 2023 saw another slight increase compared to year before (+1.1%), increasing 1.5% since before the COVID-19 pandemic, and representing a 10-year plateau of sustained high emissions and no significant reductions^[11]. Humans have emitted 40.7 gigatons of CO₂ in 2022, with a similar amount estimated for 2023. Increasing anthropogenic CO₂ emissions like this raises concerns for human cognition and well-being outside just like it does indoors^[12].

Of course, not all regions of Earth have exactly the same CO₂ concentration at all times, some can exceed others due to geographical factors. In dense urban areas, especially large cities in low-lying basins, CO₂ can build up. And with CO₂ concentration increasing by about 2.4 ppm in 2023 worldwide^[11], geographically constrained urban areas with high populations like Mexico City and Athens could see local concentrations reaching harmful levels for humans by the end of the century^[13].

Finally, claims that elevated CO₂ concentrations are not bad for humans as Corsetti suggested in his post generally ignore one more critical aspect: the rate of change. The current rate of increase is estimated to be occurring 10 times faster than any other change of similar magnitude over the past 65 million years^[14]. As this is much faster than plants, animals, and humans can evolve and adapt to (other than migration), and faster than ecosystems can sufficiently respond to, the risk of extinction is enormous in the coming decades (up to 29% of land plants and animals at risk of being wiped out with 3°C of warming; we are currently at 1.1°C of warming)^[15]. CO₂ by itself is not causing a climate crisis; elevated CO₂ emissions from human activities are.

Conclusion

The claim that because high CO₂ was fine for the dinosaurs and high CO₂ benefits plants today in greenhouses, high CO₂ can’t be bad for humans is misleading. Firstly, the optimal carbon dioxide levels for dinosaurs and plants is, in fact, harmful for humans over the long-term. The age of the dinosaurs (the Mesozoic Era) had atmospheric-CO₂ in the 2000 to 4000 ppm range, and plants typically grow best at 1000 ppm of CO₂ or greater (like in greenhouses). Humans, on the other hand, can suffer mild health effects from CO₂ as low as 1000 ppm even in the short-term, and the effects of permanent exposure to elevated CO₂ levels are unknown. This is especially relevant for vulnerable populations, and anyone living in low-lying, densely populated urban areas. Humans as a species have never before experienced the high level of CO₂ currently in the atmosphere, which is higher than it has been for as long as ice records can tell us (800 000 years). CO₂ by itself is not causing a climate crisis; elevated CO₂ emissions from human activities are.

REFERENCES

• 1 – Morris et al. (2018) The timescale of early land plant evolution. Proceedings of the National Academy of Sciences.
• 2 – Foster et al. (2017) Future climate forcing potentially without precedent in the last 420 million years. Nature Communications.
• 3 – Rae et al. (2021) Atmospheric CO₂ over the past 66 million years from marine archives. Annual Review of Earth and Planetary Sciences.
• 4 – Jacobson et al. (2019) Direct human health risks of increased atmospheric carbon dioxide. Nature Sustainability.
• 5 – Azuma et al. (2018) Effects of low-level inhalation exposure to carbon dioxide in indoor environments: A short review on human health and psychomotor performance. Environment International.
• 6 – Miller et al. (2005) The Phanerozoic record of global sea-level change. Science.
• 7 – IPCC (2019) Summary for Policymakers. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate
• 8 – Tierney et al. (2020) Past climates inform our future. Science.
• 9 – Zheng et al. (2018) The optimal CO₂ concentrations for the growth of three perennial grass species. BMC Plant Biology.
• 10 – IPCC (2021) Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.
• 11 – Friedlingstein et al. (2023) Global carbon budget 2023. Earth System Science Data.
• 12 – Karnauskas et al. (2020) Fossil fuel combustion is driving indoor CO₂ toward levels harmful to human cognition. GeoHealth.
• 13 – Lowe et al. (2018) Possible future impacts of elevated levels of atmospheric CO₂ on human cognitive performance and on the design and operation of ventilation systems in buildings. Building Services Engineering Research and Technology.
• 14 – Diffenbaugh et al. (2013) Changes in ecologically critical terrestrial climate conditions. Science.
• 15 – IPCC (2023) Summary for Policymakers. In: Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change