Recent reconstructions suggest CO2 concentrations may have ranged between 150 and 600 ppm (or more) throughout the last 4 million years.
We routinely see screaming headlines such as CO2 in the Atmosphere Climbs to Highest Level in 4.5 Million Years or Carbon Dioxide Levels Reach Highest Point in Human History.
However, these claims that modern CO2 levels are unusual or unprecedented are highly presumptive, resting on assumptions that we can accurately discern the atmospheric CO2 levels from alkenone, stomata, boron isotope…and other proxies.
It is presumed that air bubbles in Antarctic ice cores accurately record the “global” atmospheric CO2 levels for the last 400,000 years even though air bubbles from Greenland ice cores record entirely different CO2 values for the most recent millennia.
Image Source: Bern et al., 1980
A recent study (Rae et al., 2021) published in Annual Reviews suggests CO2 varied between about 150 ppm and up to 600 (to 750 ppm) throughout the last 4 million years. Presuming the atmospheric CO2 level is the only variable in the determination of the global ocean pH, the authors also claim seawater pH has varied between 7.9 and 8.3 for since the early Pliocene.
These data are graphically shown to be so imprecise that discernment of the timing or accuracy of these records is inconclusive at best. It’s more suggestive of guesswork.
Image Source: Rae et al., 2021
Another recent study (Seki and Bendle, 2021) also demonstrates just how futile it may be to discern the accuracy of CO2 estimates ranging from 150 to 600 ppm for the last 4.5 million years.
We are asked to just believe it when we’re told the modern CO2 levels are the “highest ever” in millions of years. Given the data presented, a healthy dose of skepticism is in order.