If CO₂ is classified as a pollutant (as per the U.S. EPA’s 2009 Endangerment Finding, which enabled regulation under the Clean Air Act), then there must be a scientifically defensible “safe” or “correct” threshold concentration in the atmosphere. Yet climate policy rarely specifies one beyond implying pre-industrial levels (~280 ppm) as the ideal baseline to return to.

No Clear Threshold Defined: Policies aim to limit warming (e.g., to 1.5–2°C) via emissions cuts, but don’t pinpoint an exact “safe” ppm level for CO₂. Is it 280 ppm (pre-industrial)? 350 ppm? 400 ppm?

Over the past 800,000 years (from ice cores), CO₂ fluctuated between 180–300 ppm during glacial-interglacial cycles.

For most of the Phanerozoic eon (last ~550 million years), CO₂ was often 1,000–4,000+ ppm during periods of lush vegetation and no “icehouse” Earth.

Photosynthesis responds strongly (nonlinearly) to CO₂ increases from ~280 ppm up to ~1,100 ppm. Commercial greenhouses deliberately enrich air to 800–1,000 ppm to boost crop yields.

Everyday examples undercut toxicity claims: Exhaled breath is 40,000 ppm CO₂.

Even burning all known fossil fuels might only reach ~1,000 ppm globally.

In a hypothetical “post-Endangerment” scenario, what concentration should we target?

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What Is the Correct CO2 Concentration?

From Irrational Fear

By Dr. Matthew Wielicki

A Scientific Thought Experiment in a Post-Endangerment World

The 2009 Endangerment Finding treated carbon dioxide as a pollutant that threatens public health and welfare. For more than a decade, that designation has functioned as the legal foundation for regulating fossil fuels, electricity generation, transportation, and much of the modern economy.

Now that the Finding is being reconsidered, it is worth stepping back from the regulatory framework and asking a much more basic scientific question.

What is the correct atmospheric concentration of CO2?

If carbon dioxide is truly a pollutant, then there must be a threshold. There must be some identifiable boundary at which CO2 shifts from being part of the natural carbon cycle to being harmful in a biological or climatic sense. If such a boundary exists, it should be visible in geology, in plant physiology, or in the empirical record of Earth’s climate.

But when we begin to examine those lines of evidence, the clarity we are promised does not appear.

In The Endangerment Finding Was Pre-Cooked, I walked through the Phanerozoic reconstruction of atmospheric CO2. Over the last 550 million years, concentrations frequently exceeded 1000 parts per million and at times reached several thousand.

Life did not collapse under those conditions. On the contrary, biodiversity expanded. Forests flourished. Marine ecosystems diversified. The idea that 400 ppm constitutes dirty air sits uneasily beside a geological record in which far higher concentrations coincided with biological abundance.

If today’s atmosphere is “endangering,” then vast stretches of Earth’s history would need to be reclassified as ecologically reckless. That conclusion should at least give us pause.

In Does More CO2 = More Heat?, I also explored the complex relationship between CO2 and temperature over deep time. The geological record does not display a simple linear mapping between atmospheric concentration and climate state.

Continental configuration, ocean circulation, solar luminosity, volcanic outgassing, and feedback processes all shape global temperature. CO2 plays a role, but it is not the only control knob.

If there is an upper danger threshold, it is not obvious in deep time.

The Ice Age Constraint

The more recent record complicates things further.

In Atmospheric CO2 Concentrations of the Past and Do We Really Know the Surface Temperature or CO2 Concentration of the Past?, I discussed the proxy records and their limitations. Even accepting the ice core framework at face value, atmospheric CO2 over the past 800,000 years fluctuated between roughly 180 and 300 ppm.

Those low points coincided with massive continental ice sheets. Glacial maxima were not periods of ecological flourishing. They were periods of climatic severity and instability.

Pre-industrial levels near 280 ppm did not represent a golden age of climatic balance. They were part of a cycle of repeated glaciations driven by orbital forcing. The Earth system oscillated between ice ages lasting nearly 100,000 years and comparatively brief interglacials.

That history raises an uncomfortable possibility.

Perhaps 280 ppm was not an optimal equilibrium. Perhaps it was closer to a lower boundary of climatic stability.

The Modern Biosphere Response

Meanwhile, as CO2 rose from roughly 280 ppm to more than 420 ppm, something measurable occurred.

In Greening Earth and Booming Crops, I examined satellite evidence showing widespread increases in global leaf area. Agricultural production has expanded dramatically over the past several decades. Water use efficiency has improved under elevated CO2 conditions.

Commercial greenhouse operators routinely enrich indoor air to concentrations between 800 and 1000 ppm in order to maximize plant growth. They do not regard 280 ppm as biologically ideal.

CO2 is not a trace contaminant in plant physiology. It is the fundamental substrate of photosynthesis.

If rising CO2 were inherently degrading the biosphere, we would not expect to observe widespread greening.

The Sea Level Complication

Perhaps, then, the danger lies in the sea level.

That argument is often presented as straightforward. Higher CO2 increases radiative forcing. Higher temperatures melt ice. Melting ice raises the sea level.

But in Sea Levels Were Significantly Higher in the Mid to Late Holocene at Pre Industrial Levels of CO2, I reviewed multiple peer-reviewed reconstructions showing that regional sea levels during the mid to late Holocene were, in many places, higher than today. These highstands occurred while atmospheric CO2 remained near 260 to 280 ppm.

Evidence from the Arabian Gulf, the southwestern Atlantic, and the southeast Australian coast indicates elevated sea levels under what we now describe as safe pre-industrial concentrations.

This does not mean CO2 has no climatic influence. It does mean that sea level does not map cleanly or uniquely onto atmospheric concentration. Glacio isostatic adjustment, meltwater redistribution, and regional dynamics complicate the picture.

Once again, the boundary between safe and dangerous becomes difficult to identify.

The Asymmetry

Taken together, these observations produce a striking asymmetry.

There is clear evidence that very low CO2 concentrations coincide with biospheric stress and glacial amplification.

There is far less clarity about a specific upper danger threshold anywhere near modern levels.

And yet policy treats 280 ppm as a moral baseline.

Why?

Why was the concentration that happened to exist before industrialization elevated to the status of climatic ideal?

Below the paywall, I will attempt to answer the original question directly. Not rhetorically. Not politically. But analytically.

If the objective were maximizing biospheric productivity, minimizing glacial instability, preserving adaptability, and supporting human flourishing, what atmospheric CO2 range would we rationally choose?

Subscribers receive the full stability framework, the plant physiology constraints, the radiative forcing context, and a direct answer grounded in the evidence reviewed above.