Posted on 1 March 2025 by Sue Bin Park

Skeptical Science is partnering with Gigafact to produce fact briefs — bite-sized fact checks of trending claims. You can submit claims you think need checking via the tipline.

Are high CO₂ levels harmless because they also occurred in the past?

While the Earth adapted to high carbon dioxide levels in the past, modern civilization cannot without major consequences.

Past periods of high CO₂ brought about a climate vastly different from our own. During the Eocene “hothouse” period around 55 million years ago, CO₂ concentrations peaked at 1,600 parts per million. That epoch saw ice-free poles and palm trees above the arctic circle.

The last time CO₂ was as high as today was 3 million years ago. Global temperatures were as much as 7°F (4°C) warmer and sea levels were as much as 80 feet (25 meters) higher.

Given that 40% of the population lives around 60 miles (100 kilometers) from the coast, rising sea levels from CO₂ emissions threaten a global climate refugee crisis.

The rapid CO₂ rise today, compared to the gradual rise historically, compounds the danger, with our food systems and ecosystems already struggling to adapt.

Go to full rebuttal on Skeptical Science or to the fact brief on Gigafact

This fact brief is responsive to quotes such as the one highlighted here.

Sources

NASA Carbon Dioxide

NOAA Climate Change: Atmospheric Carbon Dioxide

Nature Future climate forcing potentially without precedent in the last 420 million years

NOAA Models and fossils face off over one of the hottest periods in Earth's history

NOAA What’s the hottest Earth’s ever been?

United Nations PERCENTAGE OF TOTAL POPULATION LIVING IN COASTAL AREAS

About fact briefs published on Gigafact

Fact briefs are short, credibly sourced summaries that offer “yes/no” answers in response to claims found online. They rely on publicly available, often primary source data and documents. Fact briefs are created by contributors to Gigafact — a nonprofit project looking to expand participation in fact-checking and protect the democratic process. See all of our published fact briefs here.