What ancient climates tell us about high carbon dioxide concentrations in Earth’s atmosphere

Topics: Earth and Life Sciences
Type: Briefing paper
Publication date: May 2020



Authors:  Professor Martin Siegert, Professor Alan Haywood, Professor Dan LuntProfessor Tina Van De Flierdt and Professor Dame Jane Francis

This briefing discusses the last time our planet had the same levels of carbon dioxide in the atmosphere as it does today, and what environmental conditions were like then. Studying the geology from this and earlier periods tells us that global temperatures may rise by over 10°C if we keep emitting carbon dioxide as forecast for the next 80 years. The paper explains how, to avoid this catastrophic climate, the world must cut greenhouse gas emissions to net zero by 2050 at the latest.


  • Earth’s climate has always closely followed the concentration of carbon dioxide (CO2), and other greenhouse gases in the atmosphere. The concentration was as low as 180 parts per million (ppm) in the coldest part of the last ice age, 20,000 years ago. Around 10,000 years later, when the concentration increased to 280ppm, that ice age came to an end.
  • Over the last 800,000 years, the concentration of atmospheric CO2 naturally varied between 180-280ppm, but never rose significantly above 280ppm. In the 170 years since 1850, the concentration of CO2 has risen from 280ppm to more than 410ppm, primarily due to fossil fuel burning and changes in how humans use the land.
  • Left unchallenged, the increasing rate of change could see the CO2 concentration increase to about 1000ppm by 2100. Earth last experienced 400ppm of CO2 around 4 million years ago, during the Pliocene era. At this time, the average temperature was 2-4°C warmer than today, and the sea level was 10-25m higher.
  • The concentration of CO2 was last at over 1000ppm around 50 million years ago, when the average temperature was about 13°C warmer and sea level would have been around 70m higher than today because there was no (or very little) ice on the planet.
  • Crucially, today’s rate of change of CO2 concentration, which is 200 times greater than it was after the last ice age, may prevent living organisms from adapting to new conditions.
  • The lesson from the past is clear: sustained concentrations of CO2 in the atmosphere above 400ppm will lead to very different conditions to those we experience on Earth today. To avoid the most adverse consequences, and allow humanity to adapt to changes that are already happening, we need to step up action to curtail greenhouse gas emissions to zero by 2050 at the latest.

Download: What ancient climates tell us about high carbon dioxide concentrations in Earth's atmosphere [PDF]

'Learning from ice': Listen to a Planet Pod podcast with Professor Siegert discussing what studying core samples from over 2.5 million years ago can tell us about the likely impact of climate change today. Tune in to the podcast here.

View other Grantham Institute briefing papers.