Is Carbon Dioxide Higher Than Ever?

Episode Summary

Executive Summary: The episode answers when atmospheric CO2 last matched today’s levels: in the Pliocene, about 3 million years ago. Using observatories, ice cores, fossil leaves, and marine sediments, scientists show that while Earth has seen similar or higher CO2 before, the urgent issue now is the unprecedented speed of increase, which will drive long-term warming, ice-sheet loss, and sea-level rise.

Main Topics: Measuring today’s CO2 (Priority: 5/5): A visit to the Weybourne Atmospheric Observatory shows how modern CO2 is monitored continuously using infrared analysers and a global network of stations. Why ice cores can’t answer the full question (Priority: 5/5): Antarctic ice cores preserve ancient air bubbles and reconstruct CO2 over 800,000 years, but they never show levels as high as today’s, making them insufficient for this question. Using fossil plants as CO2 proxies (Priority: 4/5): Paleobotanist Jenny McElwain explains that stomata counts on fossil leaves can estimate ancient CO2, extending the record back over 400 million years. Marine sediments and ocean chemistry (Priority: 4/5): Gavin Foster describes using foraminifera chemistry from seabed sediments to infer past ocean pH and therefore atmospheric CO2 over tens of millions of years. The Pliocene as the closest match to today (Priority: 5/5): Scientists conclude that the last time CO2 was around current levels was during the Pliocene, about 3 million years ago, when temperatures were roughly 3°C warmer. Climate, ecosystems, and extinction risk (Priority: 5/5): The episode compares Pliocene ecosystems with today, emphasizing that life persisted under higher CO2, but rapid CO2 increases are associated with extinctions and major disruptions. Future implications of current warming (Priority: 5/5): Experts stress that climate response will unfold over centuries to millennia, including ice-sheet melt and sea-level rise, and that human choices can still alter the outcome.

Key Arguments: Modern CO2 is now above 400 ppm and rising by about 2 ppm per year, making current levels historically unusual in the observational era. Ice cores provide direct ancient-air evidence but only back to 800,000 years, and they do not capture CO2 as high as today’s. Fossil leaf stomata reflect atmospheric CO2 because plants reduce stomata when CO2 is high to conserve water; this provides deep-time estimates. Marine sediments extend CO2 reconstruction further back by linking ocean acidity to atmospheric CO2 through foraminifera chemistry. The last time CO2 was comparable to today was the Pliocene, around 3 million years ago. Higher CO2 and warmer climates have existed many times in Earth history, so the key concern is not CO2 alone but the rapidity of change. Rapid CO2 rises correlate with major extinction events, suggesting ecosystem stress is driven by the speed of change more than the absolute level. Even if emissions stopped now, climate impacts such as ice-sheet loss and sea-level rise would continue for thousands of years due to system inertia.

Data Points: Current CO2 at Weybourne: 411 ppm - Real-time measurement shown at the atmospheric observatory Recent trough at observatory: just under 400 ppm - A low point reached after summer photosynthesis Background annual CO2 rise: about 2 ppm per year - Long-term trend after accounting for seasonal and regional variation CO2 since 1958: about 25% increase - Rise since systematic measurements began at Mauna Loa Mauna Loa start year: 1958 - Beginning of continuous modern CO2 monitoring by Charles David Keeling Ice-core climate record length: 800,000 years - Ancient air preserved in Antarctic ice bubbles Oldest ice core mentioned: 140,000 years old - Berkner Island core shown in the freezer Age of fossil leaf: 200 million years old - Ginkgo fossil used for stomata-based CO2 reconstruction Stomata count on modern ginkgo: 90 to 100 per pinhead - Used as a comparison for calibrating CO2 estimates Stomata count on fossil ginkgo: 20 to 30 per pinhead - Indicates much higher CO2 in the Mesozoic Estimated CO2 from fossil ginkgo: 3 to 4 times higher than today - Derived from stomatal differences Time of last similar CO2: about 3 million years ago - Identified as the Pliocene Pliocene temperature difference: about 3°C warmer - Climate conditions when CO2 matched today Projected sea-level rise if CO2 stays ~410 ppm: 10 to 20 metres - Estimated over the next 2,000 years Projected warming beyond today: additional 2°C - If emissions stopped immediately at just over 410 ppm Recovery timescale after stabilization: 400,000 to 1 million years - Natural processes would eventually draw CO2 back down Ancient high-CO2 intervals cited: Eocene (~55 million years ago), Jurassic (~183 million years ago), 200 million years ago, 250 million years ago - Examples of earlier periods with rapid CO2 rise and extinctions

Pivotal Quotes: "This is the first time that we have observed these concentrations so high." — Dr Holly Winton: Explaining why ice cores cannot find a past CO2 level comparable to today "The real problem is the rate of change." — Professor Richard Twitchett: Summarizing the main scientific concern about modern climate change "When CO2 levels increase rapidly, animals and plants can't cope. And that's when we get extinctions." — Professor Richard Twitchett: Linking rapid greenhouse-gas increases to biological stress and extinction

Implications: The episode suggests today’s CO2 is geologically common, but the current rate of rise is not. Expect long-lasting warming, ice loss, and sea-level rise unless emissions are reduced or CO2 is removed.