Global warming currently underway is unprecedentedly rapid climate change caused by humanity. By comparing it with natural warming and cooling in past geologic periods, we can understand the unusual nature and seriousness of modern climate change.
Basic Knowledge
Current atmospheric CO₂ concentration is about420 ppm（2024）and has risen by about 50% from the preindustrial level（about 280 ppm）. In ice-core records from the past 800,000 years, CO₂ concentration fluctuated between 180〜280 ppm, so the current 420 ppm is an unprecedentedly high level. The temperature increase has reached about+1.1〜1.2℃（2023）compared with the Industrial Revolution.
Comparison with past natural warming events：
・PETM（Paleocene–Eocene Thermal Maximum, about 56 million years ago）：Rapid carbon input（the theory of massive release of seafloor methane hydrates）caused rapid warming of about 5〜8℃ over several tens of thousands of years, with a large negative δ¹³C anomaly recorded. Modern warming（more than +1℃ in about 100 years）is considered faster than the PETM.
・Cretaceous Thermal Maximum：CO₂ increased over several million to several tens of millions of years.
・Glacial–interglacial transition：A change of 4〜7℃ over about 5000〜10000 years（dozens of times slower than today）.
According to IPCC assessments, the rate of modern warming is unprecedented in at least the past 2000 years and is also among the rapid rates from a geologic perspective. The rate of increase in CO₂（2〜3 ppm/year）greatly exceeds the carbon release rate during the PETM（estimated at about 0.06 ppm/year）.
State the current atmospheric CO₂ concentration and the preindustrial concentration, and compare them with the range of natural variation over the past 800,000 years.
Answer: Currently about 420 ppm（2024）. Before the Industrial Revolution, it was about 280 ppm. Ice-core records from the past 800,000 years show fluctuations from about 180 ppm during glacial periods to about 280 ppm during interglacial periods（180〜280 ppm）. The current 420 ppm exceeds this natural range by about 50% or more and is an unprecedentedly high concentration.
- Current CO₂：about 420 ppm（280 ppm before the Industrial Revolution）
- Natural variation over the past 800,000 years：180〜280 ppm
- Modern warming：+1.1〜1.2℃ compared with the Industrial Revolution（2023）
- PETM（about 56 million years ago）：rapid warming of +5〜8℃ over several tens of thousands of years
- Rate of modern warming：faster than the PETM・dozens to 100 times faster than glacial–interglacial transitions
- Cretaceous warming：a scale of several million to several tens of millions of years（far slower than today）
- IPCC：the modern rate is unprecedented in the past 2000 years
Points to Note
① The fact that there were warm periods in the past does not mean that the present is “within the range of natural variation.” The problem today is therate. ② The PETM resulted from a rapid increase in naturally sourced carbon, such as methane hydrates, whereas modern warming is caused by human fossil-fuel combustion, with an even faster emission rate. ③ The correlation between CO₂ and temperature is clearly shown by ice-core data, but whether CO₂ preceded or followed temperature changes differs depending on the period and event.
Practice
- How does the current CO₂ concentration（about 420 ppm）compare with the range of natural variation over the past 800,000 years（180〜280 ppm）?
- What kind of climate event was the PETM? Include its period and the amount of change in your answer.
- What is the greatest feature distinguishing modern warming from geologic natural variation?