We tend to measure time by the span of a human life, making a century seem like an era and a millennium a mega-stretch of time. In this perspective, a million years is an eternity. So it can be revealing to consider our place in geologic history measured in hundreds of millions of years.
This is what a group of researchers from the United States and four other countries did recently. They reviewed existing evidence on the impact of changes in the atmospheric concentration of carbon dioxide (CO₂), the main global warming gas, from decades of research on fossilized remains and other evidence from Earth's geologic record.
Published earlier this month, their findings reinforce warnings from many climate scientists that the world's oceans, a vital source of fish food protein, may be turning acidic faster today from human CO₂ emissions than they did during four major episodes of animal and plant extinctions in the last 300 million years, when natural surges of CO₂, probably from catastrophic volcanic eruptions or meteor strikes, sent global temperatures soaring.