Climate change – ask not by how much, but how fast

Prehistoric global warming is something we know about; it is written into the geological record. Take, for example, the Palæocene-Eocene Thermal Maximum (PETM), a dramatic and singular event in the Cænozoic era some 56 million years ago, during which over the course of a few thousand years the surface temperature of the Earth rose by around five degrees.

The PETM warming shifted climate zones toward the poles, forcing plants and animals to migrate vast distances. This many species did, but some organisms, especially among those in the deep ocean, became starved of oxygen through acidification, and died.

In an article in the July issue of Scientific American, Penn State geochemist Lee Kump discusses the PETM in the context of contemporary climate change. Kump argues that prehistoric precedent has lessons for our future, with a narrative that focuses on the timescale of the PETM, and how this compares with what is happening today. Kump talks in terms of “planetary fevers”.

While there are similarities when it comes to the amount of excess carbon dioxide in the atmosphere, climate change now is far more rapid than it was 56 million years ago. During the PETM, the rate of carbon injection was around two petagrammes (2×1015, or two million billion grammes) a year, but concentrations are today rising 10 times faster. What we should be asking ourselves, says Kump, is how life will adapt to such rapid change. The implication is that we should not be fixated on the absolute degree of global warming involved.

Within the space of a few decades, the atmospheric carbon dioxide concentration has increased by more than 30 percent, and we are now pumping nine petagrammes into the atmosphere every year. That figure may reach 25 petagrammes before all remaining fossil-fuel reserves are exhausted.

Further reading

Gavin Schmidt, “PETM weirdness”, RealClimate, 10 August 2009