Blowing Up the Climate

Characterized in the crudest possible terms, the earth’s climate is simple. New heat comes in from the sun. Old heat radiates out into space. Anything that blocks incoming solar energy, like airborne volcanic ash, makes the planet colder. Anything that blocks outgoing radiation, like carbon dioxide, makes the planet warmer.

The details can get tricky, though. For example, although schoolchildren know that volcanic eruptions cool the planet, problems with the historical record have prevented scientists from constructing an accurate timeline of how often and how much volcanoes have affected the climate. Ice cores containing ash aren’t in perfect agreement, nor are the tree rings that indicate temperatures year by year.

Today, in the journal Nature, an international team of climatologists, paleoecologists, physicists, astronomers, and historians published the most complete and accurate record yet of the volcano-climate relationship over the past 2,500 years. Their work is still based on ice cores, but by incorporating new statistical techniques and a cosmic ray event of a known date (it happened in 774 or 775 CE) into the analysis, the researchers were able to anchor the timeline.

“Ice-core timescales had been misdated previously by five to ten years during the first millennium,” says Francis Ludlow, a Yale climate historian. This led to inconsistencies between when volcanoes were thought to have blown and how the climate responded to those eruptions based on tree-ring evidence and written records.

Adding this new information was like making the final move on a Rubik’s cube—the dates suddenly clicked into place. The results showed that 15 of the 16 coldest summers between 500 BCE and 1,000 CE came immediately after volcanic eruptions, and the coldest of the cold summers tended to follow the largest eruptions.

The study has also settled some longstanding mysteries. Beginning in 536 CE, Europeans and Near Easterners began writing about a mysterious dark cloud, a dry fog, or a dust veil, depending on who was describing it. The next 14 years saw strange weather and a notably cooler climate. Most historians agreed it was all the result of a volcanic eruption, but there was controversy over where the volcano was located and when it erupted. The new research has identified a pair of eruptions somewhere in the Northern Hemisphere in 536 and in either 539 or 540, which may have elevated atmospheric aerosols to levels unseen in the rest of the recent historical record.

These observations aren’t just historical puzzle pieces—they also show how damaging sudden climatic changes can be for humans. Europe suffered terrible famine in the 540s. China also suffered agriculturally. Although it’s difficult to prove, the eruptions and subsequent weather changes may have also contributed to a temporary decline of the Mayan civilization.

The researchers even speculate that the weather anomalies could have contributed to the spread of the bubonic plague during this period. The evidence on this point is thin, but it’s not a crazy suggestion, either. Yersinia pestis, the bacterium that causes the plague, is carried by certain types of gerbils, among other rodents. Researchers have shown that changes in weather can affect the prevalence of the pathogen in gerbils, making it possible that volcanic eruptions could be connected to the disease by a strange but plausible chain—volcano, weather changes, gerbil transmission, human disease.

Am I saying that carbon emissions are going to cause a gerbil-fueled return of the bubonic plague? No. (Although I’ve always suspected that gerbils are planning their revenge for the tiny cages, the smelly wood chips, and those cruel spinning wheels that never go anywhere.) But this study is more than just a great piece of cross-disciplinary scientific sleuthing. It’s a cautionary tale about how nature is bigger than we are. Changes in the atmosphere—either because of volcanic eruptions or humanity’s industrial lifestyle—have consequences on the ground. After thousands of years worth of lessons, it’s about time we learn.