Fifteen years ago the late Fred Singer and Dennis Avery published a book called Unstoppable Global Warming: Every 1500 Years. The authors presented evidence from the scientific literature showing a roughly-1,500-year cycle in the Earth’s climate system, which they attributed to solar variations, and argued that since we are on the upswing, trying to stop global warming would be futile. Over the years many scientists have come to believe in the 1500-year cycle, although it’s not smooth. The warming episodes, called Dansgaard-Oeschger (DO) events, hit hard and fast over a few decades, while the cooling is slow and lasts for centuries. Not all researchers agree that the sun is the culprit: Some figure it’s an internal clockwork mechanism in the climate. But while evidence of the global extent of the cycle during the Holocene has accumulated, there wasn’t evidence that it existed prior to the beginning of the current “Pleistocene” ice age. Until now. A new study by a team of French climate scientists published in Nature Scientific Reports found a 1,500 year cycle in sediment layers dating all the way back to the Jurassic period, 155 million years ago. It’s beginning to look like climate change is real and there’s no more room for denial.
When we say warming we’re not talking about a fiddly little warming like the single degree rise over the past century. A DO event can send the atmospheric temperatures up 15 degrees in a few decades. Nor are these things confined to a small part of the Earth’s surface. As the authors note:
“The driving mechanisms of DO events have received increasing attention in recent years because of their global record in continental and marine environments, especially in glacial archives of the last glacial period. Ice-core records from the Greenland, and deep-sea sediment records from the North Atlantic Ocean highlight these prominent DO glacial events. Paleoclimatic studies have now shown that the 1500-year climate cycle is no longer restricted to the North Atlantic Ocean of the last glacial period. The 1500-year cycle is documented in both hemispheres, in other oceans and in continents, such as in lake and river deposits, in pollen fossils, in stalagmite proxy records, and in loess–paleosol deposits.”
In this particular study the authors took advantage of a geological formation in a region called La Cluse in southeastern France. Here a portion of land is exposed that, in the Jurassic period, was under the Tethyan ocean and experienced a combination of tides and currents that led to a high sedimentation rate, leaving a detailed record of the climatic conditions of the period. They removed sections of the rock formations and with the help of sophisticated magnetic imaging equipment were able to form a detailed record of a 140,000 year slice of Jurassic history.
Their analysis showed clear evidence of a 1500-year cycle in the climatic record, strongly suggesting that DO events or something like them were taking place long before the continents were even in their current position, let alone before we were driving around them in our SUV’s. The paper’s authors also provide a nice, even-handed discussion of the current debates about what causes the DO cycles. It seems no one knows for sure. That DO events happen is now widely accepted, as is the fact that they are global in scale. But what causes is not settled.
So how do we know the current warming has nothing to do with the unknown causes of the natural warming cycle that affects the whole globe and shows up with amazingly durable persistence, in modern ice cores and ancient rock formations alike? Good question.