6 September 2018

Global warming: worrying lessons from the past

56 million years ago, an episode of global warming caused extreme floods and landscape disruption. A study conducted in the Pyrenees by an international team of researchers including Utrecht University reveals the magnitude of this environmental disaster.

56 million years ago, the Earth experienced an exceptional episode of global warming. Within a few thousand years (very fast on geological time scales) the Earth’s surface warmed by on average 5 °C, only returning to its original level about two hundred thousand years later. Based on the analysis of sediments from the southern slope of the Pyrenees, a new study lead by scientists from the University of Geneva (UNIGE) in collaboration with researchers from the universities of Lausanne, Utrecht, Western Washington and Austin, measured the impact of this warming on river floods and the surrounding landscapes in Northern Spain. Their analyses indicate that the amplitude of floods increased by a factor of about eight. Vegetated landscapes were replaced by seasonally occurring large and wild rivers that eroded soils and vegetation, leaving pebbly plains in a dry season. These conclusions, published today in Scientific Reports, for the first time provide some insight into the consequences of global warming for extreme weather from the geological record.

Prof Appy Sluijs

A Past Global Warming Phase

Previous work by Utrecht University Professor Appy Sluijs, co-author of the study, and his colleagues revealed that during the studied episode of global warming, the Paleocene-Eocene Thermal Maximum, palm trees thrived at polar latitudes and marine plankton normally restricted to tropical waters, suddenly spread across the globe. “Sea surface temperatures in the Arctic regions soared to more than 20 °C”, Sluijs says. “In fact, tropical sea surface temperatures exceeded 36 °C in some regions, too warm for many types of organisms and explaining their demise.”

But although the event is well known amongst paleoclimate researchers, its potential use for learning lessons for the future remains underexplored. Sluijs: “The research community has established that the PETM was associated with a strongly enhanced greenhouse effect and we have also reconstructed how warm it became. Now it is time to fully assess how it affected biology but also the occurrence of extreme weather”. “This is important because there is an obvious analogy with the current global warming, Sébastien Castelltort, the project leader, adds: “There are lessons to be learned from this event, even more so as the rise in temperatures we are currently witnessing seems to be much faster.”

Pebbles reveal the history of rivers

Castelltort, his student Chen Chen, first author of the paper, and their colleagues studied sediments that were deposited 56 million years ago by rivers in what are now the Spanish Pyrenees. Particularly, they determined the size of pebbles and dimensions of the preserved fossil riverbeds because these relate strongly the flow velocity and discharge of the ancient river system.

The pebbly channel layer. Credit: Appy Sluijs
The pebbly channel layer. Credit: Appy Sluijs

56 million years ago, the Pyrenees were being formed and prior to the PETM their foothills were traversed by small isolated river channels in a vegetation and soil-covered flood plain. The new paper shows that this landscape changed completely during the PETM. The water supply during seasonal extremes in rainfall was so large that rivers dramatically changed dimensions. Remarkably, these river channels were not much deeper than prior to the PETM but much wider, from 15 to 160 meters in the most extreme case, and the rivers constantly changed course. The channels discharged about 8 times as much water as the rivers did prior to the PETM and eroded the soils and standing vegetation. The landscape turned into arid extensive gravel plains, crossed by ephemeral and torrential rivers.

Lessons for the Future

“We teach a field course in the study area for our second years’ Earth Sciences students every year”, Sluijs adds. “We study kilometers of sediments, stacked onto each other in layers, but the vast pebble layers of the PETM stand out in a tremendous way and all students see how special that is.” But it is the new work that makes it relevant to climate change. “It shows the way extreme weather associated with warming climates may devastate landscapes. In many ways, it is not unlike some of the events the Earth has witnessed over the past years.” As such, the reconstructions and calculations are consistent with the expected increase in extreme weather events expected for the future. Sluijs: “For a change, however, this is not based on a model simulation but on rocks that were really deposited in northern Spain. This really happened! And it therefore supports future projections of an increase in extreme weather although perhaps the magnitude of change we observe may be larger than model projections suggest.”

Article

Estimating regional flood discharge during Palaeocene-Eocene global warmingScientific Reports, 2018. Chen Chen, Laure Guerit, Brady Z. Foreman, Hima J. Hassenruck-Gudipati, Thierry Adatte, Louis Honegger, Marc Perret, Appy Sluijs  & Sébastien Castelltort