Fossil pollen shows Earth’s vegetation is changing faster today than in the past 18,000 years

To study past plant ecosystems and their evolution, a lake or other suitable environment is cored to retrieve layered sediments that typically contain pollen grains accumulated over thousands of years. By identifying and counting the different pollen grains, researchers can then reconstruct the composition of the local vegetation. Finally, the rate of vegetation change is estimated from changes in pollen abundance over time. Credit: Work by Milan Teunissen van Manen

A global survey of fossil pollen has found that the planet’s vegetation is changing at least as rapidly today as it did when the last ice sheets retreated around 10,000 years ago. About 3,000 to 4,000 years ago, Earth’s plant communities began to change at an accelerating rate. Today, that pace rivals or exceeds the rapid turnover that took place as plants rushed to colonize once-frozen landscapes and adapt to a global climate that has warmed by about 10 degrees. Fahrenheit.

The research, published May 21, 2021, in the journal Sciencesuggests that the dominant human influence on ecosystems that is so visible today has its origins in early civilizations and the rise of agriculture, deforestation, and other ways in which our species influenced the countryside.

This work also suggests that rates of ecosystem change will continue to accelerate over the coming decades as modern climate change adds to this long history of fluxes. And by showing that recent trends in biodiversity are the start of a longer-term acceleration of ecosystem transformations, the new study provides context for other recent reports indicating that global changes in biodiversity have accelerated over the course of the last century.

Coring St. Paul, Alaska

Researchers collect a core in St. Paul, Alaska. Credit: Jack Williams

An international collaboration of scientists conducted the new analysis, which was powered by an innovative database for paleoecological data. The Neotoma Paleoecology Database is an open-access tool that collects and organizes data on past ecosystems from hundreds of scientists. Neotoma is chaired by Jack Williams, a professor of geography at the University of Wisconsin-Madison, who helped lead the new research.

The study authors analyzed more than 1,100 fossil pollen records from Neotoma, spanning every continent except Antarctica, to understand how plant ecosystems have changed since the end of the last ice age around 18,000 years ago. years, and how quickly this change has happened.

“At the end of the Ice Age, we had complete biome-scale ecosystem conversions,” says Williams, who also maintains the North American pollen database at Neotoma. “And over the last few thousand years, we’re back to that scale. It has changed so much. And these changes started earlier than we might have thought before.

Fossil pollen provides an extremely sensitive measure of past plant communities. As pollen from surrounding plants falls into the lakes, it settles in layers, from oldest at the bottom to newest at the top. Scientists can extract sediment cores and do the painstaking work of identifying pollen and reconstructing plant ecosystems over thousands of years.

Yet each sediment core only provides information about a single location on Earth, so true global-scale analyzes of past vegetation change require the collection and preservation of many such records. Neotoma has collected thousands of these data points to help scientists uncover global trends. Researchers from the University of Bergen in Norway, UW-Madison and Neotoma data managers around the world collaborated to perform the new analysis.

Using these pollen records, the team applied new statistical methods to better analyze how quickly plant communities have changed over the past 18,000 years.

They found that the rate of change initially peaked between 8,000 and 16,000 years ago, depending on the continent. These continental differences are likely caused by different rates and patterns of climate change related to retreating glaciers, increasing carbon dioxide concentrations in the atmosphere, changes in Earth’s orbit, and changes in circulation. oceanic and atmospheric.

The ecosystems then stabilized until about 4,000 years ago. Then, the rate of change began a meteoric rise that continues today, with most plant ecosystems changing at least as fast as at the peak of Ice Age-induced flux.

“It was a surprising finding, because over the last few thousand years not much has happened climatically, but the rates of change in the ecosystem were as important or more important than anything. what we’ve seen since the last ice age until now,” Williams said.

Although this analysis of pollen records has focused on detecting ecosystem changes, rather than formally determining causes, these recent ecosystem changes correlate with the onset of intensive agriculture and the world’s earliest cities and civilizations.

Williams says an intriguing feature of these analyzes is that the early rise is so early in the world, even though each continent had different trajectories of land use, agricultural development and urbanization.

Scientists have coined the term Anthropocene to describe the modern geological period, when humans exert the dominant influence on the world. “And one of the questions has been, when did the Anthropocene start?” said Williams. “This work suggests that 3,000 to 4,000 years ago humans were already having a tremendous impact on the world (and) that continues today.”

A sobering implication of this work, the scientists say, is that in the past, periods of ecosystem transformations induced by climate change and those induced by land use were widely separated. But now intensive land use continues and the world is warming at an increasing rate due to the buildup of greenhouse gases. As plant communities respond to the combination of direct human impacts and human-induced climate change, future rates of ecosystem transformation could once again break new records.

To learn more about this research, read Forests and Climate Change – “We Can’t Plant Our Way Out of the Climate Crisis”.

Reference: “Global Acceleration in Rates of Vegetation Change Over the Past 18,000 Years” by Ondrej Mottl, Suzette GA Flantua, Kuber P. Bhatta, Vivian A. Felde, Thomas Giesecke, Simon Goring, Eric C. Grimm (d. ), Simon Haberle, Henry Hooghiemstra, Sarah Ivory, Petr Kune, Steffen Wolters, Alistair WR Seddon and John W. Williams, May 21, 2021, Science.
DOI: 10.1126/science.abg1685

This work was supported in part by the National Science Foundation (grants 1550707, 1550805, and 1948926).