Follow the pollen. Records from past plant life tell the true story of global temperatures, according to research by a climate scientist at Washington University in St. Louis. Louis.
Warmer temperatures brought plants – and then came even warmer temperatures, according to new model simulations published on April 15 in The progress of science.
Alexander Thompson, a postdoctoral fellow in Earth & Planetary Sciences at Arts & Sciences, updated simulations from an important climate model to reflect the role that changing vegetation plays as a key driver of global temperatures over the past 10,000 years.
Thompson had long been plagued by a problem with models of Earth’s atmospheric temperatures since the last ice age. Too many of these simulations showed that temperatures heat up constantly over time.
But climate proxy records tell a different story. Many of these sources indicate a marked peak in global temperatures that occurred between 6,000 and 9,000 years ago.
Thompson had a hunch that the models could overlook the role of changes in vegetation in favor of influences from atmospheric carbon dioxide concentrations or ice cover.
“Pollen records indicate a large expansion of vegetation during that time,” Thompson said.
“But previous models show only a limited amount of vegetation growth,” he said. “So even though some of these other simulations have included dynamic vegetation, it was not near enough of a vegetation shift to take into account what the pollen records suggest.”
In fact, the changes in vegetative cover were significant.
Early in the Holocene, the current geological epoch, the Sahara desert in Africa became greener than it is today – it was more of a grassland. Other vegetation in the northern hemisphere, including the coniferous and deciduous forests of the middle latitudes and the Arctic, also thrived.
Thompson took evidence from pollen records and designed a set of experiments with a climate model known as the Community Earth System Model (CESM), one of the most highly regarded models in a broad class of such models. He ran simulations to account for a number of changes in the vegetation that had not previously been considered.
“Expanded vegetation during the Holocene warmed the globe by as much as 1.5 degrees Fahrenheit,” Thompson said. “Our new simulations are closely aligned with paleoclimate proxies. So it’s exciting that we can point to vegetation in the northern hemisphere as a potential factor enabling us to resolve the controversial Holocene temperature confrontation.”
It is important to understand the extent and timing of temperature changes throughout the Holocene because it is a period of recent history, geologically speaking. The emergence of human agriculture and civilization took place during this time, so many scientists and historians from different disciplines are interested in understanding how early and mid-Holocene climates differed from the present.
Thompson conducted this research work as a graduate student at the University of Michigan. He continues his research in the laboratory of climate scientist Bronwen Konecky at Washington University.
“Overall, our study underscores that vegetation change is crucial,” Thompson said. “Projections for future climate change are more likely to produce more credible predictions if they include changes in vegetation.”
Machine learning helps identify climatic thresholds that shape the distribution of natural vegetation
Alexander J. Thompson, vegetation change in the northern hemisphere drives a Holocene thermal maximum, The progress of science (2022). DOI: 10.1126 / sciadv.abj6535. www.science.org/doi/10.1126/sciadv.abj6535
Provided by Washington University in St. Louis
Citation: Changes in vegetation have shaped global temperatures over the last 10,000 years (2022, April 15) Retrieved April 16, 2022 from https://phys.org/news/2022-04-vegetation-global-temperatures-years.html
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