Higher temperatures brought the plants – and then came even warmer temperatures, according to new model simulations published in the journal Science Advances. According to Alexander Thompson, a postdoctoral fellow in earth and planetary sciences at the University of Washington in St. Louis, pollen records can tell the true story of global temperatures. Earth’s atmospheric temperature models since the last ice age have shown that temperatures have risen steadily over time. But Thompson noted that climate mediation files tell a different story. Many of these sources indicate a remarkable peak in world temperatures that occurred between 6,000 and 9,000 years ago. He said the models could overlook the role of vegetation changes in favor of the effects of atmospheric carbon dioxide concentrations or ice cover. “Pollen records suggest a large expansion of vegetation during this period,” Thompson said. “But previous models show only a limited amount of vegetation,” he said. “So while some of these other simulations involved dynamic vegetation, the vegetation shift was hardly enough to explain what the pollen files suggested.” In fact, the changes in vegetation cover were significant. At the beginning of the Holocene – in today’s geological age – the Sahara desert in Africa became greener than today, it was more of a meadow. Other vegetation in the northern hemisphere, including coniferous and deciduous forests in the middle latitudes and the Arctic, has also flourished. Thompson took data from pollen records and designed a set of experiments with a climate model known as the Community Land System Model (CESM). He ran simulations to explain a series of vegetation changes that had not been considered before. “The swollen vegetation during the Holocene warmed the globe to 1.5 degrees Fahrenheit,” Thompson said. Understanding the scale and timing of temperature change during the Holocene is important because the rise of human agriculture and civilization occurred during this period. “Overall, our study emphasizes that accounting for vegetation change is critical,” said Thompson. “Forecasts for future climate change are more likely to produce more reliable forecasts if they include vegetation changes.”
