Like in the “The Lord of the Rings” movies, trees occasionally make their way to faraway lands. Whether tropical rainforests, evergreen coniferous forests or treeless steppes: our planet’s vegetation zones are constantly changing. At the end of the last glacial period, around 11,000 years ago, the Sahara was considerably greener than it is today, while northern Germany was a tundra – an open landscape characterized by mosses, grasses and small shrubs.
When plants migrate, it almost always has something to do with the climate. Either it’s too hot or too cold, too dry or too wet. Plants are then unable to thrive in their original habitat and “move”, albeit very slowly – over several centuries – to neighboring regions. In the process, they provide a record of climate history.
Together with an international research team at the Center for Earth Systems and Sustainability at Universität Hamburg and the Max Planck Institute for Meteorology, I have investigated how past climate changes affected vegetation in Asia. The climate was unusually stable from the mid-Holocene around 6,000 years ago to the dawn of industrialization; only changes in the Earth’s orbit around the Sun – something that occurs in regular cycles – led to several regions cooling slightly during this period.
In order to investigate the changes in vegetation, we used five different models, which we then supplied with data on the changes in the Sun’s irradiation of the Earth and the slight increase in carbon dioxide emissions. Climate models use the laws of physics to describe processes like cloud formation, ocean currents, evaporation, ice melting and storms, and help us predict how the climate will develop.
We then fed these results into a vegetation model, which calculated how plants spread across the planet. Though precipitation and temperature have a major influence on plant growth, numerous other processes are also involved: higher levels of carbon dioxide in the atmosphere promote growth, tornadoes and fires destroy forests, and entire plant communities migrate, displacing established species.
Climate changes influence vegetation
This can lead to climate change feedback. For example: when the northern latitudes become colder, the treeless tundra pushes the extensive coniferous forests – the taiga – southward. Because the snow can fall between the branches, the forests remain dark even in snowy months. This means they absorb more sunlight, and they become warmer. In contrast, the treeless tundra remains completely covered in snow for several months at a time. It reflects the Sun’s warmth back into space, causing temperatures to drop even further.
Our findings show that during the study period, the tundra in Asia migrated up to 500 kilometers southward due to the cooling climate in the far north. In the transition zone between the forest, steppe and desert in northern China and Mongolia the climate has become dryer, and the Gobi Desert has expanded eastward in the last 6,000 years, displacing the steppes and forests.
We weren’t able to identify any abrupt changes in the vegetation in Asia; plants migrated in keeping with the climate changes brought about by the regular changes in the Earth’s orbit. The findings make it clear just how sensitively the vegetation reacts to even the slightest climate changes.
This content was first published as a guest article in the newspaper Hamburger Abendblatt on May 11th, 2017.
Martin Claußen is a Professor of Meteorology at Universität Hamburg and Director of the Max Planck Institute for Meteorology.
Dallmeyer, A., Claussen, M., Ni, J., Cao, X., Wang, Y., Fischer, N., Pfeiffer, M., Jin, L., Khon, V., Wagner, S., Haberkorn, K., and Herzschuh, U.: Biome changes in Asia since the mid-Holocene – an analysis of different transient Earth system model simulations, Clim. Past, 13, 107-134, doi:10.5194/cp-13-107-2017, 2017. Read more