By Alex Kirby
The amount of vegetation in the world, and the way it is spread across the planet, has changed significantly in the last three decades, researchers say.
They attribute more than half the changes they detected to the effects of the warming climate, with people responsible for only around a third. Surprisingly, perhaps, they are at a loss to attribute about 10% of the changes unequivocally to either the climate or us.
They say their work marks a scientific advance, because it has only recently become possible to quantify how far climate variability, human activity or a combination of the two are responsible for what is happening.
While the researchers, geographers from the University of Zurich, Switzerland, and colleagues from the Netherlands, say the last 30 years have seen substantial changes, satellites have during that time been recording how vegetation has altered.
An oil palm plantation at the bottom of the picture and undisturbed forest at the top (Source: Flickr/Wakx)
In a striking and perhaps unexpected development, the team found that while vegetation has declined south of the Equator, it has increased in the northern hemisphere.
The climate is what governs the seasonal activity of vegetation. In the humid mid-latitudes, temperature is the largest factor influencing plant growth.
In mainly dry areas, though, it is the availability of water and in high latitudes the amount of solar radiation that is key. And everywhere humans influence vegetation in myriad ways – and are influenced by it.
There is evidence that the arid expanses of the Sahara desert were once wet enough to support lush vegetation, so much so that the Sahara was known as the breadbasket of North Africa.
A reverse process is under way in Greenland, where the rapid warming of the Arctic means that in some southern parts of the formerly ice-bound island vegetables will now grow happily.
Vanishing forests
A pervasive human influence in many parts of the world is the pressure from growing human populations and their demand for wood for fuel and building and for plant matter for food and fodder.
The researchers have developed a model that can show the influences on vegetation of human activity and climate variability separately. Using satellite data on the increase or decline over the last thirty years, climate measurements and models, and data on the kind of land cover, they conclude that around 54% of the changes in global vegetation can be attributed to climate variability.
One of their reports, Spatial relationship between climatologies and changes in global vegetation activity, is published in the journal Global Change Biology. The other, Shifts in Global Vegetation Activity Trends, appears in Remote Sensing.
The main decline they detected has happened south of the Sahel, in countries such as Tanzania, Zimbabwe and other parts of central Africa.
“We assume that this was caused by clear cutting, the transformation of rain forest into plantations, or changes in agriculture in general,” said Rogier de Jong, a postdoctoral student at the University of Zurich’s Remote Sensing Laboratories (RSL).
But even after identifying the difference between the hemispheres and the probable reasons for it, that still leaves the tantalising 10% of change which the team cannot explain fully by either climatology or human activity.
“We suspect that this is due to unexplained effects of the interactions between humans and the climate”,’ says the head of the RSL, Michael Schaepman.
He and his team will continue to work on trying to find an explanation for what is happening under a newly created research priority programme, Global Change and Biodiversity, at Zurich.
This story was produced by the Climate News Network