Science - June 12, 2012

The Earth is losing its colours

The planet is becoming greener, literally. But certain areas have in fact become browner. Climate change is a major cause.

The Earth's discolouring can be attributed for more than fifty percent to climatic changes. This concludes physical geographer Rogier de Jong in his thesis on vegetation growth and land degradation. De Jong received his PhD on Monday.
That the Earth has become greener in the past decade due to global warming has already been known for some time. Higher temperatures and more precipitation make plants grow faster. This can be clearly mapped from space by using satellites to measure reflected sunlight. Plants use some of the sunlight for photosynthesis. Infrared light bounces back. The amount of reflected infrared light is therefore a measure of plant activity. De Jong has developed new calculation tools to show this activity more accurately.
The picture as a whole is clear: the planet is getting greener and plants are becoming more productive. But this general picture looks very different when viewed in more detail, says De Jong. 'While the northern hemisphere is becoming greener, in contrast, parts of the southern hemisphere are not.' Plant activity there is decreasing. Land degradation is a major cause. But these tendencies towards discolouration are also in a process of change and can even be reversed, says De Jong. His calculations show that this has indeed taken place in 15 percent of the Earth's surface in the past decade. 'Browning' changes to 'greening' and vice versa.
Monitoring from space enables changes in plant growth on Earth to be better mapped. But it does not provide explanations. 'You can measure vegetation activity. But what lie behind all these - the underlying processes - are very diversified.' De Jong made a first attempt towards providing an explanation by establishing links to climate change. More than half of the changes observed - particularly those in forested areas - could be caused by global warming.
Global warming prolongs the growth season in the northern hemisphere. But this extension does not necessarily mean that plant growth increases at the same rate. This is evident, says De Jong, when the change in growth is mapped for a period. 'Within a growth season, the so-called photosynthesis intensity decreases. This indicates that there are other factors which limit growth, such as the availability of water or nutrients. This can be seen especially in the northern hemisphere at the end of a growth season. Ecologists call this 'late summer stress'.