Satellite data shows the per cent amount that foliage cover has changed around the world from 1982 to 2010.
Impact of CO2 fertilisation on maximum foliage cover across the globe's warm, arid environments
Q: Can you briefly summarise your research findings?
A: We have found a strong link between rise in atmospheric CO2 concentrations and an observed 'greening' in many of the world's arid environments.
Greenhouse experiments have shown us that plants, grown in dry conditions, are more efficient at using water under higher CO2 levels than under lower levels. This leads us to expect a general increase in vegetation foliage cover across arid landscapes that is in proportion with the increase in CO2 levels.
To test if there was such an observable increase in cover, we examined satellite data going back to the early eighties. The trick was to identify, from across the globe's dry places, the maximum foliage cover that can be attained for a given rainfall. We theorised that such maximum cover values should have increased in proportion to the known rise in CO2.
What we found was that these 'maximum cover' values had increased almost exactly as predicted. This provides strong evidence that the rise in CO2 has given plants a bit of a boost in what are some of the world's lowest productivity environments.
Q: Does this mean all deserts have greened by 11 per cent?
A: No, not all. We're not saying that all deserts are greening, but that they are greener than they would otherwise be if CO2 had remained constant.
In the areas where we could isolate the CO2 effect from the effects of other drivers (such as changes in rainfall, temperature, humidity and land use), we observed an 11 per cent increase in cover. In other areas, where we were unable to isolate the CO2 effect, the overall change in cover will be the net effect of all the driving processes, including CO2 fertilisation.
So the CO2 fertilisation effect means that cover is around 11 per cent greater now than it would have been if CO2 levels were constant. In other words, there are places that have become less green (ie lower cover), but they would have had even lower levels of cover had CO2 not increased.
Q: Will this new information change the climate debate?
A: No, these results should not change the climate change debate. They do, however, provide a little more understanding on which to base the debate.
Whilst it may seem that an increase in the uptake of carbon by vegetation would serve to offset the rise in atmospheric CO2 levels, the amounts in question are too small to make a significant difference to the overall global picture.
Q: So does this mean climate change is good for the planet?
A: This does not mean that climate change is good for the planet. Whilst CO2-induced increases in cover across many of the world's deserts and semi-deserts will most likely have some beneficial effects, there will also be associated changes that are seen to be detrimental, like possible decreases in surface water availability or the encroachment of woody vegetation into native pastures.
Q: Do we understand the impacts of our deserts 'greening' in terms of water use, climate, habitat etc?
A: Our understanding of such impacts is growing rapidly. However, research into the effects on elevated CO2 in natural landscapes is an emerging field and our understanding is accordingly quite limited. Vegetation links the carbon and water cycles, and is tied to biodiversity and agricultural productivity. Understanding the many flow-on and feedback effects induced by a change in vegetation is a challenge that warrants considerable effort.