Nick Carne
Two papers just published in the journal Nature present more grim news about our changing planet.
The first – and the more general in scope – suggests that global warming could cause sudden and potentially catastrophic losses of biodiversity in regions across the globe throughout the present century.
It predicts when and where severe ecological disruption could occur – and suggests that the first waves could already be happening.
The second paper, reporting on an Australian study, presents evidence that mature forests are limited in their ability to absorb “extra” carbon as atmospheric carbon dioxide concentrations increase, which has implications for climate change modelling.
Alex Pigot from University College London, the lead author of the first paper, says it is clear that climate change risks to biodiversity don’t increase gradually.
“Instead, as the climate warms, within a certain area most species will be able to cope for a while, before crossing a temperature threshold, when a large proportion of the species will suddenly face conditions they’ve never experienced before,” he says.
“It’s not a slippery slope but a series of cliff edges, hitting different areas at different times.”
Pigot and colleagues from the US and South Africa gathered climate model data from 1850 to 2005 and cross-referenced it with the geographic ranges of 30,652 species of birds, mammals, reptiles, amphibians, fish, and other animals and plants.
They then used climate model projections for each year to 2100 to predict when species in each grid cell will begin experiencing temperatures that are consistently higher than the organism has previously experienced across its geographic range, for at least five years.
The results, they say, suggest that in most ecological communities across the globe, a large proportion of organisms will find themselves outside their comfort zone within the same decade.
Across all the communities, on average 73% of the species facing unprecedented temperatures before 2100 will cross that threshold simultaneously.
As always, it’s a question of degrees.
The researchers predict that if global temperatures rise by four degrees Celsius by 2100 (noting that this is plausible), at least 15% of communities across the globe, and potentially many more, will undergo an abrupt exposure event where more than one in five of their constituent species crosses the threshold beyond their niche limit within the same decade.
Such an event could cause irreversible damage to the functioning of the ecosystem.
However, if warming is kept to two degrees or less, potentially fewer than 2% of communities will face such exposure events – although that 2% includes some of the most biodiverse communities on the planet, such as coral reefs.
The second paper reports on the EucFACE (Eucalyptus Free Air CO2 Enrichment) experiment led by Australia’s Western Sydney University.
With CO2 concentrations in the atmosphere increasing steadily due to human emissions, there is ample evidence that plant photosynthesis is going up, the researchers say, and experiments have shown that single young trees use the extra carbon acquired through photosynthesis to grow faster.
However, scientists have long wondered whether mature native forests would be able to take advantage of the extra photosynthesis, given that the trees also need nutrients from the soil to grow.
And the answer appears to be “no”. When Belinda Medlyn and colleagues exposed a 90-year old eucalypt woodland to elevated CO2 levels, the trees took in about 12% more carbon, but they didn’t grow any faster.
Analysis showed that the extra carbon was quickly cycled through the soil and returned to the atmosphere, with around half being returned by the trees themselves, and the rest by fungi and bacteria in the soil.
“The trees convert the absorbed carbon into sugars, but they can’t use those sugars to grow more, because they don’t have access to additional nutrients from the soil,” Medlyn says. “Instead, they send the sugars below-ground where they ‘feed’ soil microbes.”
These findings have global implications, the researchers say, because current models used to project future climate change and its likely impact on plants and ecosystems assume that mature forests will continue to absorb carbon over and above their current levels, acting as carbon sinks.