New research from the University of California has found that as the rate of emissions increases, the ability for the Earth to absorb those emissions is lowered – rendering well-meaning carbon-offset programmes useless.

The study, posted online this week by the Proceedings of the National Academy of Sciences journal, is based on a computer simulation of the global carbon cycle developed by Inez Fung and colleagues with support from the National Science Foundation’s climate dynamics program.

It examined how rising carbon dioxide emissions affect the ability and capacity of the Earth’s natural carbon repositories – such as plants, soil, rain, clouds, bacteria, phytoplankton and oceans. The researchers used observations from the past two centuries to project the coming century.

Their major finding was an inverse relationship between the rate at which carbon dioxide is emitted from the burning of fossil fuels and the capacity of land and ocean to absorb it.

The faster the emissions, the less effective were the carbon sinks.

The reasons are various. On land, global warming tends to dry out the tropics and reduce plant growth, which in turn reduces photosynthesis and carbon uptake.

In the oceans, carbon dioxide from the atmosphere mixes fairly rapidly into the upper layers, down to about 100 metres or so, from where it slowly leaks into the deep ocean and remains sequestered.

However, rising temperatures warm the upper layers making the ocean more stratified and meaning the carbon dioxide cannot mix so readily downward.

Lead researcher Fung says that taking all these effects together, “our finding implies that carbon storage by the oceans and land will lag farther and farther behind as climate change accelerates with growing carbon dioxide emissions, creating an amplifying loop between the carbon and climate systems.”

Overall, said Fung, the model agrees with others predicting large ecosystem changes, especially in the tropics.

By David Hopkins

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