About half the 24 billion tons of carbon dioxide generated by human activities each year are absorbed by forests and oceans — a process without which the world might already be several degrees warmer.
But as CO2 levels rise and soils dry, microbes can start breaking down accumulated organic matter, so forests become net producers of greenhouse gases. The sea’s power to absorb CO2 also falls sharply as it warms.
The latest research suggests the threshold for such disastrous changes will come when CO2 levels reach 550 parts per million (ppm), roughly double their natural levels. This is predicted to happen around 2040-50.
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