Newly identified side to global warming

An atmospheric mechanism has emerged that could lift global temperatures by even higher than the 2-5?C rise predicted by the end of the century as a result of greenhouse gas emissions.

The newly discovered mechanism involves the gas ozone (O₃), which, like CO₂, has been accumulating in the atmosphere as a result of fossil fuel burning.

Ozone was already known to have a significant direct greenhouse gas (GHG) action. But the new effect is an indirect one, resulting from the toxic effect of high ozone levels on land plants.

Ups and downs of CO₂ levels

Atmospheric CO₂ levels are determined by two competing factors. New CO₂ is constantly being generated by processes like volcanic activity and fossil fuel burning, and at the same time it is absorbed or ’sunk’ by the oceans and the land surface, including plants. 

Land plants absorb CO₂ through pores in their leaves called stomata.   When atmospheric ozone levels reach about 40ppm - already reached in many parts of the world - these stomata contract, reducing the plants’ CO₂ uptake. Moreover, ozone levels are expected to reach 70ppm in many parts of the world by the year 2100. This will suppress the land carbon sink, push up the equilibrium level of atmospheric CO₂, and boost the greenhouse effect, say UK researchers.

Soaking up the rays

Climatologists use a parameter called radiative forcing (RF) to measure the earth’s ability to retain solar heat. Standard climate models used by the researchers predicted that the indirect ozone effect will generate an extra RF of between 0.62 and 1.09 Watts  per m² (W/m²) by the end of the century. By comparison, the total RF due to all GHGs is about 2.9W/m² today, and is expected to rise to about 8W/m² by 2100.

According to lead researcher Stephen Sitch of the Met Office’s Hadley Research Centre, an RF of 1W/m² could boost average world temperatures by a further 0.5-1.25?C - compared with the 2-5?C that will result from a doubling of atmospheric CO₂.

Ozone needs to be factored in

The indirect effect makes ozone a far more important GHG than was previously thought, said Sitch. That could prompt new climate control measures, for example limits on the industrial NO_x emissions that are precursors of atmospheric ozone.

Other GHGs also have indirect forcing effects, he said. CO₂ itself exerts a cooling indirect effect by protecting plants against ozone and increasing ocean uptake of CO₂, but it also helps heat the earth indirectly by reducing evaporative cooling of the land surface. It is not known which is more important. 

Peter Mitchell

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