Opinion Another powerful negative-feedback mechanism which acts to reduce the effects of global warming has been identified, as scientists say that rising temperatures cause plants to emit higher levels of planet-cooling aerosols.

"Aerosol effects on climate are one of the main uncertainties in climate models," explains Pauli Paasonen of Helsinki uni. "Understanding this mechanism could help us reduce those uncertainties and make the models better."

Aerosols - suspended particles - in the atmosphere have various effects. Black carbon soot absorbs sunlight and heats the world up, but most other kinds of aerosol tend to cool things down, mostly by presenting nuclei for clouds to form on and so reflecting heat back into space. There is widespread scientific agreement, even among climate researchers, that aerosols have powerful effects - but just how much aerosol can be expected in the atmosphere of the future is not at all well known, and current models aren't thought to handle this factor at all well.

Paasonen and his colleagues' new work indicates that there is and will be a lot more aerosol in the air and cooling the planet than had been thought, as they have carried out detailed measurements showing that a slightly warmer climate makes plants give off much increased amounts of cooling aerosols. This had long been suspected, but previous studies had been able to identify such an effect only in very specific and limited locations: Paasonen and his colleagues have now shown that the feedback acts on continental scales, overcoming earlier difficulties in accounting for the always tricky effects of the boundary layer where the atmosphere interacts with the surface beneath.

"One of the reasons that this phenomenon was not discovered earlier was because these estimates for boundary layer height are very difficult to do," explains Paasonen. "Only recently have the reanalysis estimates been improved to where they can be taken as representative of reality."

On its own, according to Paasonen, this newly quantified feedback would not wipe out global warming (logically enough, as it needs some warming before it appears). But he and his colleagues consider that it means forecasts should be adjusted downward by around a degree, which is a big deal in global-warming terms. Various other previously unknown cooling or negative-feedback effects have been identified in recent research - for instance, melting ice sheets are said to create a massive carbon sink and the mysterious "Criegee intermediates" are also thought to be a powerful planet coolant.

Paasonen and his colleagues' new paper is published in the journal Nature Geoscience. ®