Plants, clouds and climate

Plant–cloud interactions have the potential to both cool and warm the climate. Ascertaining how these processes balance out at the global scale will require close collaboration between climate scientists and plant biologists.

When studying the Earth's climate, biology, chemistry and physics cannot be separated. The interaction between terrestrial plants and the surrounding atmosphere is a case in point. Transpiration from land vegetation dominates terrestrial water fluxes (Nature 496, 347–350; 2013) and land ecosystems take up and sequester large amounts of atmospheric carbon dioxide (Proc. Natl Acad. Sci. USA 104, 18866–18870; 2007). But land plants also exert more subtle effects on the climate: they release large quantities of organic vapour into the atmosphere that, upon oxidation, yields aerosol particles that affect cloud properties (Nature Geosci. 5, 453–458; 2012).

Two studies in this issue suggest that the climatic significance of these biogenic emissions may be greater than thought. A Letter on page 443 shows that the number of cloud droplets is not only affected by aerosols in particulate form, but also by semi-volatile organic compounds. These vaporous compounds contribute to droplet growth directly by condensing on growing particles and facilitating further water uptake. The process could have significant consequences for the reflectivity of clouds.

Regardless of the pathway from leaf to cloud, organic vapours seem to lead to more droplets and brighter clouds — a process that could intensify on warming, according to another Letter on page 438. Here, it is shown that the number of particles in the lower atmosphere large enough to serve as seeds for cloud droplets rises exponentially with temperature, which the authors attribute to growing vapour emissions from plants in warmer conditions. If this plant-driven increase in the atmospheric aerosol burden translates into an increase in the number of cloud droplets as expected, a negative feedback ensues between the terrestrial biosphere and climate warming.

However, it has also been suggested that a reduction in the plant-mediated transfer of water to the atmosphere under higher atmospheric carbon dioxide concentrations could lead to a reduction in water vapour levels in the lower atmosphere (Nature Geosci. 5, 701–704; 2012). The resultant suppression of cloud formation could enhance warming.

How these two parallel but mutually counteracting feedback loops balance out at a global scale will be difficult to quantify, given the regional nature of many of the processes involved. There is clearly much to learn about how plants affect clouds — mediated by water vapour, organic vapours and aerosol particles — in a changing climate.