Global greening may be a double-edged sword
Research into the effects of increased plant cover is a reminder of the complexity of the systems that influence Earth’s climate. Tim Wallace reports.
Carbon dioxide is plant food. It’s a botanically self-evident truth that has been weaponised in the lingering debate about the dangers of human-driven climate change.
A new global assessment on the effect of changes in planetary vegetation over the past three decades, drawing on satellite data, shows that large parts of the world are greening due to the CO2 fertilisation effect, as well as human contributions of nitrogen and land-use changes.
Much is still unkown, however, about whether the fertilisation effect will offset other impediments to plant growth arising from rising temperatures; and what this will mean for global warming itself. Will it mitigate the impact of humans pumping carbon into the atmosphere?
The new research, published in Science, demonstrates that it is difficult to generalise about the effects. The analysis of satellite data on global leaf abundance led by Giovanni Forzieri of the European Union’s Directorate for Sustainable Resources indicates starkly divergent regional effects from increases in vegetation.
Using a metric known as the leaf area index, or LAI, Forzieri and his colleagues have calculated the effect of greater leaf abundance on temperature, through absorption of the Sun’s shortwave radiation and reflection of longwave radiation.
Their findings are that increased leaf area in warm regions (where the average mean annual air temperature is more than 16º C) is associated with cooling, mostly due to plant transpiration, evaporating water that subsequently condenses in the troposphere. By contrast, in cold-temperate and boreal regions (where average mean annual air temperature is less than 8º C) increased leaf area boosts warming, mostly due to making the surface reflect away less sunlight.
“Ultimately, for about 60% of the global vegetated area, greening has buffered the dominating warming signal, with a local mitigation effect of about 14%,” the study concludes. “For the remaining areas, mostly located in the boreal zone, LAI trends have amplified the raise in air temperatures, leading to an additional warming of about 10%.”
Pep Canadell, CSIRO Research Scientist and executive director of the Global Carbon Project, commends the Forzieri paper for shedding light on the importance of global greening. “The trend is among of the most compelling evidence of human influence at the planetary scale,” he says. “Yet we know little about its consequences for the function of natural ecosystems and ‘ecosystem services’ relevant to society.”
He notes that greening also affects biochemical processes such as the cycling of carbon and nitrogen, which need to be factored into the big picture. Nonetheless, the analysis shows how the planet is transforming “in ways that are multifaceted, complex and regionally specific”.
Bill Laurance, director of the Centre for Tropical Environmental and Sustainability Sciences at James Cook University, in Queensland, concurs, noting the importance of reflection and evaporative cooling. “It’s not just how much carbon dioxide and other greenhouse gases are in the atmosphere,” he says. “It’s the combined effect that determines how much global warming we’ll see now and in the future.”
On that point – whether the marginal overall cooling effect of global greening calculated for the past three decades will continue in the future – the Forzieri paper warns that changes in environmental conditions (such as further increased atmospheric CO2) could “alter the dominant mechanisms observed in today's climate”.
Importantly, the paper finds that LAI influences on temperature are amplified by a factor of five during extreme warm-dry and cold-wet years: in cold climates, the authors write, “rapid greening in combination with the positive sensitivity of surface temperature to LAI are contributing to the accelerated warming of ecosystems that are particularly vulnerable to climate change.”
The marginally positive results of the past three decades should not therefore be taken to mean that everything will be relatively okay in the future, says Cate Macinnis-Ng, of the School of Biological Sciences at the University of Auckland. “Extreme events will be more frequent and more severe in future climates. While we may be seeing a trend of increasing leaf area across the globe, widespread loss of forest due to severe weather events could quickly reduce or reverse global greening.”