When Steve Running co-authored a 2003 paper that found warmer temperatures had led to increased plant growth over the preceding two decades, he expected that terrestrial net plant productivity – "which is really just plant growth" – would continue to rise with temperatures.
The warmer temperatures did continue, making 2000 to 2009 the warmest decade on record, but plant growth actually declined.
This is the finding of a new study out in this week's issue of the journal Science, co-authored by Running, the director of Numerical Terradynamic Simulation Group at the University of Montana, and his colleague Maosheng Zhao.
The study picks up where the 2003 one left off. Put together, they form a cohesive picture of climate change's effects on terrestrial plant life. The story is fairly straightforward: for 20 years the amount of carbon-storing plant matter on the Earth's land surface had continued to increase as warmer temperatures led to a longer growing season, but somewhere around the start of the last decade, it began to decline.
Drought appears to be one of the main culprits.
"Global plant growth is now overall declining and this is because, while some areas are still benefiting from an increased growing season, other areas are starting to be retarded by drought and water deficits," Running told SolveClimate News in a phone interview.
Net primary productivity, or NPP, is an indicator based on satellite images that is used to track vegetation cover. It refers to the amount of carbon from the air or water that plants "fix" and turn into compounds used by other organisms. As carbon dioxide levels have increased, so has NPP, but only to a point.
Plants and carbon: a give-and-take relationship
Other recent studies have pointed to similar correlations between an increase in carbon in the atmosphere eventually leading to a decreasing ability of plants to store and use all that carbon. A paper in the peer-reviewed online journal PLoS ONE in July challenged the idea of "carbon fertilization" whereby more atmospheric carbon yields more plant growth, including greater agricultural yields.
In that study, researchers from the University of Guelph said that trees in temperate and boreal forests have been both storing less carbon dioxide than expected and growing more slowly over the past century. Like Running and Zhao, they found this was largely related to stress caused by the need to use water more efficiently.
While some carbon fertilization effect exists, "the effect is really pretty small compared to climate variables, so it is not the main driver that is controlling the rate of plant growth," said Running.
Those climate variables include higher temperatures and water-related stress – like large-scale droughts – which cause more plant respiration, which in turn actually returns carbon to the atmosphere rather than allowing it to be absorbed by the vegetation.
Not only is decreased NPP a symptom of climate change, it might also make warming worse.
"Right now the terrestrial biosphere absorbs about a quarter of the fossil fuel emissions produced by mankind," explained Running, "so that's CO2 that stays out of the atmosphere because of the biosphere. If the biosphere starts absorbing less of that CO2 that means that atmospheric CO2 rises even faster."
His paper's findings also concur with the PLuS ONE's contention that plant growth is being affected differently in different regions. The Science study finds the warmer temperatures of the past decade have led to greater plant growth at higher latitudes and elevations and, significantly, in the rainforests. But a "drying trend" in the Southern Hemisphere has decreased growth there, thus negating the increase in Northern Hemisphere regions and making for a negative global rate of plant growth.
To determine the effect of climate variability on NPP in recent years, Running and Zhao analyzed 10 years of data acquired by NASA's Terra satellite alongside data on the global climate.
Finite potential for plant growth
The main question now is whether this slower plant growth is just a cyclical phase or a trend that we should expect to continue.
"The easy answer is to say we don't know and we better keep monitoring it," said Running. "The somewhat more detailed answer is we know on an overall basis that the global climate is warming and what we're seeing is that the warming is now starting to impede plant growth unless additional rainfall comes with the warmer temperatures,"
"In some parts of the world [additional rainfall] is occurring and in some parts it's not, so our best estimate right now is that a trend like this is likely to continue as an overall global average with a lot of variation from one region to the next," he said.
If the trend does continue, it is likely to have serious implications for food security. This is because the planet has what Running calls a "finite plant growth potential." Decreasing NPP would set a stricter bound on that potential, meaning society will have to make hard decisions on how to use a given potential for plant growth.
Adding biofuels into the equation only further complicates that debate.
"We don't imagine that global food production will immediately fall short – I wouldn't want to claim that there's going to be widespread starvation anytime soon – but what this does do is suggest a trend that if it plays out over decades as the population continues to grow we may be at a point where food production becomes tighter," Running said.
(Photo: Walter J. Pilsak, Waldsassen)