Global atmospheric carbon dioxide levels are higher than at any point in the past 800,000 years, according to the National Oceanic and Atmospheric Association. Through the process of photosynthesis, plants play a critical role in the carbon cycle, which includes reducing excess CO2 in the atmosphere caused by human emissions—a process known as the CO2 fertilization effect.
“Plants convert CO2 into biomass through photosynthesis, and with increasing CO2 in the atmosphere, plants in theory should be able to grow better,” explains William Kolby Smith, an assistant professor in the School of Natural Resources and the Environment with expertise in ecosystem ecology, remote sensing, and high-performance computing.
“For the most part, this theory has held up—today plants are taking up roughly a quarter of the total amount of the CO2 that we're putting into the atmosphere every year.”
However, whether this effect will persist into the future remains unclear and largely unexplored.
Smith teamed up with researchers from across the U.S., China, Spain, Belgium, France, and the United Kingdom to examine satellite observations from the past three decades in order to better understand how CO2 fertilization affects vegetation photosynthesis overtime. The study was recently published in the journal Science and found the fertilizing effect of CO2 has been declining on a global scale since the 1980s.
“Essentially, what it looks like is plants are becoming less efficient at pulling excess CO2 out of the atmosphere. This is supported by the satellite data as well as ground observation data collected through a network of flux towers measuring the rapid exchange of carbon and water across sites globally,” Smith said.
“Another factor we have to consider is as you put CO2 into the atmosphere, it increases temperatures as a key greenhouse gas,” Smith said. “With increased temperatures, the atmosphere gets more arid and the demand for water from the plants intensifies. This combination of higher temperatures and drier atmospheric conditions results in an enhanced water stress constraint on vegetation growth.”
In support of this view, the authors found that plant nutrient concentrations in leaves have been declining since the 1990s in parallel to the CO2 fertilization effect. The team also found increasing sensitivities of photosynthesis to water supply in parallel to the declining trends in the CO2 fertilization effect.
“This has big implications because current policy decisions are based on models that don't incorporate or accurately represent these nutrient and water constraints,” Smith said. “Past models are thus potentially over optimistic in terms of what we need to do in the future to limit climate warming.”