Warmer springs are leading to substantially reduced plant productivity across the northern hemisphere in the later months of the year, a new study has revealed.
The results call into question the validity of climate models that include plant productivity when assessing the amount of carbon captured by vegetation and what remains in the atmosphere.
Using 30 years of satellite images, an international team of scientists examined 41 million square kilometres of land in northern regions.
They found the early onset in plant productivity caused by warmer springs did not continue into the summer and autumn months.
Previously it was believed the earlier start to the growing season due to increasing global temperatures extended the growing season for vegetation. Now the team has found the adverse effects caused by a warmer spring, particularly those linked to depleted water supply, substantially reduced any benefit from longer warm seasons.
In many areas plant biomass decreased in the summer and autumn months, significantly limiting carbon capture.
Professor Pierre Friedlingstein said: “Satellite images are providing unique information on the dynamic of terrestrial biosphere.
“Here they show that warming in spring enhances plant productivity in many regions of the northern hemisphere, something most land carbon models achieve to simulate.
“However, they also show that many regions show the opposite later in the season, for reasons that are not fully clear yet, and poorly represented by those models.”
Satellite images across the entire globe north of the 30th parallel were studied from southern Europe and Japan to the tundra regions in the far north. It allowed the team to determine how much photosynthesis takes place and how much biomass is gained.
The team assessed the correlations between temperature, time of year and extent of greenness across the northern landscape, including areas in the UK, Canada, Germany, France and Russia.
The satellite observations showed the northern hemisphere becoming greener in the spring, but between 13 per cent and 16 per cent of the total land area showed adverse effects in later months.
Negative effects of the warmer springs were particular localised in western North America, Siberia and parts of eastern Asia. The study suggests depleted water resources associated with a warmer spring season could be a cause of the reductions.