By Rod Minchin, Press Association
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 current 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 that the early onset in plant productivity caused by warmer springs does 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.
The study shows that the current climate models underestimate the reduction in plant productivity – and therefore overestimates the amount of carbon being absorbed by terrestrial ecosystems throughout the year.
Professor Pierre Friedlingstein, from the University of Exeter, 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.”
For the research, 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 point by point how much photosynthesis takes place and how much biomass is gained.
With satellite images the team was able to survey the northern regions for greenness associated with healthy productive vegetation.
They 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% and 16% of the total land area showed adverse effects in later months.
This is contrast to current carbon cycle models which show adverse effects between 1% to 14%.
Negative effects of the warmer springs were particular localised in western North America, Siberia and parts of eastern Asia.
The study suggests that depleted water resources associated with a warmer spring season could be a significant cause of the reductions.
Abundant plant growth leads to increased water demand and evaporation which prevents plants from having enough water later in the year to maintain productivity.
Certain plants may also have a naturally predetermined growth period, which cannot be prolonged and an earlier growth spurt results in an earlier decay.
The paper, Widespread Seasonal Compensation Effects Of Spring Warming On Northern Plant Productivity, is published in the journal Nature.