Browning of the Earth: Land Plant Growth Decline Since 1998: Part 1 of 2 // Aug 23, 2019
Earth stopped getting greener 20 years ago. A new research study used satellite images to determine that plant growth on land increased in the 1980s and 1990s, but reached a turning point in 1998, and has since been decreasing.
The decrease is mostly attributed to decreasing moisture in the air, as measured by a Vapour Pressure Deficit (VPD) parameter; which is the difference between the actual amount of moisture in the air versus the maximum amount of moisture possible in the air (saturation) at the given temperature.
Ref 1: ‘Earth Stopped Getting Greener 20 Years Ago’, Scientific American, By Chelsea Harvey, E&E News on August 15, 2019. Declining plant growth is linked to decreasing air moisture tied to global warming:
‘The world is gradually becoming less green, scientists have found. Plant growth is declining all over the planet, and new research links the phenomenon to decreasing moisture in the air—a consequence of climate change. * The study published yesterday in Science Advances points to satellite observations that revealed expanding vegetation worldwide during much of the 1980s and 1990s. But then, about 20 years ago, the trend stopped. *
‘Since then, more than half of the world’s vegetated landscapes have been experiencing a “browning” trend, or decrease in plant growth, according to the authors. * Climate records suggest the declines are associated with a metric known as vapor pressure deficit—that’s the difference between the amount of moisture the air actually holds versus the maximum amount of moisture it could be holding. A high deficit is sometimes referred to as an atmospheric drought‘.
Ref 3: ‘Increased atmospheric vapor pressure deficit reduces global vegetation growth‘, Science Advances 14 Aug 2019:
‘Atmospheric vapor pressure deficit is a critical variable in determining plant photosynthesis. Synthesis of four global climate datasets reveals a sharp increase of VPD after the late 1990s. In response, the vegetation greening trend indicated by a satellite-derived vegetation index, which was evident before the late 1990s, was subsequently stalled or reversed… Six Earth system models have consistently projected continuous increases of VPD throughout the current century‘.
Atmospheric Drying Reducing Terrestrial Vegetation Growth Since 1998: Part 2 of 2 // Aug 23, 2019
It is well known that global vegetation decline is worsening from:
- land-use forest clearing,
- soil degradation …
But some regions like the Arctic are greening. We also know that the maximum amount of moisture air can hold at saturation goes up by 7% per degree C temperature rise.
This temperature rise increases atmospheric water vapour content via ocean, lakes, rivers, and soil water evaporation and evapotranspiration from plants.
If this increase is under 7%, a Vapour Pressure Deficit occurs, plant stomata shrink, and vegetation growth slows reducing global primary productivity.
Ref: ‘Global water cycle amplifying at less than the Clausius-Clapeyron rate‘, Nature-Published: 09 December 2016, ‘Understanding and quantifying observed global water cycle change is key to predicting future climate. The Clausius-Clapeyron relationship predicts an increase in the water holding capacity of air (the saturation water vapor pressure) of approximately 7% per degree Celsius rise in temperature2. It has been suggested that this would lead to a strengthening of the global evaporation (E) minus precipitation (P) pattern with global surface warming…‘
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