As winter wanes and spring ushers warmth, trees get busy: putting leaves on branches and drawing more water from the soil. With global warming, spring is arriving earlier. This is likely to spell drier soils in summer.
Evapotranspiration is the transfer of water from the land and oceans via plant transpiration (water usage) and evaporation. Plants draw water from the soil and respire it from their leaves.
The growing season in the Northern Hemisphere is ~10 days longer than it was 30 years ago. “Earlier vegetation greening under climate change raises evapotranspiration and thus lowers spring soil moisture,” reports Chinese climate scientist Xu Lian.
With plants active earlier, soils are drier in the summer than they have been in decades past. “For almost the entire Northern Hemisphere, earlier springtime greening can significantly alter summer water content” says Swiss climate scientist Sebastian Sippel.
This summer dryness can raise local temperatures and trigger or intensify heat waves – which in turn can kill vegetation, which negatively affects the soil and furthers warming.
Xu Lian et al, “Summer soil drying exacerbated by earlier spring greening of northern vegetation,” Science Advances (3 January 2020).
Jonathan Lambert, “Climate change is bringing earlier springs, which may trigger drier summers,” Science News (3 January 2020).