The early arrival of spring is often cause for
celebration in northern climates. But it may come at the cost of drier, hotter
days in some areas in summer.

As winter wanes and leaves start to peek out from branches, trees increasingly draw water from the soil and move it
into the sky — a process known as evapotranspiration. But when this greening
starts earlier in the calendar year, scientists worry that more moisture could
be sucked from the soil than if the season starts later.  

Now, analyses of satellite data and climate
simulations show that earlier spring greening can leave
soils drier in summer
across much of the Northern Hemisphere. That, in turn, could
lead to more frequent and intense summer heat waves, researchers report January
3 in Science Advances.

As the climate warms, scientists expect to see earlier
and longer growing seasons.
One study found that, already, the growing season in the Northern Hemisphere
has been extended by about
10 days
on average over the last three decades.

“More green on the
ground causes evapotranspiration to go up,” says Chris Huntingford, a climate
modeler at the U.K. Centre for Ecology and Hydrology in Wallingford, England. But
it wasn’t clear if a local increase in water being pumped into the atmosphere
due to evapotranspiration would be offset by rain falling back to Earth, or
whether certain geographic areas might be more affected than others.

So Huntingford, climate
researcher Xu Lian at Peking University in Beijing and their colleagues
analyzed satellite data of vegetation cover and soil dryness across the
Northern Hemisphere from 1982–2011. Across much, but not all, of the top half of the globe, they
found that earlier spring greening was associated with soils being drier in
summer than in years when spring arrived later.

The association did
not hold in places dominated by croplands, such as parts of central Europe and
the Great Plains in the United States that are under intense irrigation, the
researchers say. Weather also may swamp these effects of an earlier spring in
certain regions. Siberia, for instance, experienced many early springs without
summer dryness, perhaps because weather patterns there consistently bring in excess
moisture from Europe.

Soil drying can have myriad
consequences, including raising local air temperatures near the Earth’s surface
and triggering heat waves or making them worse (SN: 3/8/16). The researchers estimate,
based on climate simulations, that soil dryness due to earlier spring greening
could increase the number of extremely hot summer days for a region by nearly one
day per decade, and could raise maximum temperatures there by 0.07 degrees Celsius
per decade.

“That doesn’t seem
like a lot, but in four or five decades, heat waves could be so strong that
minor increases like this could matter,” says Sebastian Sippel, a climate
scientist at ETH Zurich who was not involved in the study. The new study shows
that “for almost the entire Northern Hemisphere,
earlier springtime greening can significantly alter summer water content” of
soils, he says.

However, the study
can’t weigh the relative impact of springtime greening on soil dryness compared
with other factors, like an especially hot or dry season, Sippel says.

The researchers plan
to repeat the analysis for the Southern Hemisphere, to potentially determine
whether earlier greening and soil dryness there might be increasing the risk of
heat-associated disasters, such as wildfires.