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 springs 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.

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“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.