How Planting Trees Can Change Water Availability as Climate Warms

Planting trees has become one of the most widely supported responses to climate change.

Governments advocate it, businesses pay for it, and carbon-offset schemes frequently depend on it. The belief also extends beyond carbon capture.

More forest cover is commonly thought to make a landscape somewhat wetter. However, a new study indicates that the reality is more complicated.

The researchers discovered that an identical forest planted in the identical location may raise or reduce water availability, depending on the extent of climate warming.

A matter of warming levels

The research was conducted by a team headed by Tao Tang, a climate scientist at the Institute of Atmospheric Physics, which forms part of the Chinese Academy of Sciences (CAS).

Their aim was to examine an assumption underlying almost every reforestation commitment.

Instead of considering the number of trees to plant, Tang’s team modelled the same vast tree-planting programme using detailed climate simulations for two possible futures.

In the first, humanity limits emissions and warming remains moderate. In the second, emissions continue to rise and the planet becomes much hotter.

They measured water availability as rainfall in a location minus the water returned to the atmosphere through evaporation.

This represents the moisture remaining for people, crops and rivers. The outcomes in the two futures were directly opposed.

Forest planting in a cooler future

If warming is kept moderate, planting forests slightly increases the world’s overall water supply. At first glance, that appears to be an uncomplicated benefit. Yet the extra water is not distributed equally.

Humid areas receive most of the additional water, whereas land that is already dry gains very little. Wet places become wetter while dry places remain dry, reinforcing a divide identified by earlier research into global tree restoration.

The added trees draw water from the soil and release it into the atmosphere. Under moderate warming, enough of that moisture returns as rainfall to produce a small surplus, although the increase is limited. Wet regions take most of the benefit.

A hotter world

In a warmer future, the outcome is reversed. The very same forests reduce water availability rather than increasing it.

Trees need more water in hot conditions and lose more moisture than the additional rainfall can restore.

In a warmer atmosphere, leaves release water more rapidly, and a study of historical land-use change found that this process can outweigh any rise in rainfall. Higher temperatures shift the balance towards a loss.

There is one unusual advantage to these losses. With strong warming, the reduced water supply is spread more evenly around the world.

However, this scenario also assumes a far larger global population by 2100. There would be less water and many more people needing access to it.

What drives the divide

To identify where the water was going, the team tracked the movement of air itself. They calculated how moisture entered and left every region, comparing rainfall with the moisture carried away by winds.

This pointed to atmospheric circulation: the large-scale flow of air that determines where moisture accumulates and where it declines.

Across the simulations, these wind systems shifted moisture over wet regions in opposite directions at the two levels of warming. The forests did not change, but the winds did.

Why winds respond so differently as the planet warms is still unresolved.

The researchers are cautious on this issue, saying that the underlying explanation lies beyond what their analysis can currently establish. The pattern is robust, but its cause is uncertain.

What is new

Scientists had already established that forest planting can direct water towards wetter regions, and that trees requiring more water in a warming climate may take more than they return.

What had not been clearly established was just how fully the result is determined by the degree of warming.

When the same planting programme is modelled in a cooler future and a hotter one, its effect does not simply become weaker or stronger. It switches direction. The decisive factor is warming rather than the number of trees.

The same young tree, planted in the same location, might help replenish a reservoir in one future but reduce it in another. The only difference is the amount of carbon the world continues to burn.

Location and timing

For those designing a tree-planting commitment, tree numbers alone do not tell the full story. Junji Cao, one of the study’s authors, says the key lesson concerns location and timing as much as scale.

“Reforestation is not a one-size-fits-all solution,” said Cao. The same programme that increases water supplies in a low-emissions world could gradually drain them in a hotter future.

The study’s conclusion is notable. The climate in which a forest develops can turn its impact on water availability from a benefit into a loss, an idea also reflected in newer research on how location influences the result.

For communities downstream, this changes an apparently positive gesture into a planning decision with serious implications for fields and water supplies.

Future forests may therefore be selected not just for the carbon they store, but also for the water they hold back or release.