On a hillside overlooking cabbage fields outside the northern Thai city of Chiang Mai, a drone’s rotors begin to whir, lifting it over a patch of forest.

It moves back and forth atop the rich canopy, transmitting photos to be knitted into a 3D model that reveals the woodland’s health and helps estimate how much carbon it can absorb.

Drones are part of an increasingly sophisticated arsenal used by scientists to understand forests and their role in the battle against climate change.

The basic premise is simple: woodlands suck in and store carbon dioxide, the greenhouse gas that is the largest contributor to climate change.

But how much they absorb is a complicated question.

A forest’s size is a key part of the answer – and deforestation has caused tree cover to fall 12 percent globally since 2000, according to Global Forest Watch.

But composition is also important: different species sequester carbon differently, and trees’ age and size matter, too.

Knowing how much carbon forests store is crucial to understanding how quickly the world needs to cut emissions, and most current estimates mix high-level imagery from satellites with small, labour-intensive ground surveys.

“Normally, we would go into this forest, we would put in the pole, we would have our piece of string, 5 metres [16.4 feet] long. We would walk around in a circle, we would measure all the trees in a circle,” explained Stephen Elliott, research director at Chiang Mai University’s Forest Restoration Research Unit (FORRU).

“[But] if you’ve got 20 students stomping around with tape measures and poles … you’re going to trash the understory,” he said, referring to the layer of vegetation between the forest floor and the canopy.

That is where the drone comes in, he said, gesturing to the Phantom model hovering overhead.

“With this, you don’t set foot in the forest.”