A new study suggests that trees’ ability to soak up carbon could expire. Mirjam Guesgen explains.

Trees have long been held as the saviour for climate change. Plant enough trees and we might be able to balance out some of that carbon-emmitting flying or driving. But a new scientific study says that trees only buy us a certain amount of time. Push a tree too far and it’ll turn on you.

How do trees fight climate change?

The reason trees make such excellent climate fighting machines has to do with chemistry. They suck up carbon dioxide from the atmosphere and use it as one of the ingredients to make sugar, their form of fuel. That’s the basics of photosynthesis.

Trees and other land plants are kind of saving our bacon right now, absorbing around a third of human-made carbon emissions.

So what’s the problem?

A paper in the journal Science Advances says that’s likely to change in the next few decades. Instead of land plants helping fight climate change, they may start contributing to it. Instead of soaking up carbon dioxide, they’ll start spitting it out.

How?

Again, chemistry. As well as absorbing carbon dioxide through photosynthesis, plants and bugs in the soil naturally release it too. When the plant or bugs-that-break-down-plant-stuff use up that sugar, the reaction creates some carbon dioxide again. That’s respiration.

So plants’ ability to be carbon fighters depends on the balance of those two processes: photosynthesis and respiration. In a nutshell, more photosynthesis = more carbon absorbing. More respiration = more carbon releasing.

Right now, most land plants are doing more photosynthesising more of the time. In the future, that will change, this paper says.

What’s the difference between now and the future?

One word: temperature. Warmer global temperatures come with a whole heap of repercussions but a warmer climate also affects how fast photosynthesis and respiration happen.

Both processes depend on enzymes. As temperatures rise, those enzymes work better and make the photosynthesis and respiration happen faster. Up to a point though. Eventually those hot conditions start to mess with the enzymes, making them too flexible and wobbly. The enzymes don’t work as well anymore and the reactions slow down again.

The trick is photosynthesis and respiration don’t speed up at the same rate as temperatures rise. That’s where you start to get this imbalance between the two and the trees turn on us. 

When will the trees turn on us?

The Science Advances paper applied a theory for understanding plant chemistry (macromolecular rate theory) to real-life temperature and carbon-exchange data from all over the world to estimate at what point photosynthesis and respiration would top out.

Photosynthesis rates maxed out at 18 degrees celsius for trees, rice and wheat, and 28 degrees for plants like sugarcane and corn. Respiration rates on the other hand just keep increasing and the scientists estimated it wouldn’t max out until 60 or 70C. That tipping point for when there’s more respiration than photosynthesis was around 25C.

So based on a business-as-usual carbon emission scenario (and the rise in temperatures that come with it) by 2100 more than half of all land plant systems could be past that tipping point.

But, because some parts of the globe are better at sucking up carbon than others — and those will be the first to tip over — it could be even sooner than that. Within the next two decades, or 2040.

Any silver linings?

There’s still a couple of unknowns, according to one of the researchers involved in the study, plant biologist Vic Arcus. 

Oceans are also great carbon-absorbers, mopping up at least 40% of human-made emissions. “They’re also saving our bacon,” the University of Waikato scientist says. His team are working with researchers overseas to figure out how land and ocean systems work together, or not, to store carbon dioxide.

There have also been suggestions that more carbon dioxide in the atmosphere will fuel photosynthesis making trees better at taking up carbon. It makes sense theoretically, you get a bigger fire if you put more wood on it. But in reality that’s not what’s happening, according to Arcus, because temperatures have gone up at the same time and offset any possible positives. 

The biggest silver lining is that reigning in global temperature rise should halt the flip. Planting trees is still still good, says Arcus, but it only buys us a certain amount of time.