SOURCE: Inside Climate Change

DATE: April 25, 2020

SNIP: Tim Brodribb has been measuring all the different ways global warming kills trees for the past 20 years. With a microphone, he says, you can hear them take their last labored breaths. During blistering heat waves and droughts, air bubbles invade their delicate, watery veins, cracking them open with an audible pop. And special cameras can film the moment their drying leaves split open in a lightning bolt pattern, disrupting photosynthesis.

“We really need to be able to hear these poor trees scream. These are living things that are suffering. We need to listen to them,” said Brodribb, a plant physiologist at the University of Tasmania who led a recent study that helps identify exactly when, where and how trees succumb to heat and dryness.

The study, published April 17 in the journal Science, reviewed the last 10 years of research on tree mortality, concluding that forests are in big trouble if global warming continues at the present pace. Most trees alive today won’t be able to survive in the climate expected in 40 years, Brodribb said. The negative impacts of warming and drying are already outpacing the fertilization benefits of increased carbon dioxide.

Trees and forests can be compared with corals and reefs, he said. Both are slow-growing and long-lived systems that can’t easily move or adapt in a short time to rapid warming and both have relatively inflexible damage thresholds. For corals, a global tipping point was reached from 2014 to 2016. In record-warm oceans, reefs around the world bleached and died.

The new paper shows that the hope that rising carbon dioxide would green the planet is probably misplaced. Studies have shown that increased carbon dioxide in the atmosphere boosts photosynthesis, spurring plant growth by chemically combining the carbon with water and ground nutrients.

But there will “probably be more browning than greening,” said University of Arizona forest scientist Dave Breshears, who was not involved in the new research.

“The review ends on a hard note, with high confidence that we’re going to have a lot of impacts with hotter droughts in the future,” he said. Mass forest die-offs will proliferate and expand. The trend toward more extreme heat waves and droughts is lethal for forests. But despite the grim outlook, it’s important not to paint an entirely desperate picture, he said.

At the current pace of warming, much of the world will be inhospitable to forests as we know them within decades. The extinction of some tree species by direct or indirect action of drought and high temperatures is certain. And some recent research suggests that, in 40 years, none of the trees alive today will be able to survive the projected climate.

The stakes are high, since trees are the foundation for terrestrial biodiversity and because they capture and store about one-third of human-caused CO2 emissions within their dense wood frames. A global loss of forests could lead to a surge in heat-trapping carbon dioxide, causing more warming, and would also eliminate habitat for countless other animals, plants and fungi, with a rippling effect that reaches humans.

The new paper reinforces the observational evidence that global warming has pushed many of the world’s forests to a knife edge, said University of Utah forest researcher Bill Anderegg. In the West, you can’t drive on a mountain highway without seeing how global warming affects forests, from wildfires to die-offs caused by beetles or other pathogens, he said.

“The risks of climate change to forests are substantial and going up faster than we thought,” Anderegg said.