Forests face climate change tug of war

Forests face climate change tug of war

In a world of rising levels of atmospheric carbon dioxide, plants should be happy, right? Experiments have shown that, yes, increased carbon dioxide does allow plants to photosynthesize more and use less water.

But the other side of the coin is that warmer temperatures drive plants to use more water and photosynthesize less. So, which force, CO2 fertilization or heat stress, wins this climate tug of war?

The answer, University of Utah researchers write in a new study in Proceedings of the National Academy of Sciences, is that it depends on whether forests and trees are able to adapt to their new environment. The study, they say, incorporates aspects of a tree's physiology to explore how trees and forests respond to a changing climate.

"It's taking the physiology of individual cells and scaling it up in a computer to make projections of a continents' worth of forests," says study co-author William Anderegg.

Stemming water loss

To set the stage for this tug of war, it's important to understand how trees and plants use water.

In a tree, water is pulled up from the roots through the xylem, the tree's vascular system. The water moves to the leaves, where photosynthesis happens. On the underside of leaves, small pores called stomata open to admit CO2 for photosynthesis. Water vapor can escape through the stomata, though, so closing stomata is required to guard against water loss during dry or hot times.

During an intense drought, trees have to work harder to pull water into the tree and through the xylem. If the soil is dry enough, the tension on the water causes a bubble of air to form in the xylem, effectively reducing water transport and injuring or killing the tree. It's akin to a heart attack.

A physiological model

John Sperry of the U's School of Biological Sciences spent decades studying the physiology of tree water use, and in recent years has been joined by Anderegg and postdoctoral researcher Martin Venturas, along with other colleagues. Together, they've developed a model of how trees' physiological traits, primarily the regulation of stomatal opening, influences photosynthesis and water loss in response to a changing environment, including drought.

This model, Sperry says, has now enabled a new way of predicting the outcome of the climate tug of war, quantifying the competing effects of CO2 fertilization and heat stress to find the balance point.

But it's also enabled another advance in understanding: Anderegg says that the model allows them to simulate the ability of trees to acclimate to heat and drought -- both at short time scales, by closing or opening stomata, or at long time scales, by extra tree growth or forest dieback. "We're assuming the plants are adapted to be somewhat smart about responding to the climate and the environment," Anderegg says.

Some acclimation was seen in previous experiments where trees were bathed in CO2-enriched air, Venturas adds, and is also seen in forests that are similar to each other but are located in slightly different climates.

"Our present-day models don't do physiology or acclimation," Anderegg says. "They matter absolutely enormously to the future of forests. We came up with ways to incorporate those."

It's all about the ratio

The model results, Sperry says, suggest that the winner of the tug of war doesn't depend on the absolute amount of CO2rise or warming -- just the ratio between the two.

"So you can have the same forest moving across big gradients in climate change if that ratio is at the neutral point," Sperry says. "But anything that pushes that ratio to the warming side is going to have potential for serious negative impact."

If forests aren't able to acclimate, the researchers write, then the ratio must be above 89 parts per million CO2 per degree C of warming to avoid significant stress and tree die-off. Only 55% of climate forecasts show this scenario occurring. But if forests are able to acclimate, then they can tolerate a lower ratio: 67 parts per million CO2 per degree of warming, which occurs in 71% of forecasts.

Other tipping factors

But even with acclimation, other factors can tip the balance toward forest catastrophe. The model doesn't take into account forest fires or insect infestation, Venturas says, only the physiology of the trees -- although stressed forests are more susceptible to both fires and insects.

"It's improving one piece of the puzzle, but we still need to learn a lot about the other pieces and how they're integrated," he says.

The researchers also write that exceptionally dry years can also tip the balance. "In those cases, if we drop below a soil moisture threshold, we could have the whole forest die," Venturas says. The mortality can happen relatively suddenly. "You see this in your flower pot at home if you forget to water," Sperry says. "It'll look fine up to a certain point but then you hit that moisture threshold and in a matter of days the plant can die. If you don't get rain in that period, the system goes into a cycle where the soil's drying out too fast and sends the trees into vascular failure."

Sperry adds that the study predicts a precarious tightrope of climate conditions for future forests to navigate. "The study by no means gives a green light to the status quo."

Stalled weather patterns will get bigger due to climate change

Street flooding

Climate change will increase the size of stalled high-pressure weather systems called "blocking events" that have already produced some of the 21st century's deadliest heat waves, according to a Rice University study.

Atmospheric blocking events are middle-latitude, high-pressure systems that stay in place for days or even weeks. Depending upon when and where they develop, blocking events can cause droughts or downpours and heat waves or cold spells. Blocking events caused deadly heat waves in France in 2003 and in Russia in 2010.

Using data from two sets of comprehensive climate model simulations, Rice fluid dynamicists Ebrahim Nabizadeh and Pedram Hassanzadeh, and colleagues found that the area of blocking events in the northern hemisphere will increase by as much as 17% due to anthropogenic climate change. The study, which is available online from Geophysical Research Letters, was co-authored by Da Yang of Lawrence Berkeley National Laboratory and the University of California, Davis, and Elizabeth Barnes of Colorado State University.

Hassanzadeh, an assistant professor of mechanical engineering and of Earth, environmental and planetary sciences, uses computational, mathematical and statistical models to study atmospheric flows related to a broad range of problems from extreme weather events to wind energy. He said researchers have increasingly been interested in learning how climate change might affect blocking events, but most studies have focused on whether blocking events will become more frequent as the atmosphere warms because of greenhouse gas emissions.

"Studies in the past have looked at whether you get more or less blocking events with climate change," he said. "The question nobody had asked is whether the size of these events will change or not. And the size is very important because the blocking events are more impactful when they are larger. For example, if the high-pressure system becomes bigger, you are going to get bigger heat waves that affect more people, and you are likely going to get stronger heat waves."

Nabizadeh, a mechanical engineering graduate student in Rice's Brown School of Engineering, set out to answer the question two years ago. Using a hierarchical modeling approach, he began with experiments on a model of atmospheric turbulence that's far simpler than the real atmosphere.

The simple model, which captures the fundamental dynamics of blocking events, allowed Nabizadeh to do a great deal of exploration. Making slight changes in one parameter or another, he ran thousands of simulations. Then the data was analyzed using a powerful dimensional analysis technique called the Buckingham-Pi theorem, which is often used in designing large and complex engineering systems that involve fluid flows.

The goal was finding a scaling law, a mathematical formula that described the size of a blocking event using variables that climate scientists already study and understand. Nabizadeh started with scaling laws that have been developed to predict the size of day-to-day weather patterns, but he found that none of the variables were predictive for blocking events.

His persistence eventually paid off with a simple formula that relates the area of blocking events to the width, latitude and strength of the jet stream, all of which are well-studied and measured.

"I gave a talk about this recently, and one of the people came up after and said, 'This is magical, that these powers add up and suddenly you get the right answer.' But it took a lot of work by Ebrahim to get this elegantly simple result," he said.

At a one point, Nabizadeh had analyzed the data from many simulations and produced a comparison that included page upon page of figures, and Hassanzadeh said the scaling law discovery was encouraged by an unlikely agency: the Texas Department of Motor Vehicles (DMV).

"Ebrahim went to the DMV one weekend, and I went to the DMV the week after, and at the DMV you have to sit and you don't have anything to do," he said. "So after staring at these numbers for hours, we realized this is the right scaling."

They also compared the simple-model results with the output of increasingly complex models of the Earth's weather and climate. Nabizadeh said the scaling law predicted changes in the size of future winter blocking events in comprehensive climate model simulations with remarkable accuracy.

"It performs better for winter events than summer events for reasons we don't yet understand," Nabizadeh said. "Our results suggest future studies should focus on better understanding summer blocks and also how larger blocking events might affect the size, magnitude and persistence of extreme-weather events like heat waves."

The research was supported by NASA (80NSSC17K0266), the National Academies' Gulf Research Program, the Department of Energy (DE-AC02-05CH11231) and the National Science Foundation (NSF) (AGS-1545675). Computing resources were provided by the NSF-supported XSEDE project (ATM170020) and Rice's Center for Research Computing in partnership with Rice's Ken Kennedy Institute for Information Technology.

---

Story Source:

Materials provided by Rice University. Original written by Jade Boyd. Note: Content may be edited for style and length.

Stalled weather patterns will get bigger due to climate change

Street flooding

Climate change will increase the size of stalled high-pressure weather systems called "blocking events" that have already produced some of the 21st century's deadliest heat waves, according to a Rice University study.

Atmospheric blocking events are middle-latitude, high-pressure systems that stay in place for days or even weeks. Depending upon when and where they develop, blocking events can cause droughts or downpours and heat waves or cold spells. Blocking events caused deadly heat waves in France in 2003 and in Russia in 2010.

Using data from two sets of comprehensive climate model simulations, Rice fluid dynamicists Ebrahim Nabizadeh and Pedram Hassanzadeh, and colleagues found that the area of blocking events in the northern hemisphere will increase by as much as 17% due to anthropogenic climate change. The study, which is available online from Geophysical Research Letters, was co-authored by Da Yang of Lawrence Berkeley National Laboratory and the University of California, Davis, and Elizabeth Barnes of Colorado State University.

Hassanzadeh, an assistant professor of mechanical engineering and of Earth, environmental and planetary sciences, uses computational, mathematical and statistical models to study atmospheric flows related to a broad range of problems from extreme weather events to wind energy. He said researchers have increasingly been interested in learning how climate change might affect blocking events, but most studies have focused on whether blocking events will become more frequent as the atmosphere warms because of greenhouse gas emissions.

"Studies in the past have looked at whether you get more or less blocking events with climate change," he said. "The question nobody had asked is whether the size of these events will change or not. And the size is very important because the blocking events are more impactful when they are larger. For example, if the high-pressure system becomes bigger, you are going to get bigger heat waves that affect more people, and you are likely going to get stronger heat waves."

Nabizadeh, a mechanical engineering graduate student in Rice's Brown School of Engineering, set out to answer the question two years ago. Using a hierarchical modeling approach, he began with experiments on a model of atmospheric turbulence that's far simpler than the real atmosphere.

The simple model, which captures the fundamental dynamics of blocking events, allowed Nabizadeh to do a great deal of exploration. Making slight changes in one parameter or another, he ran thousands of simulations. Then the data was analyzed using a powerful dimensional analysis technique called the Buckingham-Pi theorem, which is often used in designing large and complex engineering systems that involve fluid flows.

The goal was finding a scaling law, a mathematical formula that described the size of a blocking event using variables that climate scientists already study and understand. Nabizadeh started with scaling laws that have been developed to predict the size of day-to-day weather patterns, but he found that none of the variables were predictive for blocking events.

His persistence eventually paid off with a simple formula that relates the area of blocking events to the width, latitude and strength of the jet stream, all of which are well-studied and measured.

"I gave a talk about this recently, and one of the people came up after and said, 'This is magical, that these powers add up and suddenly you get the right answer.' But it took a lot of work by Ebrahim to get this elegantly simple result," he said.

At a one point, Nabizadeh had analyzed the data from many simulations and produced a comparison that included page upon page of figures, and Hassanzadeh said the scaling law discovery was encouraged by an unlikely agency: the Texas Department of Motor Vehicles (DMV).

"Ebrahim went to the DMV one weekend, and I went to the DMV the week after, and at the DMV you have to sit and you don't have anything to do," he said. "So after staring at these numbers for hours, we realized this is the right scaling."

They also compared the simple-model results with the output of increasingly complex models of the Earth's weather and climate. Nabizadeh said the scaling law predicted changes in the size of future winter blocking events in comprehensive climate model simulations with remarkable accuracy.

"It performs better for winter events than summer events for reasons we don't yet understand," Nabizadeh said. "Our results suggest future studies should focus on better understanding summer blocks and also how larger blocking events might affect the size, magnitude and persistence of extreme-weather events like heat waves."

The research was supported by NASA (80NSSC17K0266), the National Academies' Gulf Research Program, the Department of Energy (DE-AC02-05CH11231) and the National Science Foundation (NSF) (AGS-1545675). Computing resources were provided by the NSF-supported XSEDE project (ATM170020) and Rice's Center for Research Computing in partnership with Rice's Ken Kennedy Institute for Information Technology.

Change overtime.