Under a business-as-usual scenario of greenhouse gas emissions, the average global temperature may increase by almost 5°C through the end of the century. This climate change could cause a 1-meter increase in sea levels, possibly wreaking havoc on coastal regions and demanding hundreds of billions of dollars every year in adaptation and mitigation measures. As grim as this scenario may sound, it might be optimistic.
According to recent research, there are carbon cycle feedbacks not accounted for by current climate models. The reason is that forests, which can absorb about a third of greenhouse gas emissions, may be relatively short-lived carbon stocks in the future as trees live fast and die young.
Scientists are concerned because carbon uptake is a “critical ecosystem service that our forests are providing by effectively slowing the rate of climate change—and buying us time while we figure out policies to address it,” said Andrew Reinmann, an assistant professor of geography at the City University of New York.
Carbon dioxide (CO2) stimulates the growth of trees due to carbon uptake during their development. This process, which scientists call CO2 fertilization, can accelerate tree growth, with more carbon available in the atmosphere (especially under higher temperatures) causing trees to have shorter life spans. The trees die sooner because higher metabolism rates can cause them to age faster and invest less in defenses or a more efficient hydraulic architecture or simply cause them to reach their maximum size sooner in life. The entire process means that trees will store carbon for a shorter time, accelerating the carbon cycle and potentially increasing carbon concentrations in the atmosphere.
“This can have an important effect on forest carbon sinks in the future,” said Roel Brienen, a professor at the School of Geography at the University of Leeds in the United Kingdom.
Brienen led a study showing that the trade-off between tree longevity and growth rate is almost universal, extending from high latitudes to the tropics. An international team of researchers observed ring data sets on 110 tree species all over the world and noticed that on average, 50% of early growth increase meant a 23% life span reduction. “While this relation across species was already known, we found this difference also occurs within species,” Brienen said.
The finding is important, but the paper doesn’t account for the variation in tree reproduction and seedling production, observed Oswald Schmitz, a professor of population and community ecology at Yale University’s School of the Environment not involved in the study.
It’s possible that there could be a carbon balance, with enough sprouting seedlings to replace dead trees, said Schmitz—but the carbon cycle is rarely that simple. “Carbon cycle models don’t really account for such nuanced dynamics, especially if there’s regeneration failure—by continuing deforestation, for instance,” Schmitz explained.
Temperature might play an important role in the relationship between tree longevity and growth as well: According to another paper from the same group, published in the Proceedings of the National Academy of Sciences of the United States of America, tropical trees grow twice as fast as those in temperate and boreal regions but live half as long. The study analyzed tree ring data of more than 3,300 tree populations and 438 species across different biomes.
Lead author Giuliano Locosselli, a researcher at the Biosciences Institute at the University of São Paulo, Brazil, said there is only so much heat trees can withstand without having their life spans shortened. The hotter it gets, the more water evaporates from trees. “We saw that mean annual temperatures above 25.4°C affect tree longevity because [trees are] already operating in their limit—too much evaporation might cause water stress and affect their survival,” Locosselli said.
Climate Modeling Challenges
According to Reinmann, merging these and other processes that happen to trees into global carbon cycle–climate feedback models would make them a lot more accurate than they are now. Overall impacts could be immense. “If, under a future climate, forest carbon absorption plummets and we didn’t account for that, we would throw off the effectiveness of our climate change policies,” he said.
Incorporating and verifying such diverse data, however, is a Herculean task. Pieter Tans, a senior scientist at NOAA’s Global Monitoring Laboratory not involved in the research, points out that in addition to data about forest carbon, assessing a forest’s effect on climate models must also include soil moisture conditions and carbon storage, aboveground plant canopy structure, and myriad other variables.
A particularly difficult assessment to make involves the gas itself. “It’s easier for us to see something in methane than in CO2,” Tans said. “Carbon dioxide has large sources and large sinks, with huge fluxes of net photosynthesis and seasonal uptake, for example—but the respiration that returns that gas to the atmosphere is equally large, and there’s also a large interannual variability.”
The research community is rising to the challenge. Brienen has been working with a group in France to incorporate tree longevity trade-off data in the Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface model. “It only started quite recently, so we don’t have any results to show yet,” Brienen said.
As more accurate climate models are still in the works, researchers were unanimous about one thing: We have to do more as a society to curb greenhouse gas emissions to avoid further strangling natural carbon uptake capacities.
“We know what to do,” Tans said. “We can still have a good life while consuming a lot less energy. Renewables technology is ready to go. Furthermore, if we think about the work it’ll take to reform our entire energy infrastructure, it’s really a jobs program.”
—Meghie Rodrigues (@meghier), Science Writer