Don't count on forests to keep us cool
New research suggests that boreal forests (shown here in dark green) might not help curb global warming as scientists once thought. Boreal forests cover 15 percent of Earth's land surface. That, plus their their ability to store or release carbon, might have a big impact on global warming.
JB: This is Earth & Sky. Scientists have discovered that over the past 25 years, many trees in the far north have been growing more slowly in the summertime.
DB: And that’s bad news if you’re concerned about global warming. Andy Bunn is an ecologist at the Woods Hole Research Center in Massachusetts. Bunn studies boreal forests: conifer trees growing in Alaska, Canada and northern Eurasia. He told Earth & Sky that scientists expected to find trees growing faster as Earth warms. In other words, he said, these trees should be “greening.” Instead, biologists say, they’re growing more slowly or “browning.”
Andy Bunn: One of the few things that could make global warming less intense than it is is the assumption that as the climate warms, the boreal forests are going to be able to grow more intensely and soak up more carbon dioxide from the atmosphere.
JB: As plants grow faster, they breathe in more of the heat-trapping gas carbon dioxide and store more of it in their roots, leaves and wood. But the northern forests aren’t growing faster, as expected. They appear to be stressed by increasingly drier air and soil in the summer, which slows their growth and makes them more susceptible to fire and pests. Bunn told us he’s worried.
Andy Bunn: We thought that forests were going to be able to take more carbon dioxide out of the atmosphere and store it in the ground, and we’re in fact seeing that that might not be the case.
DB: More at earthsky.org. Our thanks to NASA: explore, discover, understand. We’re Block and Byrd for Earth & Sky.
Read Forest on the Threshold, from NASA.
Learn more about Boreal Forests of North America, from Woods Hole Research Center.
Learn more about Taiga or Boreal Forests, from Susan Woodward, Radford University.
The browning effect that Andy Bunn is reporting is too subtle to see from the ground. The browning is revealed by satellites – which can see large swaths of Earth over long time periods. The satellite data is matched to tree ring data taken at ground level in sample sites. That allows scientists to accurately analyze the satellite data.
Andy Bunn explained why scientists had expected northern forests to grow more intensely in the summer: ?We thought that logically it seems like these forests should grow longer over the course of the year and more intensely during the summer because they live in very cold environments. So their growth is limited by the growing season temperatures. They only have a few months in which it’s warm enough above freezing essentially to grow over the course of the year. So we thought that as the climate warms that the trees would be able to grow earlier in the springtime and grow later into the fall than they have been able to in the past. And what we found is that the trees are growing earlier in the springtime as spring arrives earlier and earlier every year – but they are not able to grow longer into the fall or more intensely during the summer, probably because of drought.?
Our thanks to:
Andy Bunn
Post Doctoral Fellow
Woods Hole Research Center
Falmouth, MA
Additional Teacher Resources
NASA Earth Observatory Northern Forest Affected by Global Warming
By most predictions, the Northern forests that cover much of North America, Europe, and Asia, should be getting greener. Scientists have always thought that plant growth in the world’s Northern forests was limited by temperature. Arctic summer provides a brief period in which plants can develop before the cold of winter ends the growing season. Over the past century, however, temperatures have gone up and the length of the growing season has increased, nearly doubling in sections of Alaska. With carbon dioxide, one of the key ingredients in photosynthesis, also on the rise, plants should be thriving. But they are not.
NASA Earth Observatory Forests on the Threshold
What is happening to the forests of northern Alaska, Canada, Europe, and Siberia? Why have they slowed their growth when everyone thought they should be expanding for several more decades? Is it a sign that global warming is changing Northern forests more quickly than anyone thought possible? The answers to these questions impact more than the sparsely populated forests of the North.
EPA Climate Change
The EPA Climate Change Site offers comprehensive information on the issue of climate change in a way that is accessible and meaningful to all parts of society – communities, individuals, business, states and localities, and governments.
