Photo: NOAA

The boreal forests of the world—that vast swath of woodland that circles the globe across the top of North America, Europe, and Siberia, in between the Arctic tundra to the north and the temperate forest to the south—aren’t likely to grow as colder latitudes warm up. Rather, they are more likely to simply shift northward and shrink, releasing more stored carbon into the atmosphere than expected.

Previous studies have shown that boreal forest was likely to extend farther towards the North Pole than it currently does today, as warming made more land suitable for this type of plant growth. According to research from Lawrence Berkeley National Laboratory, boreal forest may still grow farther north than it does today, but it’s range isn’t likely to expand.

As the northern regions warm, the present southern border of the boreal region will also get overtaken by warmer and drier conditions. In some places, current boreal forests will likely turn to grassland; in others, other types of forest. In either case it’s less carbon storage than today—and more carbon released from the ground.

According to lead researcher Charles Koven, “Most Earth system models don’t predict this, which means they overestimate the amount of carbon that high-latitude vegetation will store in the future.”  

Though grasslands can store lots of carbon in their soil, it accumulates at a much slower rate than in boreal forest. And boreal forests store an enormous amount of carbon. Though tropical forests get a lot of the attention when it comes to carbon storage in wooded areas, boreal forests actually store about twice as much carbon as tropical forests, and nearly six times as much as temperate forests. Just over 20 percent of all the carbon stored on the Earth’s land surface is in the boreal region—which isn’t surprising considering it’s the largest biome on the planet.

In the case of tropical forests, most of this carbon is stored in the lush vegetation above ground—with the exception of peat forests in parts of Indonesia and Malaysia. But boreal forests store so much carbon because they slowly accumulate carbon below ground, keeping it out of the atmosphere for several millennia at least.

For example, Canada’s boreal forests store around 208 billion tons of carbon, with two-thirds of that stored below ground. Not all of that will get released as forest shift to the north, of course. But some of it will, and it will be more difficult to trap as ecosystems change. It’s the broader realization that’s most important. Forests may shift to higher latitudes as the Earth warms, but that hardly means that Earth’s forested area is going to grow.