At Rocky Mountain National Park, mountain pine beetle has killed off swaths of the forest’s trees. (Photo by Juliet Eilperin/The Washington Post)

We tend to have a greatly oversimplified view of the planet’s carbon problem — and therefore, of what we have to do to solve it.

The general notion is that it’s all about fossil fuels, and so if we stop driving so much and using so much coal, we solve climate change. But there are other major players involved in putting carbon in the atmosphere and removing it, and a recent piece of research shows how one of them  — forests — could make it harder for the United States to reach its express goal of reducing emissions by 26 to 28 percent below their 2005 levels by the year 2025. That’s the same pledge the country’s negotiators will take to Paris later this month for a much anticipated global climate meeting.

new study in Nature Scientific Reports, by U.S. Forest Service researchers David Wear and John Coulston, finds that U.S. forests, which currently store more carbon than they lose each year — lowering the country’s net emissions — could store less of it in the future. And that means fewer of our emissions would be offset by trees and plants pulling carbon back out of the air again.

“Looking to the future there is a diminishment of carbon over time, due to the diminishment of growth” of trees, says Wear, who is a project leader at the Forest Service’s southern research station in Raleigh, N.C.  “That’s very gradual, but it reflects the aging of forests in the United States. And what could be more dramatic, if you will, is shifts in land use.”

U.S. forests as a whole are actually faring pretty well at the moment, the study finds — particularly in the East where they have seen net growth due to more timber plantations and also the reclamation of agricultural land, Wear says. As a partial result, currently U.S. forests store 173 teragrams or million metric tons of carbon a year, the researchers find.

However, that figure is in units of carbon — if you convert to carbon dioxide, which has a greater molecular weight, it’s actually 634 million metric tons.

That’s enough to offset “9.7% of C emissions from transportation and energy sources,” the researchers write. Indeed, the figure is roughly equivalent a third of all the carbon dioxide emissions caused by transportation in the U.S. in 2013 — 1,718 million metric tons — according to the U.S. Environmental Protection Agency.

However, Weir and Coulston project that our forests will change, and their carbon storage will decline, because of factors like worse wildfires, pest outbreaks, and simple aging of trees and changing land use. Granted, the effect varies greatly by region. In the Rocky Mountain region, they find, forests could actually come close to becoming a net source of atmospheric carbon, rather than a “sink,” as they currently are.

“Forests in the Rockies are close to a balance between forest growth gains and disturbance losses,” the authors write.

The net result would be only 112 teragrams (or million metric tons) of carbon storage annually by 2040, the study finds. This means that in effect, over 200 million fewer tons of U.S. carbon dioxide emissions would be offset by forests annually. The change is not enormous, but it is certainly substantial.

The researchers are fairly blunt about explaining the policy implications of this result. As they put it:

The projected changes in forest sequestration under the reference scenario implies that, without forest-based mitigation activities, emission offsets decline and emissions in other sectors would need to be reduced further to reach emissions targets (e.g., the US intends to reduce net emissions by 26–28% between 2005 and 2025).

“It is impressive that the forests in the U.S., in spite of fire and insects, we project that on net, they will remain a robust carbon sink,” Wear says. “It’s just that the strength of that sink is diminishing over time.”

However, it is important to note that the study did not include Alaska, a region where there are not only vast forests, but also sharp concerns that more of their carbon could end up in the atmosphere in the future due to worse wildfires, and emissions from carbon rich soil layers and the permafrost layer beneath it. Wear says that right now, the data on Alaska’s forests aren’t good enough to perform an analysis similar to the one performed for the lower 48 United States.

[Alaska’s terrifying wildfire season and what it says about climate change]

Granted, concerted future actions to restore forests or plant new ones could move the ledger in the other direction — toward greater carbon storage at a time when we’re definitely going to need it. According to the U.S. National Climate Assessment, afforestation, or active planting of trees, could lead to the successful storage of several hundred million additional tons of carbon a year.

So far this year, gigantic Indonesian peat fires — also the result of human land use change — have put a staggering 1.726 billion tons of estimated carbon dioxide equivalent emissions into the atmosphere. That’s the extreme form of what can happen if a country doesn’t adequately protect its land and forests.

The Indonesia story is far worse than anything happening with forest and land in the United States, and yet at the same time, the two are linked. For both show that how we manage our land and forests on a global level will have a great impact on whether we can keep global warming under control.