The sooner that carbon dioxide emissions get to net zero, the less climate damage the world will suffer. That is a simple statement of fact.

The “net” in net zero in this case refers to what happens when you take all the activities that add carbon dioxide to the atmosphere—such as fossil fuel burning or deforestation—and include the effects of naturally occuring and engineered “carbon sinks,” which remove carbon dioxide from the atmosphere. Carbon added to the atmosphere minus carbon removed with the help of sinks is the net—which needs to get to zero, as soon as possible.

One natural land-based sink that has attracted a lot of attention lately is that associated with the rapid turnover of carbon in vegetation, or biomass—especially in the form of forests.

It is likely to play a role because the task of achieving net zero is so big that it will need to be tackled by a wide range of approaches—akin to the many wedges of a pie—each of which may contribute only a small part to the solution. One is to take advantage of the relatively fast turnover of the natural land carbon cycle, by encouraging the use of forest products—not only logs, but bark, roots, stumps, leaves, sawdust, lumber mill waste, and the rest of the biomass of a tree—in place of fossil fuels to generate heat or electricity for industrial processes.

The reason that the burning of biomass as a fuel can be considered an activity that gets us closer to net-zero was essentially identified by the 16th-century alchemist and proto-chemist Jan Baptist van Helmont. In his famous “willow tree” experiment, van Helmont found that almost none of the mass of a tree grown from seed was taken out of the soil in which it is grown. He thought that the mass came from the water that the tree took up while growing—and to a certain extent this is true—but in fact about half of the dry weight of a tree is carbon, and that carbon came from carbon dioxide in the air. (Vegetation does require tiny amounts of nutrients such as nitrate and phosphate from soil, but these make up a minuscule fraction of their mass.)

In other words, the structural part of a tree is literally made out of thin air.

Carbon gets into the tree through oxygenic photosynthesis, a metabolism that took hold on Earth some 2.5 billion years ago, starting out in blue-green algae and eventually making its way onto land when land plants emerged about two billion years later. Oxygenic photosynthesis works by using the energy in sunlight to convert carbon dioxide and water into organic carbon (think “wood”) and oxygen. Burn a tree, release usable energy, and you put carbon dioxide into the atmosphere. Re-grow the tree over the next 20 or 30 years, and the same carbon dioxide goes back into the tree. It’s just a rather inefficient (but rather inexpensive way) of harvesting solar energy.

Fossil fuels, like coal or oil or natural gas, work on the same principle, except that the carbon involved came from a trickle of organic carbon that was buried and accumulated over millions of years; we are digging up and releasing that carbon over a mere few centuries, but it will take millions of years to re-stock.

So far so good, but when it comes to deciding whether biomass burning is good for the climate the devil is in the details—and there are a lot of possible devils lurking in these particular details.

Many of the issues are highlighted in the controversy over the conversion to biomass fuel of the massive Drax power plant in the United Kingdom. As a result of this and similar projects, the United Kingdom has become one of the world’s largest importers of wood pellets, and has contributed to a rapidly growing worldwide wood pellet industry. The supply chains involved in providing these wood pellets are long and complex, and doing the carbon accounting to figure out the net effect on forest carbon stocks going into the next few decades is far from easy. Given rampant crony-capitalism in the United Kingdom, there are plenty of reasons to be suspicious of the way Drax does its carbon accounting, and of whether the subsidies it has received for biomass use are justified, but it is far from proven that biomass burning at Drax is “worse than coal,” as some activists have claimed. Still less does it mean that biomass utilization cannot, in principle, be done in a way that contributes to the net-zero goal.

There are several ways that the promise of biomass as a net-zero fuel can go wrong. The source of the wood pellets being burned, and the forestry practices involved in producing them, are both crucial factors. The least problematic source is waste wood, such as sawdust from lumber mills, which would be discarded and release their carbon dioxide back into the air through natural decomposition if not used as fuel. Still, the definition of what is “waste” is rather fluid, and in some cases can extend to large branches and even whole (but otherwise economically unusable) trees that would otherwise be left on the forest floor and store their carbon for a time.

The effect of such practices on net stocks of forest carbon is difficult to assess. There is generally a moral hazard in making a “waste” product economically valuable through increased demand for wood pellets, since that holds out the temptation of artificially increasing the amount of “waste.” Drax imports wood pellets largely from the United States and Brazil. Much of the Brazilian source is nominally from wood waste produced as a by-product of producing tannin from acacia trees for the leather industry. Even though the wood pellets in question are a waste product, it needs to be assured that the trees in question are grown sustainably, and that the increased demand for this waste does not indirectly lead to deforestation. Given the anti-environmental stance of the current Brazilian government and lax monitoring of environmental disruption, there are plenty of reasons to be concerned about the life-cycle carbon accounting of such wood pellet sources. Worse, looking more broadly beyond Drax, there are very few policies in place to assure that the wood pellets feeding biomass power are really sourced from trees that will be allowed to re-grow. If wood pellets are produced from deforestation, that is just tree-mining and is no more net-zero than is the burning of coal.

The concept of “payback time” for the carbon dioxide released by biomass burning engages yet more subtle and challenging issues. It is fairly straightforward to estimate the time it takes a tree to regrow and re-stock the carbon released by burning it. However, most of the carbon stock in the forest is not in living above-ground biomass, but in soil carbon below ground. Temperate forests contain 650 metric gigatons of carbon in living biomass (equivalent to roughly 70 years of worldwide carbon emissions at current rates), but 2,300 metric gigatons of soil carbon. Almost none of that soil carbon is in chemically recalcitrant forms that resist being oxidized back into carbon dioxide through microbial processes (“respiration” — just the same as we do when eating food). Forest activities disturb this carbon pool and expose it to air, and shortening rotation periods in forests can also affect the amount of carbon in the soil carbon pool in complex ways. The time taken to re-stock soil carbon is highly variable from one forest to another, and difficult to accurately account for.

None of these things are necessarily show-stoppers for the biomass wedge. What they call for is more stringent controls on biomass sources, and better and more transparent carbon accounting.

It is far too soon to throw up our hands and just discard the entire biomass wedge as an uncurable evil, as was done in a rather facile and partisan feature article on Drax published by the New Yorker last December. The New Yorker article naively views carbon emissions from biomass burning as a “giant loophole” in climate protection protocols, dismissing the regenerative possibilities on the grounds that it will take decades for forest regrowth to recoup the emitted carbon—but a few decades’ delay is in reality not a serious issue in comparison to the nearly irreversible climate disruption caused by the burning of fossil fuels. Such discourse, which is all too common in criticisms of the biomass wedge, is unhelpful and detracts attention from the serious policy issues that need to be confronted if biofuels are indeed to be net-zero.

Even if biomass burning can be done in an essentially carbon-neutral way, the expanded use of biomass as a fuel comes into conflict with a number of other important environmental and societal goals. Monoculture tree plantations managed on a short harvest rotation are not natural forests and are not good for biodiversity. Furthermore, the exploitation of forests in this manner can trample the rights of indigenous peoples. This comes into play in a stark way in Sweden, which relies on biomass for nearly a quarter of its energy supply; this is an attractive source of arguably renewable energy in Sweden, which has a high ratio of forest land to population. Unfortunately, a great deal of this forest lies in the ancestral homelands of the Sami people of Northern Sweden. Industrial exploitation of these forests threatens reindeer husbandry and other aspects of the traditional Sami way of life, as highlighted by the Sami joik-singer and rights advocate Sofia Jannok. (See the web page of her Arvas Foundation and also check out some of her music.)

There is an attractive simplicity in journalist and climate activist Bill McKibben’s principle: “Just Burn Nothing.” It avoids the whole morass of ways that reliance on biofuels can go wrong. But we may not have that luxury. The question is—if some regions of the world find they need to use biofuels as part of their decarbonization strategy, what is the most effective and least harmful way of doing so?