Last month, the United Nations released a report detailing how humanity can limit global temperature rises to no more than 1.5 degrees Celsius.

To achieve this, scientists say we need not only to drastically reduce carbon emissions, but also take out the carbon dioxide we've already emitted into the atmosphere. Almost all (PDF) of the U.N.'s scientific scenarios that ensure the future of human civilization itself are staked on this "negative emissions" theory.

But the route to negative emissions many governments are banking on has fundamental flaws. It combines biomass — burning wood and other organic material for energy — with carbon capture and storage, a process known as BECCS for short.

Biomass, without CCS, already accounts for around half of the European Union's total "renewable" energy consumption and is a key plank of the bloc's plan to decarbonize energy.

The BECCS theory is that as trees grow, they absorb carbon and — once burned — technology is used to capture and store the CO2 released. This results in more carbon being removed from the atmosphere than is emitted from the process of producing biomass energy. Simple? Let's consider just how many trees we're talking about.

The injustice of land use

For BECCS to be commercially viable, it would require a vast amount of fast-growing bioenergy crops. Much of the biomass feedstock is therefore likely to come from the global south, where tropical climates enable trees to grow much more quickly.

But the scale of the land needed to achieve the negative emissions from BECCS assumed in almost all of the IPCC's scenarios does not square with the land globally available. One scenario aiming to limit global warming to 1.5C would require bioenergy crops to be planted over an area almost twice the size of India (PDF).

The BECCS theory is that as trees grow, they absorb carbon and — once burned — technology is used to capture and store the CO2 released.

This poses major threats to people in the global south, where land rights and land tenure are less secure. There are many examples of large-scale investments on land that previously has provided local communities with food, water and forest-based products such as timber, rattan or medicines. The grant of this land to companies stems from the gross misconception that land in the global south is unused and therefore available for investment ventures. In truth, local people use the land differently: for shifting cultivation or for harvesting forest products, for example.

Land use conflicts — or "land grabbing" — lead to loss of homes, loss of livelihoods and food insecurity. There are serious concerns about whether the land laws of sub-Saharan Africa, South America and Asia are equipped to protect against land grabs for BECCS.

And there are serious justice questions about whether the land in the global south should be turned into tree plantations to help the global north reduce its emissions.

Does the carbon accounting even add up?

Currently, hopes are firmly pinned on BECCS. It is touted as a win-win that provides a carbon-neutral alternative for fossil fuel-based energy, while removing additional CO2 from the atmosphere to achieve negative emissions.

But the belief that BECCS could reduce carbon in the atmosphere is based on the unjustified assumption that burning wood and other plant matter for energy is carbon-neutral. But the whole biomass supply chain — from harvest to power plant stack — produces carbon emissions. If you add all of that up, biomass just isn't carbon neutral. And it is not even a given that trees will be replanted afterwards.

The whole biomass supply chain — from harvest to power plant stack — produces carbon emissions. If you add all of that up, biomass just isn't carbon neutral.

To achieve negative emissions, the carbon from the entire BECCS supply chain would need to equal zero or less. So, if generating energy with biomass cannot be truly considered "carbon neutral," then BECCS cannot be considered "carbon negative."

We also must remember, trees grow slowly. It takes decades to up to a century for new trees to reabsorb the carbon emitted through the biomass supply chain. Most scientists agree (PDF) that emissions must peak between 2020 and 2030 to keep to climate goals. In any global supply chain, BECCS is very likely to actually increase carbon emissions and contribute to global warming within that key timescale.

Can forests provide the answer?

As well as the serious concerns around available land and the justice in its use, the assumption that BECCS can remove additional CO2 from the atmosphere is clearly flawed.

So, what could work?

The more straightforward option is to protect and restore our forests and double down on true renewable energy sources.

When it comes to cutting carbon, forest restoration is cheaper, easier and better understood than BECCS and has a host of co-benefits that help to fight global warming.

In the past few decades, forests have absorbed as much as 30 percent of man-made global carbon emissions annually.

Around a quarter of global greenhouse gas (GHG) emissions (PDF) are from the land sector, and half of these are from deforestation and forest degradation. While EU forest cover is slowly growing, forest harvests are likely to increase again by 30 percent before 2020.

Reforestation is not without controversy. There are already many competing uses for land on which forests could be restored or regrown. These include agriculture (both food and feed), infrastructure development and bioenergy, not to mention biodiversity and other ecosystem functions and services. Restoring forests needs good-quality land and there will be trade-offs. Importantly, reforestation will achieve only negative emissions if it does not result in another area of land being degraded or another forest being cut down elsewhere. Displacing the problem is not an option.

Preserving the carbon stocks in forests is one of the most cost-effective forms of climate mitigation we have. Left undisturbed, whole trees can continue to grow and absorb carbon. Cutting them down results in a lost potential for carbon to be removed from the system.

We cannot sacrifice irreplaceable ecosystems as part of an unproven global experiment. When it comes to cutting carbon, forest restoration is cheaper, easier and better understood than BECCS and has a host of co-benefits that help to fight global warming — beyond just removing CO2 from the atmosphere.

Surely forests' worth is far greater than to just prop up the fragile future of the biomass industry?