Most of the countries of the world have ratified the Paris Climate Agreement, which confirmed the target to limit climate change to below 2°C warming since the mid-19th century, and introduced a new aspiration to limit warming to 1.5°C. But adding up current annual emissions and future committed emissions from existing infrastructure reveals we will likely reach 1.5°C within this decade, meaning that reducing emissions (though crucial) will not be enough. Most likely, we also need to remove carbon dioxide (CO2) from the atmosphere, and there are at least two ways the land can help achieve this goal.
One option is to produce bioenergy from plants (like switchgrass or wood pellets) at power stations equipped for Carbon Capture and Storage (CCS), where CO2 is stored in bedrock deep underground. This is called BioEnergy with CCS (or BECCS). A second set of options relies on forests to remove CO2 from the atmosphere – either by growing more forests or by avoiding future deforestation. Both of these strategies require large land areas to make a dent in climate change – we need new forests roughly the size of India, or bioenergy croplands twice the size of India, to get us toward meeting the Paris ambitions.
Recently, my colleagues and I investigated the effectiveness of these two strategies for removing CO2 from the atmosphere. We used a computer model that calculates carbon stored in plants and soils due to changes in climate, land use, and atmospheric CO2 (which enhances plant growth). We then calculated how much carbon could be stored with BECCS, compared to the carbon stored in forests. Our results show that although BECCS could be effective, in many places it’s much better to protect or regenerate forests.
These results have some implications for how we use the land to mitigate climate change. First, our modelled bioenergy crop produced yields similar to corn, but average yields need to be about 50% higher to make BECCS an attractive option globally. Clearly, we need continued innovations for producing more food and biomass on our limited land, and for using land more efficiently and sustainably. For example, lifestyle changes such as using less CO2-intensive transport and a healthy diet (e.g. one serving of beef and pork per week) could decrease the need for negative emissions by 3-4 times. These actions both reduce greenhouse gas emissions and reduce the need for land-hungry livestock.
And second, where we put forests and bioenergy crops is really, really important. Forests accumulate and store carbon in vegetation and soils, and this is lost when forests are converted to crops. In our study, this loss made it very difficult for BECCS to “pay off” the initial carbon debt due to deforestation in these locations. On the flip side, the recent fires in California are a reminder that forests are susceptible to the forces of nature. So the location of forest-based mitigation is also key - so we don’t invest in planting trees in places where that investment could literally go up in smoke.
We already knew that forests have a big role to play in limiting climate change, but our work has shown this role is bigger than we thought for the highly ambitious Paris targets. It’s not impossible to meet the ambitions from Paris, but it’s going to take careful consideration of the mitigation methods and a concerted global effort.