5 Gt of negative emissions by 2050?

Meeting the Paris Agreement climate goals was never going to be easy. But there has been relatively little published analysis attempting to understand exactly what it will take to make the ambition of the Agreement a reality. A group of European scientists recently published a paper titled, “A roadmap for rapid decarbonization” in the journal Science that attempts to change just that. The paper lays out what the authors dubbed the “Carbon Law,” which explains a simple heuristic for what we need to do meet our climate goals. The “Carbon Law” proposes that we will have to halve our CO2 emissions each decade starting in 2020 while also ramping up carbon removal rapidly starting in only a few decades time to achieve net zero emissions by 2050 (and reducing land sector emissions to zero) as described in the chart, below. And while the Paris Agreement targets will be challenging to meet, the “Carbon Law” analysis reveals important points about carbon removal -- and just how critical it will be for the Paris Agreement.

Above: The red line shows a halving of CO2 emissions each decade starting in 2020, the blue line shows carbon removal scaling up to the 5Gt CO2/yr level by 2050 to get us at net-zero global emissions. Source: Rockstrom, et al. 2017.

Above: The red line shows a halving of CO2 emissions each decade starting in 2020, the blue line shows carbon removal scaling up to the 5Gt CO2/yr level by 2050 to get us at net-zero global emissions. Source: Rockstrom, et al. 2017.

1. Without carbon removal, decarbonizing as quickly as is needed to meet climate goals looks highly implausible. Say you disagree with the “Carbon Law” authors about the likelihood of getting large scale negative emissions scaled by 2050. The natural question, then, is how does the Carbon Law change to meet a 2C goal if we don’t have negative emissions? Back of the envelope math shows that we’d need to cut emissions in half every five years (i.e. twice as fast as the original Carbon Law) starting in 2020 to reach net zero (defined as <1Gt CO2) by 2050.

To even the most ardent supporter of renewable energy, energy efficiency, and electrification, this rate of decarbonization likely does not pass the laugh test. Even if we had the will to spend the massive amounts of money to achieve this, the physical act of transforming all of our energy, transportation, building, and industrial infrastructure will take time -- major construction and building efforts are not trivial undertakings. Large-scale carbon removal by 2050 will face major challenges, but those challenges pale in comparison to the alternative of having to decarbonize twice as fast.

2. If we can’t decarbonize as quickly as the Carbon Law proposes, we will need lots more carbon removal to meet Paris Agreement goals. As Brad Plumer over at Vox writes “This road map is staggering.” Most climate experts would likely agree: cutting emissions in half each decade starting in 2020 will be really challenging. Even cutting emissions in half every 15 years will be a challenge. So what happens if we don’t cut emissions in half every decade, but rather every 15 years -- how much carbon removal would we need then to meet our Paris Agreement goals? A back of the envelope calculation shows we’d need double the amount of carbon removal proposed by the authors, ending up with 10 GtCO2/year by 2050 to reach net zero emissions. Given how hard it will be to achieve the halve every 10 years, the Carbon Law shows how valuable carbon removal will be as a hedge against meeting our climate goals

3. The Carbon Law tells us about meeting a 2C target… what about a 1.5C target? Without carbon removal, we need to halve emissions every year, starting immediately. This isn’t happening, so the only way to get 1.5C is with major carbon removal. Basically, the 1.5C roadmap is “reduce emissions as fast as possible to zero, scale up carbon removal to the 10Gt+ scale as fast as possible.”