This article originally appeared on the author's personal blog, and was written in that capacity. Italics are the author's.
The only sure path to stop climate change is to zero out greenhouse gas emissions as soon as possible. That’s it. As simple as this sounds, it’s going to be an enormous job, requiring hard work over the coming decades.
But I find that most people don’t understand the time dimensions of the problem very well.
A useful way to think about the effort and timescales required is to consider the "Carbon Law," which was coined by my friend Johan Rockström. Despite the name, this isn’t a physical "law" of the universe but rather a set of recommendations.
So, what does the Carbon Law say?
It says to limit global warming to less than 2 degrees Celsius, as outlined in the Paris Accords, we need to severely restrict the total, cumulative amount of greenhouse gases we release into the atmosphere moving forward. This idea is called the "remaining carbon budget" and refers to how much carbon dioxide (and other gases) we can still emit before warming the planet beyond a particular target. The more we burn, the warmer the planet gets.
To keep within the remaining carbon budget for 2 degrees C, we have to cut our emissions drastically, reaching net-zero emissions as soon as possible.
But cutting emissions takes time, so we have to find a balance between the severity and speed of these efforts.
The Carbon Law outlines a possible path forward. It shows how we can limit the cumulative amount of greenhouse gases we emit in the future and quickly reach "net-zero" emissions. The path illustrated by the Carbon Law limits the warming of the planet to less than 2 degreesC while giving us some time to make the transition.
But the speed and severity of the required cuts are still breathtaking.
According to the Carbon Law, we need to peak greenhouse gas emissions roughly now — and then cut them in half in the 2020s.
That’s not all. The Carbon Law says we need to cut them in half again in the 2030s. And then in half again in the 2040s.
Alongside these deep emissions cuts, the Carbon Law suggests ramping up carbon removal projects, which will take many years to develop and deploy at sufficient scale, between now and 2050.
Together, leading with steep emissions cuts early on, with carbon removal building up later, we can get to "net-zero" emissions around 2050, limit our cumulative emissions moving forward, and limit global warming to 2 degrees C.
Let me illustrate how this might work with a simplified version of the Carbon Law.
Historically, greenhouse gas emissions rose from about 27 Gigatons-CO2equivalent/year in 1970 to about 50 Gt-CO2e/yr in 2020. According to the Carbon Law, we need to stop this rise and hit peak emissions as soon as possible (Figure 1).
Figure 1. Historical Greenhouse Gas Emissions. This includes all anthropogenic greenhouse gases, not just CO2. The total is expressed as an equivalent amount of CO2, using a single “global warming potential” for a 100-year window. Data from IPCC and the Global Carbon Project. Graphic by Jonathan Foley © 2021.
Then we should cut emissions by about 50 percent in this decade, bringing them down to about 25 Gt-CO2e/yr around 2030 (Figure 2). Notice that this is a much steeper decline than the emissions rise that came before.
It’s a big cut, no matter how you look at it.
Figure 2. A simplified version of the Carbon Law, where we cut total emissions by ~50 percent in the first decade. (In the original Carbon Law paper, the authors considered energy & industrial emissions separately from land use. Here I combined them for simplicity. The general lesson is the same.)
To achieve such rapid cuts in emissions, we need to deploy the fastest possible climate solutions.
To me, this would include halting climate-destructive practices such as tropical deforestation, flaring and fugitive emissions of methane, and "black carbon" emissions from biomass burning, dirty cookstoves and other sources. These would have an immediate effect on the atmosphere.
Other "quick wins" can come from rapid and cost-effective improvements in efficiency. There are enormous opportunities to be more efficient with electricity (especially in buildings and industry), food (where about 30–40 percent is wasted globally), industrial processes, transportation (higher fuel efficiency, more alternative transportation), and buildings (improved building envelopes, building automation and reduced refrigerant leaks).
In addition, we will have to rapidly shut down fossil fuel energy sources and deploy renewable energy systems across the planet as quickly as possible. But given the enormous physical infrastructure and capital involved, this inevitably will take time. Even the most aggressive scenarios of this energy transition require the 2020s and 2030s to complete.
We are in a race to stop climate change, and we will have to use the fastest solutions we’ve got. And those are usually the ones already on the shelf.
After cutting emissions by about 50 percent in the 2020s, we have to keep going and cut emissions in half again in the 2030s and in the 2040s (Figure 3).
Figure 3. And then we cut emissions by another ~50 percent in the 2030s and 2040s.
I wish we could cut emissions to zero, period, before 2050, but this framework acknowledges that it may be very difficult to eliminate all greenhouse gas emissions by then. We’ll see.
But if we assume that some emissions may continue in the 2040s, we will need to start relying on carbon removal — powered by nature (with trees, soil, or oceans) or technology.
A lot of business and technology leaders are very enthusiastic about carbon removal right now. But don’t get too excited just yet. It’s going to take a long time to make a difference. In fact, the total sum of carbon removal projects done to date — whether with trees, crops, cattle, rock weathering, or technology — isn’t even measurable in the atmosphere yet.
Because carbon removal projects are still very small, the Carbon Law allows time for them to spin up between now and 2050 (Figure 4). In this scenario, carbon removal starts to take off in the 2030s and 2040s.
Figure 4. As we cut emissions heavily in the first decades of the Carbon Law approach, we allow time for carbon removal projects to scale up by the 2040s, balancing out the remaining emissions.
Together, the drastic cuts in emissions, front-loaded to the 2020s, with ongoing cuts in the 2030s and 2040s, combined with the ramp-up of large-scale carbon removal by the 2040s, would help us achieve net-zero emissions around 2050 (Figure 5).
Figure 5. Together, the steep emissions cuts today and gradual increase in carbon removal later lets us reach net-zero by 2050.
It’s important to stress this is one possible way we can stop climate change in the future. How we actually get there will likely be different. But the Carbon Law teaches us to focus on deep and rapid emissions cuts first, with continued cuts for decades, followed by the gradual build-out of carbon removal later.
This sounds reasonable, but the most challenging part — that worries me the most — is that we have to cut emissions in half this decade.
That’s a huge job, no matter how you look at it. To put this in perspective, the Carbon Law says we have to cut emissions more in this decade than emissions grew in the previous five decades combined.
Figure 6. A huge amount of the work we need to do today, according to the Carbon Law, is reduce emissions by 50 percent before 2030.
How are we going to cut emissions in half in a decade? Simply put: We need to act fast, without delay. We have to start with tools on hand, and not wait for new ones that may (or may not) appear in the future.
This is important to remember.
Time is the most crucial parameter here, not whether we have the best possible tools. We have already squandered decades debating and denying climate change — a form of "predatory delay" that benefitted big polluters. But we’ve wasted all the time we can, and we cannot delay any longer.
We will need to do everything we can to cut emissions in half during this decade. That means no more waiting. No more delays. Not even well-intended ones, including waiting for better technologies that can help reduce emissions a little better. We have to get started today and fold in any new tools that become available as we go along.
As venture capitalist and entrepreneur Ibrahim AlHusseini likes to say, "Now is better than new." And he’s right.
I’d maybe add, "Time is as important as tech."