The ongoing anthropogenic climate change is largely due to increasing levels of greenhouse gases in our atmosphere. These greenhouse gases (GHG for short), with carbon dioxide (CO2) and methane (CH4) as the most important ones, affect how much of the solar energy our planet receives stays here as heat energy instead of radiating back into space.
The climate discussion often confuses annual GHG emissions, GHG emissions reductions, the GHG levels of our atmosphere and how these levels develop over time.
The very first thing we need to do is to stop annual emissions from growing. The latest measurements from spring 2019 show that the annual emissions grew at near-record speed last year (2018), by roughly 3.5 parts per million (ppm).
While it is prudent not to draw too far reaching conclusions from a single year, it is relatively safe to say that so far, we have failed even in the first step of climate mitigation, which is to stop the growth of annual emissions.
Walking towards a catastrophe
When we finally do manage that first step of stopping annual emissions growth, the climate project is only getting started. We are still walking towards a climate catastrophe, but now at a more constant speed, instead of the previous, accelerating speed.
The next step is to reduce the amount of annual GHG emissions we release, and preferably do that consistently and at an increasing rate. At this step, we are still increasing the amount of greenhouse gases in the atmosphere, but now at a decreasing rate. We are still moving towards a potential catastrophe or a self-reinforcing tipping point. Such a tipping point can start to accelerate the climate change despite what we humans might do.
For example, the ice sheets might melt in such a way that they stop reflecting sun’s light back to space, leaving that energy here instead, where it will warm our planet further. Or perhaps the enormous carbon stores in permanently frozen tundra peat start to melt and release that carbon into the atmosphere . Or perhaps ocean acidification disturbs the food chain in such a way that the oceans’ ability to sequester carbon from the atmosphere is severely reduced.
There are many of these mechanisms. We can’t accurately predict how and when they start, or if they start at all. We don’t even know for sure what their precise net-effects will be for the climate, but we do know that they can be enormous.
The coming centuries of negative emissions
When we reach the point where our annual emissions are net-zero, we enter the endgame. We then need to capture and store the excess carbon from the atmosphere somehow and somewhere. The GHG concentration of the atmosphere is already higher than what is likely to be compatible with a stable and reasonably benign climate in the long run.
This is an enormous undertaking that will likely take us centuries. How many centuries, it depends greatly on when we get started and what is the GHG concentration in the atmosphere when we do.
Today the concentration of CO2 is roughly 410 parts per million (ppm), and it has been increasing at roughly 3 ppm per year, or even faster. At this pace (if we manage to stop annual emissions from increasing), the concentration will reach 500 ppm by mid-century. The somewhat safe level in the long run has been estimated to be at around 350 ppm, although there are severe uncertainties in this number. This number was given by James Hansen, who is one of the most well-known climate scientists in the world. My guess is that the number would be between 300 and 400 ppm, depending on our tolerance for risk and our luck with the climate feedback mechanisms.
So if we manage to start a massive carbon capture and storage -industry at some point, capable of reducing our CO2 levels by one ppm per year, each 100 extra ppm at the starting point will mean another century of work, and another century of waiting if some self-reinforcing mechanism kicks in and takes the matter out of our control for good.
To summarize, we need to start with these three steps: Stop growing our annual emissions, start reducing our annual emission, and reach carbon neutrality, where the GHG concentrations are no longer increasing. After that, we need to start capturing and storing much more CO2 than we release to become effectively carbon negative. Not just in our imperfect CO2 emissions bookkeeping, but in the measured levels of CO2 and other greenhouse gases in the atmosphere.
We are in a situation where we need to both cut our current emissions rapidly and plan for ambitious long-term measures to ensure centuries of negative emissions.
 More than 40 % of the carbon on our soils is stored in peat. That is more than 550 billion tons of carbon. If transformed into carbon dioxide, it would use our remaining emissions budget many times over. https://www.iucn.org/resources/issues-briefs/peatlands-and-climate-change
Four steps in stopping climate change
1. Reducing the growth of annual emissions
2. Reducing annual emissions
3. Get net neutral on annual emissions
4. Remove excess carbon from the atmosphere