Let’s just start from the state of the carbon sinks right now. If I am not in error, they have been absorbing around 55% of human CO2 emissions over the last ten years. My source for this is Glen Peters. In the graph he shows, we can see that carbon sinks have increased in capacity over the last hundred years. We also see that the terrestrial biosphere carbon sink swings more wildly than a bitcoin price chart, and the ocean carbon sink has increased its capacity in a relatively steady way, clocking in now at around ten billion tons of CO2 per year (up massively from around 2 billion in 1900).
Since the biosphere carbon sink swings so wildly, taking a ten year average seems to be a good idea. Anyway, let’s just start with the assumption that right now carbon sinks removed 55% over the last ten years. Assuming around 35 billion tons per year of emissions, 55% of those 350 billion tons would amount to 192.5 billion tons of CO2.
Under these numbers for the human emissions and the carbon sinks, “human net zero” would mean that someone rides a time machine to 2006 to stop all human emissions (hi, Angel). They then use the Carbocadabra spell to turn all fossil fuel reserves into spaghetti carbonara. As a result, all 350 billion tons of CO2 emissions go to zero.
This would of course mean that the 192.5 billion tons removed by carbon sinks would have come out of the atmosphere. So “human net zero” doesn’t mean “net zero” for the atmosphere. It means a large negative value, as long as there is a working carbon sink.
In contrast, “climate net zero” means that human emissions are exactly the same as the carbon sink capacity at that particular time. So if our time traveler didn’t remove all fossil fuel reserves by magic not existing in the real world but instead just explained to fossil fuel reserves owners that they stand to make more money by selling only half of the amount (hi, Phaseout Profit Theory) and that slowed down emissions to exactly 192.5 billion tons, the net increase for the atmosphere would be exactly zero.
With a positive climate sink (as now), it is easy to see that “climate net zero” is a much easier goal than “human net zero”. It is also easy to see that climate net zero (avoiding to make the problem worse) still allows for a huge chunk of fossil fuel use.
That might change in the future. Carbon sink capacity may go further up, like it has done over the past century, or it may go down. It may even become negative. If we get some feedback loop going that changes the existing carbon sinks to new carbon sources, it will be exactly the other way around. Human net zero will not be enough to achieve climate net zero, with the latter becoming the more ambitious goal.
Anyway, it should be useful to get some clarity on exactly what you are talking about when mentioning a “net zero” goal. Is “net zero” measured by the results (CO2 in the atmosphere remains unchanged) or by one of the factors (human emissions)? I for one think it is useful to aim for clarity in this way.
And I am going to use the terms “human net zero” and “climate net zero” here when talking about this.
A figurative way to explain this is to think of an overweight person. They have 100 kilos where they should have only 70.
The radical solution would be to stop eating at all until they get down to their goal. That’s the equivalent of stopping all human CO2 emissions (human net zero).
However, if the goal is only to avoid making things worse, it would be enough to limit food intake to whatever the body burns. That’s the equivalent of limiting emissions to whatever the carbon sinks absorb (climate net zero).
And if the goal is to get back to a healthy 70 kilos, some value between eating nothing and eating less than the body consumes would be reasonable.
Going back to the climate, “climate net zero” only assures that the problem doesn’t get even worse (which would already be progress). While it is the easier goal to achieve, it probably is not enough of an effort.