Roger Pielke Jr had a recent guest post on his substack by Tom Wigley called Net-Zero Does Not Mean What You Think it Does. The post concluded that CO2 emissions do not need to be eliminated to meet the Paris targets and that uptake by the natural sinks means that CO2 emissions can remain positive for a very long time. I was slightly surprised, since this didn’t seem consistent with our current understanding, but it was based on some slightly older work and I thought it may not be quite up-to-date with our current understanding.
However, I’ve since watched some of a recording of a debate between Roger Pielke Jr and Steve Koonin on “Is net zero by 2050 both achievable and necessary to address climate change?” The first question you reasonably might ask is why they would arrange a debate on this topic between these two speakers, but that isn’t the point of this post. In the recording Roger explains what net zero is and suggests that it’s the point at which emissions are balanced by the natural sinks (oceans and biosphere) and, hence, that net zero requires emission reductions of about 80%, rather than 100%.
This is simply wrong. It’s very clear that net-zero means that anthropogenic emissions are zero, with any residual positive emissions balanced by negative emissions that lead to essentially permanent geological storage, hence the “net”. The latest IPCC report says explicitly:
Net zero CO2 emissions: Net zero carbon dioxide (CO2) emissions are achieved when anthropogenic CO2 emissions are balanced globally by anthropogenic CO2 removals over a specified period.
The scientific under-pinnings of net-zero go back to papers published in about 2008 (e.g., Matthews & Caldeira 2008) demonstrating that when anthropogenic emissions go to zero, global average surface temperatures would soon stabilise. The reason for this is that we currently have a planetary energy imbalance that – all else being equal – would lead to continued warming so as to return the system to energy balance. When anthropogenic emissions go to zero, the natural sinks continue to take up some of what has been emitted so that atmospheric CO2 concentrations go down in such a way as to compensate for this unrealised warming.
Of course there are uncertainties (there could be some additional warming, or even some cooling) and there are other slower feedbacks that are not considered (carbon released from the permafrost, for example). However, the best estimate today is still that, on multi-decadel timescales, the zero-emission commitment is close to zero.
Roger’s definition of net-zero would lead to stabilised atmospheric CO2 concentrations, rather than stabilised global surface temperatures. Given that the system is not yet in energy balance, stabilising concentrations would lead to continued warming. Since atmospheric CO2 concentrations are already 420 ppm, even if we could achieve ~80% reductions in emissions by 2040, we’d probably have concentrations of ~440 ppm. If you calculate the equilibrium warming for such an atmospheric CO2 concentration, assuming an ECS of 3oC, it’s about 2oC. So, Roger’s definition of net-zero is essentially committing us to at least 2oC of warming. Also, if emissions don’t eventually go to zero, concentrations would start rising again, and warming would resume.
So, to clarify, net-zero does not mean that positive anthropogenic emissions are balanced by uptake by the natural sinks so that emissions just need to be reduced by ~80%, it means reducing anthropogenic emissions by 100% with any positive anthropogenic emissions being balanced by “permanent” anthropogenic negative emissions.
You might be surprised that someone could engage in a high-profile debate about net-zero without properly understanding what the term means and the scientific understanding that underpins the term. Of course, if you’re a long-standing veteran of the public climate debate, this is entirely expected. Evidence would suggest that Roger doesn’t regard being properly informed about a topic as being a pre-requesite for making confident assertions about the topic.