By Rudy Rogalsky

This is another short note in a series about aspects of the climate crisis. 


Canada, like many other countries dealing with climate change, has pledged to reduce national emissions of CO2 to net-zero by mid-century. This pledge is an acknowledgement that the burning of oil and other hydrocarbons is the main source of CO2 in the atmosphere and must be reduced if we are to mitigate the worst effects of global warming. But the term “net-zero” needs some explaining.

First, it must be understood that the term refers, not to the quantity of CO2 in the atmosphere, but rather to the amount we add to that concentration each year. Right now, humanity emits annually nearly 40 billion tons of CO2  resulting in current CO2 concentration of 420 parts per million (It was 270 ppm over 200 years ago). Another consideration is that the 2050 target is not a total elimination of those emissions. For some industrial activities such as aircraft and other transportation, it will still be necessary to burn hydrocarbons and continue to emit some portion of those 40 billion tons. Those emissions will have to be offset by physically withdrawing CO2 from the atmosphere either by expanding our forests, improving our land use practises or by developing technologies that can withdraw and safely store significant amounts of CO2 . Some large emitters of CO2 (companies and countries) seem to be counting on those technologies being developed in the distant future to avoid having to lower emissions in the short term even though they are quite aware that we don’t yet know if current-day experiments with CO2 capture and storage can be scaled up to a size that would be effective and economically feasible as an offset. In other words, they are once again kicking the can down the road. 

It is important to note that heat entrapment by CO2 and other greenhouse gases (GHGs) is directly related to the stock of those gases in the atmosphere. Our flow of CO2 (annual emissions) indirectly affects warming by increasing the stock. Thus, should we actually achieve net-zero, annual emissions until then will have added to the concentration of GHGs and to heat entrapment. In 2023, the IPCC reported that the world has a carbon budget of about 250 billion tons of CO2 that we can add to the atmosphere to have a coin-flip (50%) chance of keeping global warming to no more than 1.5o C above pre-industrial levels [Johan Rockstrom, 44th Macaulay Lecture]. Emitting 40 billion tons per year, we will have exhausted that budget in less than a decade.

What would happen should we ever actually achieve net-zero? At that point, we would have mostly stopped emitting CO2 (and abated what we couldn’t) but we will still have to contend with the phenomenon of Earth Energy Imbalance (EEI). Global warming happens, and has been happening because the inflow of energy from the sun is greater than the energy in the earth’s infra-red radiation back into space. In the 11,000 years before the industrial revolution, earth’s temperature remained essentially constant at 14o C indicating that EEI was more or less steady at zero. So, to reduce warming, we will have to continue withdrawing CO2 from the atmosphere until our energy-out matches the sun’s energy-in. The prevailing view among climate scientists is that we will be aided in this by our oceans which will continue to absorb 90% of excess energy inflow and 25% of CO2 emissions. The scientific models supporting this view unfortunately do not include detailed consideration of increasingly probable climate tipping points such as permafrost melting [Zeke Hausfather,”Explainer: Will Global Warming ‘stop’ as Net-zero Emissions are Reached”]. And it is too soon to know if confidence in the prevailing view will be affected by scientists’ admitted inability to understand the sudden and unprecedented 2023 increase in ocean surface temperatures [New York Times, Feb.27, 2024, Scientists are Freaking Out About Ocean Temperatures]. 

And finally, in addition to such imponderables, we are faced with an important socio-political reality: after years of talking about emission cuts at international meetings, actual emissions are still growing. There is still a rapidly diminishing window of time to reverse our direction. It’s improbable but still possible to do.