Spurred by some new studies there is once again the notion making the rounds that we should maybe focus on short-term climate forcers such as soot and methane rather than on carbon dioxide in the fight against climate change. For example, a recent Nature article has the title “Pollutants key to climate fix” and opens with:
Buses spew clouds of black exhaust fumes in Mexico City while, in India, wood burnt in rudimentary stoves fills houses with sooty smoke. Methane leaks from gas pipelines in Russia and rice paddies in China, eventually breaking down in sunlight and contributing to the production of smog and ozone. In each of these cases, simple steps to curb air pollution would promote public health; scaled up, they may offer the only realistic way to tame global warming over the next few decades.
As I have related in an earlier post, climate scientists like Raymond Pierrehumbert say that nothing could be further from the truth. Focusing on short-lived forcers at the expense of CO2 means losing time rather than buying time. For instance,
the methane concentration in the atmosphere is determined by the methane emission rate averaged over the previous ten years, and the methane component of warming disappears quickly after emissions cease. In contrast, about half of CO2 emitted disappears into the ocean fairly quickly, while the other half stays in the atmosphere for thousands of years. Therefore, the atmospheric burden of CO2 in any given year is determined by the cumulative emissions going back to the beginning of the Industrial Revolution, and the warming persists for thousands of years after emissions cease. (…)
Suppose we are outrageously successful, and knock down anthropogenic methane emissions to zero, which would knock back atmospheric methane to a pre-industrial concentration of around 0.8 ppm. This yields a one-time reduction of radiative forcing of about 0.9W/m2. (…) This gives us a one-time cooling of 0.4ºC. The notion of “buying time” comes from the idea that by taking out this increment of warming, you can go on emitting CO2 for longer before hitting a 2 degree danger threshold. The problem is that, once you hit that threshold with CO2, you are stuck there essentially forever, since you can’t “unemit” the CO2 with any known scalable economically feasible technology. (…)
It’s cumulative carbon that counts, and pretty much it is the only thing that counts.
To be fair, the Nature article does note that “aggressive mitigation of carbon dioxide would still be needed to keep the atmosphere from warming more than 2 °C”, but that’s rather a footnote in the whole article and others do not even make that distinction.
And I would add to the natural science analysis that not only are most non-CO2 forcers short-term but so are the actions to reduce them. Giving somebody a new cooking stove that produces much less soot is very cheap and easy. Conversely, changing course in our energy and transport systems is extremely difficult. For some like the authors of the „Hartwell Paper“ this is exactly the reason why other things should be prioritised. To me it’s exactly the reason why changing our energy and transport systems needs to be top priority. Once a coal power plant has been built it is going to emit CO2 for decades and decades. So these investments need to be stopped and redirected to clean alternatives now, not in 10 or 20 years. The International Energy Agency (IEA) in its latest World Energy Outlook warned that the door to keeping global temperature increase below 2°C is closing fast. If the world continues on a “business as usual” trajectory, by 2017 enough carbon-intensive infrastructure investments will have been locked in to generate all the energy-related emissions that are permissible up to 2035 to stabilise the atmospheric CO2 concentration at 450ppm, which gives a 50% of staying below 2°C –
leaving no room for additional power plants, factories and other infrastructure unless they are zero-carbon, which would be extremely costly. Delaying action is a false economy: for every $1 of investment avoided in the power sector before 2020 an additional $4.3 would need to be spent after 2020 to compensate for the increased emissions.