While Morrison crab walks to net zero (part V)

(this is part v of a series and builds on knowledge in parts i to iv. i reckon it is worth reading them before reading this, but you do you. live dangerously if you want.)

How quick can I do the recap?

1. Australia is a big emitter, and the burning of coal, oil and gas is the real problem (read it here)
2. Our pathway to net zero emissions matters as much, if not more than the date that we reach net zero (read it here)
3. But the date is also important, and hitting zero emissions earlier is better than hitting it later (read it here)
4. ‘Net zero’ isn’t the be-all-and-end-all: what sectors you have zeroed, and what you have not, matters (read it here)

In this piece, I’m going to continue with the ‘what zero’ theme of the last piece, but this time focussing on question of which gases are being zeroed, rather than which sectors.

As of 2020, the world has warmed by around 1.2°C above pre-industrial levels above the average temperature of the years 1850 to 1900. This has mostly been driven by increases in the level of carbon dioxide in the atmosphere. carbon dioxide is emitted from a range of man-made sources, including the burning of coal, oil and gas.

But carbon dioxide is very, very far from being the only human-emitted substance that jiggers with the temperature of the planet. Alongside carbon dioxide there are dozens and dozens of other gases, both natural and human-caused, that either heat the planet – like methane and nitrous oxide – or cool the planet – like sulfate and nitrate aerosols. It’s simplifying things, but I’m going to refer to these heating and cooling substances collectively as ‘emissions’, ‘cooling emissions’ and ‘heating emissions’ based on whether they generally nudge the temperature bar up or down overall.*

(* This series is intended to complicate the over-simple, it is not an atmospheric chemistry PhD thesis. If that’s what you’re into, you can start with Chapter 8 of Working Group I’s Contribution to the IPCC’s Fifth Assessment Report.)

While there is a lot of uncertainty on this – and I am intentionally simplifying complicated dynamics of the atmosphere (see above) – among the human-caused substances that jigger the climate, the heating emissions and cooling emissions other than carbon dioxide roughly balance each other out.

This coincidence means that we can guess-timate the impact of our stuffing with the climate using carbon dioxide to a reasonable degree of accuracy. A climate model will always be much better, and that’s why climate scientists use them. That said, the balance between heating jiggerers and cooling jiggerers is a coincidence. There’s no rule saying that the non-carbon dioxide heating emissions and cooling emissions will for now and all time balance. Realistically, the balance won’t stay the same, but how much it changes depends on a range of factors.

The dynamics of this get especially complicated because many things that we do that create large amount of carbon dioxide also produce large amounts of other climate-jiggerers in significant quantities. For instance, the production and use of coal, oil and gas is the largest source of human-caused carbon dioxide (which heats the planet), as well as being a significant source of sulfate and nitrate aerosols (which cool the planet) and one of the largest sources of methane (which heats the planet). The very-much overdue shift to zero carbon dioxide emissions from producing and using coal, oil and gas will unevenly shift the balance of the the other climate jiggerers as well.

Below I have shoddily redrawn (and simplified) a chart from chapter 1 of the IPCC’s special report Global Warming of 1.5°C. It shows the impact on the temperature human civilisation hypothetically hitting net zero carbon dioxide emissions last year, and the different outcome on our temperature of zeroing warming emissions, zeroing cooling emissions, zeroing both and zeroing neither.

Here’s the original chart, in context.

Baring in mind that each of these assessments contains quite large uncertainty:

1. Zero cooling emissions (red): If we had reached net zero carbon dioxide emissions in 2020 and emitted no more cooling emissions, but kept pumping out the warming emissions other than carbon dioxide at today’s rates, this would lead to the world continuing to warm over the next eighty years. By 2100 this would lead to around half a degree more than we have seen so far.
2. Zero warming emissions (purple): If we had reached net zero carbon dioxide emissions in 2020 and emitted no more of the other warming emissions either, but kept pumping out cooling emissions at today’s levels, this would cool compared to where it is today. By 2100, the world would be about four-tenths of a degree cooler than it is today.
3. Only zero carbon dioxide (orange): If we had reached net zero carbon dioxide emissions in 2020, but kept both warming and cooling emission at today’s levels, the world would continue to gently warm over the next eighty years. By 2100 though, this would result in less than one-tenth of one degree more than today.
4. (Net) Zero everything (brown): If we (net) zeroed all carbon dioxide, warming emissions and cooling emissions in 2020, this would lead to a short-term bump in global temperatures – because cooling emissions generally move out of the atmosphere much faster than the warming emissions – but as short-lived powerful warming emissions like methane are broken down over the next decade, and carbon dioxide moves from the atmosphere into the oceans, the temperature rebalances at below today’s levels. By 2100, this leads to a drop in temperature of a little less two-tenths of a degree.

Ignoring the possibility of climate feedbacks (aka ‘tipping points’, aka ‘unholy hellfire’), the difference between future scenarios for emissions other the carbon dioxide are stark. While a more realistic estimate is somewhere in the middle, it’s worth noting that there’s the best part of a full degree celsius difference between the two most extreme scenarios.

Basically, what happens with emissions other than carbon dioxide matters.

Net zero carbon dioxide emissions is important, but if we tackle that while ignoring the fact that atmospheric concentrations of methane and nitrous oxide in the atmosphere are not just increasing, but accelerating, then we’re going to get ourselves into a bit of trouble.

I had intended to cover another topic here, the issue of ‘carbon dioxide equivalence’ between greenhouse gases. However, in the interests of keeping things bite-sized and (relatively!) simple, I have decided to add another post to the ‘what zero’ series, and give the topic its own dedicated post, rather than tack something else on here. [edit: I’ve decided to cover something else entirely that is more interesting.]

That will be part VI. See you then!

(I don’t ask for payment, but claps, likes and shares keep me writing: just in case you were wondering.)

Part VI is here.