Cutting emissions immediately is more important than ‘net zero’ policies.

Friends, I have been obsessing these last weeks over the concept of ‘Net Zero’. This is the idea that we should aim to minimise the risks of climate change by:

  • First reducing carbon dioxide emissions to around 10% of current values.
  • Then doing ‘something else‘ that will actually reduce the concentration of atmospheric carbon dioxide enough to compensate for the residual emissions.

Nobody quite knows exactly what this ‘something else‘ actually consists of, but it might possibly involve land use changes, or some high-tech system of capturing CO2 directly from the air and burying it.

I have been thinking about this for the last couple of weeks and I think I have changed my opinion about ‘Net Zero’.

Previously, I had considered that ‘Net Zero’ was an essential policy. But now I think it is a fine goal, but it is not worth having an argument about. At least not at the present time.

At the present time, our overwhelming priority should to start cutting carbon dioxide emissions as soon as possible and as dramatically as possible.

The Physics

The reality of our situation at the start of the 21st Century is that if we do not stop emitting carbon dioxide into the atmosphere, the surface of the Earth will continue to warm in ways that are at best alarming, and at worst catastrophic.

The graphs below taken from this article in the Proceedings of the US National Academy of Science – illustrate the need to act sooner rather than later.

Click the image for a larger version. The graphs above show projections over an entire millennium – out to the year 3000. The 21st Century is shown as a light-blue band and 2020 is shown red The left-hand graph shows projections of the possible ways in which the concentration of atmospheric CO2 might vary based on 2% annual emissions growth, with the number highlighting the peak value. After the peak emissions are assumed to (unrealistically) fall to zero. The right-hand graph shows the estimated global warming resulting from the emissions. It is clear that failing to take early action commits us a warmer Earth for a very very very long time. The graph is taken the journal PNAS.

First of all note that the graphs above show projections over an entire millennium – out to the year 3000. The 21st Century is shown as a light-blue band and 2020 is shown red.

The left-hand graph shows projections of the possible ways in which the concentration of atmospheric CO2 might vary based on 2% annual emissions growth. Actual emissions do not appear to be growing at this rate any longer, but neither are they dramatically falling. After emissions have peaked, they are assumed to (unrealistically) fall to zero.

The right-hand graph shows the estimated global warming resulting from the corresponding emissions pathway. What is most striking is that the peak warming is more or less ‘locked-in’ for the subsequent millennium. So although the emissions pathways are not particularly realistic, the model makes clear that failing to take early action commits us a warmer Earth for a very very very long time.

The emissions in the atmosphere now have already committed us to centuries of significant climate change with significantly harmful outcomes no matter we do.

To minimise these harms, it would be desirable to rapidly reduce our emissions to zero. If we did that then we would stop making the climate worse. It would not restore the climate to how it was. The climate in which we have all lived is now part of geological history.

Reducing emissions by somewhere between 80% and 90% is definitely achievable by doing more of the things we are doing now: more wind, more solar, more batteries, more insulation, more heat pumps, less gas, more veg, less meat, more EVs and less ICEVs, and less flying.

But truly zero carbon dioxide emissions are probably not possible. It is very hard to completely eliminate emissions from processes that we might plausibly want to keep doing e.g. making concrete or steel.

The concept of ‘Net Zero’ is that in addition to reducing direct emissions by (say) 90%, we also take steps to remove carbon dioxide from the atmosphere at a rate which matches our residual emissions. We are only in the infancy of being able to measure the rates of absorption of carbon dioxide by bogs, wetlands and trees, but the hope is by the time we have reduced emissions by 90%, we will have figured out how to do this ‘final step’.

But surely ‘Net 10%’ would be almost as good?

A 90% reduction in carbon dioxide emissions requires significant societal changes, and achieving it would represent a massive success. It is definitely do-able using known technologies and although our lifestyles would likely be somewhat different, they would not necessarily be massively disrupted.

But since eliminating that last 10% of emissions ‘net-zero’ is going to be hard, mightn’t reducing emissions by 90% be good enough!

Click the image for a larger version. Estimates of the annual emissions of carbon dioxide since 1750. The graph is taken from Our World in Data and also shows in green the year in which emissions were 10% and 20% of their current value.

For the globe as a whole, a 90% cut would return emissions to their value in ~1920 and an 80% cut would return emissions to their value in ~1950. Either of these options would slow the pace of the climate change to which we are committing ourselves. But would it be enough in itself?

‘Net zero’ versus ‘Net 10%’

It is the concentration of CO2 in the atmosphere that determines the average surface temperature of the Earth.

But the concentration of CO2 is itself the result of a balance between activities which emit carbon dioxide and activities which remove it from the atmosphere – mainly by dissolving in the oceans or increasing the amount of biomass.

Based on a very simple model I have made a qualitative estimate for how atmospheric CO2 concentration will vary on a variety of emissions scenarios.

Click the image for a larger version. Estimates of the future atmospheric carbon dioxide levels on various scenarios. The red dotted line shows likely levels if we continue to emit 35 Gt CO2 per year. The black lines show the expected results of ‘net zero’ policies and the green lines ‘net 10%’ policies. The full lines are based on the unrealistic assumption that emissions are reduced to zero immediately, and the dotted line assumes (optimistically) that emissions taper to zero in 2050.

The graph above shows what we might expect if CO2 emissions taper from their current 35 giga tons per year (Gt/year) to either 0 (black dotted line) or to 10% of their current value (3.5 Gt/year) (green dotted line) in 2050.

What strikes me most about these outcomes is – for both my lifetime and the lifetime of my children, there is not much difference between the two policies.

The graph above also shows what we might expect if CO2 emissions were somehow miraculously immediately reduced to either 0 (black full line) or 3.5 Gt/year (green full line) in 2050.

Over my lifetime, this (impossible thing) would immediately begin to reduce the concentration of atmospheric CO2.

What this shows is that cutting CO2 emissions now has immediate benefits compared with eliminating the last 10% of emissions. But as the PNAS graph at the head of the article shows, once we reduce emissions to zero – or a very low level – the warming does not abate – it just doesn’t continue to get worse. This means cutting CO2 now has long-lasting benefits – very long-lasting benefits.

Summary

I like the idea of ‘net zero’. It’s a great goal and we should plan to make it happen.

But for the next few decades our overwhelming priority should be to reduce carbon dioxide emissions as rapidly as we possibly can.

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