What would happen if emissions stopped today?

I was under the impression that if emissions stopped today, global temperatures would keep on rising because of lags in the system.  But that turns out to be incorrect.

In a reply to a comment on his post in   '...and Then There's Physics', the blog owner reproduced this graph from the IPCC Special Report on 1.5C (SR15):

This shows various projections for the rise in temperatures after 2020, depending on what we (the world) do.

The purple line shows temperatures if we stop all greenhouse gas emissions now, but continue emitting aerosols (basically, sulphur dioxide which is produced as a by-product to burning coal).  This is driven by the decay of methane already released into the atmosphere, and the continued absorption of CO₂ by carbon sinks.  Temperatures would fall by nearly half a degree over 30 years.

The yellow line shows the same scenario but with aerosol emissions stopping too—which is more logical, as aerosols mostly come from coal burning.  Sulphur dioxide aerosols in the atmosphere reflect some of the sun's incoming rays back into space.  In the past, large volcanic eruptions have led to global temperatures dropping for a couple of years after the eruptions because of this effect.  We could, of course, artificially produce SO₂ and release it into the atmosphere to mimic the release from burning coal.  And it would be relatively cheap—but still a big environmental negative.

The solid blue line shows what would happen if we stopped emitting CO₂ but continued to emit other greenhouse gases (presumably including methane, but also CFCs).  The dotted blue line shows the temperature trajectory assuming we stop emitting CO₂ by 2050, but continue emitting other greenhouse gases.  The dashed blue line shows the continuation of the trend of the last 20 (in fact 50 years)  The green line (wrong colour!) shows what happens with an instant end to CO₂ and SO₂ emissions but a continuation of other greenhouse gas emissions.

To reduce emissions to zero by 2050, which we need to do to limit further rises in global temperatures to 0.5 degrees (dotted blue line), they would need to fall by 3.2 percent of the current level each year.  That's 100%/31 years.  Measured by the decline in each year on the previous year, the percentage decline would get steadily bigger, year by year, because the base each year would be getting smaller.  In the last year, with an equal absolute decline each year, the percentage decline would be 50%.  In the tenth year (2029) , the annual percentage decline would be 4.7%.   By 2039, the annual percentage rate of decline would be 8.9%.

So the world should aim to cut CO₂ emissions by 3.5% per annum (compounded, i.e, with the percentage decrease being calculated on an ever declining number as emissions fall) starting now, stepping up to 5% by 2030 and 9% by 2040.

Can we do that?  Yes, easily.  According to this chart from the EPA, global electricity and heating plus transport emissions make up 40% of total greenhouse gas emissions (includes methane)  By 2040, there will be no coal power stations left, and most of the world's vehicle fleet will have transitioned to EVs.  If we can also halve the GHG emissions of iron/steel, cement and chemicals manufacturing, we would exceed the 3.2% per annum pathway.  Can we halve industry emissions?  Yes.  We could produce iron and steel using hydrogen produced from renewable electricity via electrolysis.  One CEO of a global cement company stated that we could easily produce cement with far fewer CO₂ emissions, but no one has asked them to.  Marvellous.  And my guess is that the same applies to the chemical industry.  A carbon price would be a good way to force this shift.

And of course, you can take powerful personal action by eating less meat.

Source: EPA

As Churchill used to say during the war in his memos: Action this day.

Giving up is not an option.