Pious piffle

 Net-zero by 2050 is now, apparently, the orthodoxy.  Everybody, except for a few climate denialist loons, apparently accepts it.  But that goal is nearly 30 years away.  It's easy to make pious promises about future virtue.  If everybody who is so "committed" to cutting emissions actually cut emissions by a small amount each year, we would achieve that goal.  But that would mean an annual target, not some vague promise of compliance 30 years in the future.

How much would emissions have to fall each year to achieve net-zero by 2050?  Well, they'd have to fall by 3.4% of the 2021 level -- in other words, 100% divided by 29 years.  Each year, every year.  But of course, as emissions fall each year, the annual percentage decline on the previous year would rise.  Look at the chart below.

The 'straight-line' decline is what it says.  Each year, emissions would fall by 3.4% of the 2021 level (and not 3.4% on the previous year's level).  The grey line shows the 3.4% decline on the previous year, not on 2021.  Since emissions are falling each year, the absolute decline each year is smaller than the year before.  So, the slope of the line lessens.  Initially, the 'straight-line' and the '3.4% compound' line are close, but by 2030 start to diverge radically.  

The '5% compound decline' means a decline of 5% each year on the previous year.  Its slope also lessens over time.  But absolute emissions under the '5% compound decline' are lower than under the straight-line decline until 2038.  

A relative or percentage decline is much more realistic than an absolute decline, because we'll pick the low-hanging fruit first.  The easiest sector, which is also the largest, will be electricity generation.  Getting the same absolute decline that we'll get from transitioning electricity generation from other sectors will be harder because these sectors individually are smaller than electricity generation is.  Land transport, for example, is only ±20% of world emissions, whereas electricity generation makes up 30%.  So even if you reduce emissions from land transport by 5% a year,  that still only reduces total emissions by 1% a year.  Iron and steel make up ±7% of global emissions.  Even if we cut emissions in the iron and steel sector by 10% a year, that still only reduces total emissions by 0.7% a year.  And so on.

In short, governments should be aiming for an annual percentage decline in emissions, not some distant, airy-fairy target 30 years away.  As we have seen, to get to zero by 2050, we have to cut emissions by 3.4% per annum, stepping up each year as emissions fall.  If we just achieve a 3.4% cut in emissions every year on the previous year, the 1.5 degree maximum increase in global temperatures that it is vital we attain,  will be out of reach.  A 3.4% decline per annum will mean two to three decades where average temperatures rise by 0.2 degrees, and another two where temperatures rise by 0.1 degree per decade.  On the other hand, a 5% per year compound decline would lead to a much more rapid decline in the decade-by-decade temperature increase.

However, emissions are not falling by even 3.4% per year.  They're actually still rising, and most forecasts are that they will lift to new highs in 2022.  And until emissions actually start falling, year-by-year, all this talk of net-zero by 2050 is so much pious piffle.