How fast will emissions decrease?
Tags: Climate Change, Emissions, Global Warming, Climate, Zero Carbon

The left graph shows you how electricity has been generated over time; the black line is for coal. Now look at the right graph; the black line shows the total emissions produced in the UK. Both black lines drop sharply around 2010–2015: as renewables let us retire coal plants, emissions dropped very rapidly. What about the future? You can see from the left hand graph there’s very little coal left, so there’s no easy source of emissions reductions left either.

You can see the overall effect of the transition clearly in this graph showing emissions from different sectors, with electricity in red:

From here.

Technology adoption

We’d expect similar effects to happen in the future. At some point electric vehicles, storage technology, heat pumps, and other technologies will go mainsteam, following adoption curves like those in this image:

From here. This one is for the US, but we’re only looking at the general trends.

That’s a century worth of technology; each curve shows you how one particular technology spreads. Typically adoption is slow until about 10% of a population is using a technology, then very fast until 80% or so, then slow again. The next graph shows how different countries are progressing along the electric-car adoption curve:

From here.

The UK isn’t marked on that curve, but is near the bottom: in 2019, 3.3% of new registrations were battery electric vehicles. In the future we can expect the UK to

Move slowly up to the point G; in this phase emissions reduction will be small as few petrol cars are being replaced. Move rapidly up the curve until electric cars account for about 80% of cars on the road. During this rapid adoption phase, emissions will drop rapidly as petrol cars go off the road. Once about 80% of new registrations are electric, emissions will go back to declining slowly, as number of petrol cars isn’t dropping that fast.

All in all, this gives us a rapid burst of emissions reductions. We saw the same kind of burst when renewables replaced coal, and we’d expect similar bursts when technologies like large-scale electricity storage and heat pumps go mainstream.

So: emissions won’t actually drop at a steady rate. They will lurch downwards, dropping quickly when a new technology is mid-adoption and slowly the rest of the time. Each new technology will take a quick ‘bite’ out of emissions in some sector, just as renewables did in the electricity sector (red line below):

This again.

As emissions are reduced, it will become harder and harder to reduce them; emissions from air travel, for example, are very hard to reduce. So the absolute pace of emissions reductions, in megatons per year,* will slow down. We’ll come back to this later.

*I lied a little here. Emissions are measured in megatons per year, so the rate of change of emissions is measured in (megatons per year) per year. Don’t worry about it if it’s confusing.

From here.

Modelling future reductions

How should we model these fast-slow-fast-slow emissions reductions? Looking at each of electric cars, heat pumps, etc., in turn and figuring out when they’ll become mainstream is an enormous task, and hard to do with any accuracy.

Thankfully, economics gives us a better method. Per-capita economic growth happens as a result of many technological improvements.* As we saw in the 20th-century graph above, each technology is adopted in an S curve, so each technology gives a burst of economic growth. But in the long run, all of these average out to give fairly steady trend growth.

*If you’re an economist: yes, I should be talking about total factor productivity here if it weren’t too technical. Well done. Have a lolly.

We’ll use the same idea here, and use the average annual emissions drop over a long period to estimate the future trend. 1990 is the usual reference point for emissions reductions, so we’ll start there. Using data from here and here, we get an average drop of 1.76% between 1990 and 2035. What would that look like if we extrapolated it out to 2100?

I noted above that the more we reduce emissions, the harder it is to keep reducing them. If you like, we grab the low-hanging fruit first, so progress slows down as there are fewer, higher fruit left. One of the advantages of modelling reduction as a fixed percentage per year is that it captures this slowdown in reduction: you can see that the red line above becomes flatter and flatter over time.

Politics vs the Facts

The UK recently announced a new target of net zero emissions by 2050. You can see that the red line in my projection graph above doesn’t come anywhere close to that. In fact, the old UK target for 2050 was 120 megatons of emissions, and the UK is not remotely on track for that. The UK is even due to miss its goals for 2030. Even those 2030 goals are misleading because they exclude aviation emissions for UK flights, which have doubled since 1990 and are still rising.

What this tell us is that when governments set aspirational multi-decade goals, even ‘legally binding’ ones, those are political theatre. Another example is China — the central government has committed to net zero emissions by 2060 but its regional development plans include substantial construction of coal plants over the next decade. If we want to know how fast emissions are likely to decrease, we need to look at policies rather than goals.

Negative Emissions

Aspirational goals generally rely on negative emissions technologies, technologies that remove greenhouse gases from the atmosphere. I wrote a detailed review of the research on direct air capture, which tells us that it’s not realistic to expect it to reduce emissions by a significant amount. The same unfortunately applies to other negative emissions technologies. The research shows that they all have capacity constraints that mean their contribution will be quite limited. For example, to make a substantial dent in emissions by growing plants, we’d need to plant over agricultural land the size of India, causing massive food shortages.

I have yet to see any realistic negative emissions plans — typically proponents show that something could work on a small scale, and then assume everything be scaled up arbitrarily. (In economics lingo, they assume that supplies of their inputs are ‘perfectly elastic’.) As thing stand, reaching net zero carbon before 2100 is an unrealistic, techno-utopian goal; we might be lucky with some as yet undreamt of scheme, but it’s unlikely.

Correcting for Population

There is one reason to think emissions might drop faster than the red line in my graph above. At the moment, the UK population is rising relatively fast; by 2100, it will be rising much more slowly (if at all), as the next graph shows:

More people means more travel, more buildings, more food, and generally more emissions. And emissions-reduction technologies generally act by reducing emissions per person. For example, switching to electric cars reduces emissions per driver; if the number of drivers were to go up fast enough, overall vehicle emissions could still go up.

So we should get a better picture of the effect of technology by thinking of emissions per person dropping by a fixed amount per year. Before we do that, though, there’s one other improvement we can make…

Correcting for Consumption

Here’s another graph from the UK Dept. for Business and Other Gubbins:

From here.

Like all the graphs we looked at earlier, this shows a pretty substantial decline in emissions. That picture isn’t wrong, exactly, but it’s very misleading. Over time, more and more emissions-intensive manufacturing has shifted overseas — but people in the UK still buy things from abroad, which emitted greenhouse gases when they were made.

If you look at the UK’s consumption emissions, that is the emissions in all the things bought by people in the UK, you get a much less optimistic picture:

From here.

Between 1990 and 2017, there’s been an increasing gap between CO₂ emissions produced in the UK and CO₂ emissions consumed in the UK, showing that people in the UK have been importing more and more emissions-heavy goods. (I wish I could find consumption statistics for all greenhouse gases, but I can’t, so I’m using the CO₂ figures. For long-terms trends that should be fine as 75% of warming is from CO₂.)

That gap has made the production-based statistics the government likes to quote look very good, but it’s a little misleading; ‘exporting’ emissions to other countries is zero-sum and doesn’t actually global warming. It’s also not possible to keep exporting more and more emissions. In that respect consumption statistics will give us a better measure of long-run trends.

Putting it all together

Using data from here, we find consumption emissions per person dropped by 18% from 1990 to 2017, which is 1.2% a year.

I can’t find projections of consumption emissions, but as I just noted they can’t keep diverging from production emissions, so we can use the production projections from the start of this article. Correcting those for population, we get a projected drop of 17% from 2017 to 2035, which is 1.7% a year.

The US

We’ve covered all the key ideas while discussing the UK, so this discussion of the US will just focus on numbers. The ‘bible of energy projections’, which everyone cites, is from the independent Energy Information Administration’s Annual Energy Outlook 2020 (AEO2020):

This doesn’t cover all emissions (and I’ll confess I don’t understand what’s left out), but it covers most of the ~6.5 billion tons of current emissions listed by the EPA. So it’s a decent source for trends.

The central black Reference line in the graph is the case the AEO2020 considers most likely to happen, given current policies and expected technological advances. The Low Renewables Cost line is for the case where the prices of solar panels and wind turbines drop faster than expected.

If you’re surprised by how flat those emissions lines are, there are quite a few reasons:

“Costs for renewables such as wind and solar have continued to decline as experience is gained with more builds. How long these high cost reduction rates can be sustained is highly uncertain.”

As renewables technology improves, the cost of electricity will drop; as it drops and also incomes go up, people will use more electricity.

As electricity becomes cheaper, some coal plants will become uneconomic and close. But there’s a core of coal plants that are inexpensive to run, which are projected to stay in use until 2050.

Wind and solar aren’t always available. Although the AEO2020 models the growth of storage (batteries), a lot of gas will still be needed.

The graph is for the US as a whole; the population will grow a lot by 2050. Emissions per person are projected to drop from 15.0 tons per person in 2020 to 12.7 tons per person in 2050 (Reference) or 12.1 tons per person in 2050 (Low Renewables Cost).

The AEO may be the bible of energy projections, but it has been criticised for consistently overestimating wind and solar costs. On the other hand, this will mostly affect emissions from electricity generation, which only account for about 20% of US emissions:

In case you’re wondering, the transportation projection does take into account growth in electric cars and increasing fuel efficiency; they’re just balanced out by economic growth and increasing air travel. The projection doesn’t take into account the fact that high-altitude emissions cause about 1.9x as much global warming — so the actual effect of transportation will be even higher.

Also, as the green bars in the next chart show, the scope for use of renewable power in industry is relatively small.

So while emissions may drop faster than the AEO projects, the difference is unlikely to be dramatic; even if emissions from electricity generation halve, that should only affect the final figure by 10%. Still, it would be good to check against another objective source. The AEO2020 so ubiquitous that I can’t find any other projections, so let’s try looking at historical data instead. Here are US emissions:

From here.

The sharp drop around 2009 is due to the financial crisis, so we don’t want to read too much into it; we either want to exclude it or take a wide window around it. I’ll do the latter. Using data from here, I find per-capita US consumption emissions decreased 14% from 1990 to 2017, or 0.54% a year.

How does that compare to the AEO2020? That projects an annual drop of 0.55% (Reference case) or 0.71% (Low Renewables Cost). The historical data is at least in the same ballpark, which is good.

Incidentally, if the US had stayed in the Paris Agreement, it would have committed to reducing its (production) emissions from by 80% from 2005 to 2050. That’s a 3.5% reduction each year; you can see that both projections and historical data fall very far from that lofty goal.* One major reason for that is that increases in efficiency are counterbalanced by GDP growth, as the graph below shows. To put that more simply, as time passes, each thing we consume involves less emissions, but we consume more things.

From here.

*Per-capita US consumption emissions decreased 21% from 2005 to 2017, or 1.97% a year. But much of that drop was due to the financial crisis, so it’s not a great basis for extrapolation.

Putting it all together

Here’s a quick recap of all the per-capita emissions figures we’ve found…

UK consumption emissions dropped 1.2% a year from 1990 to 2017 .

from . UK emissions are projected to drop 1.7% a year from 2017 to 2035.

US consumption emissions dropped 0.54% a year from 1990 to 2017 .

. US emissions are projected to drop 0.71% a year from 2019 to 2050 under the AEO2020’s Low Renewables Cost scenario.

Any attempts to project emissions to 2100 are inherently uncertain, but the best estimates we can make using history and science will rely on those numbers.

Divergence

The most obvious fact that jumps out from those numbers is that emissions have decreased much faster in the UK than in the US; UK and US emissions have diverged. You can see that divergence between the USA and UK lines in this next graph:

From here. Not per capita.

In the graph, the divergence is represented by the UK and the US moving further apart over time. We need to be careful when extrapolating because this divergence is very unlikely to keep happening all the way to 2100. One reason for that is that because the UK has made more progress to date, it’s now harder for it to reduce emissions; the low-hanging fruit are gone.

Another way of looking at things is that in the long run, emissions decreases are driven by technologies like electric vehicles; adoption in the US may be slow right now, but it will get happen eventually. You see something similar with GDP per capita; countries that are behind tend to grow faster, in a ‘catch-up effect’, because it’s easier to copy technologies than invent them from scratch.

So in the long run, the USA and UK emissions lines are likely to drift down at roughly the same rate; perhaps USA emissions will always be above UK ones, but they won’t get further and further apart.

Projections

Here are those key numbers again.

This means it would be a mistake to use our UK and US numbers to project emissions for each country separately; i.e. we shouldn’t estimate that per-capita emissions to 2100 will decrease by 1.7% a year in the UK and 0.7% a year in the US.

A better model is to estimate that in the long run, emissions will decrease at roughly the same rate in all developed countries. The reason for this is that the long-run rate of decrease will mostly depend on green technologies, just as long-run economic growth depends mostly on technological progress, and green technologies should eventually spread to all countries.

And here’s that graph again.

You can see from the last graph that the UK has reduced emissions faster than other developed countries, while the US has been slower than any country except Canada. That tells us that the US and UK are good choices to bracket the projected rate of emissions decrease in developed countries; it’s unlikely to be faster than the UK or slower than the US.

So —deep breath — our best estimate is that emissions per person will decrease between 0.5% a year and 2% a year in developed countries. It’s possible that there will be some miraculous technological breakthrough that results in faster reductions, and it’s possible that major wars or other geopolitical crises will displace climate cooperation and result in slower reductions or increases. But the range that’s supported by past data and expert projections is 0.5% to 2% a year.

There are a couple of reasons to think 0.5% is pessimistic. First, the AEO has been consistently bearish on renewables and it’s possible that even their Low Renewables Cost scenario is too pessimistic. Second, all the technologies that have been used in the UK will be adopted in the US eventually, so we’d expect the US to follow the faster UK trajectory sooner or later. Given those points, I think I’ll go out on a limb and make a…

…final prediction!

In developed countries, the average annual per-capita emissions reductions to 2100 will be between 1% and 2%.

Let’s see what that actually looks like. Using population projections from here, I put together these graphs showing total emissions for each country.