What are the prospects for a 4C global temperature rise by 2100?
I want to offer up my view on this by starting with five underlying points.
1. Take-off growth is unpredictable.
Check out the amazing chart below. In black and white are actual 1990 carbon emissions plus projections for 2000 and 2010 from EIA’s IEO 1994. I’ve modified it by adding, with red stars, EIA’s actuals for 2010 for the USA and China. The projection made in 1994 for the USA in 2010 was pretty good, but China’s actual 2010 emissions are almost literally off the chart: instead of doubling, they almost quadrupled! This highlights what I think is a fundamental uncertainty in macroeconomics: it’s hard to tell when a poor country is going to get on the path to rapid growth.
PS. Two technical notes: First, this chart is in billion metric tons of carbon; to convert to CO2 multiply by 44/12. Second, this chart is similar to one I posted in a Twitter thread back in 2021, but I think I confused some of my numbers then; to try to reduce confusion I’ve only included actuals here for the USA and China.
2. Lots of countries could be the next China.
The World Bank lists 2022 GDP per capita (at Purchasing Power Parity) for the top 10 most populous countries in the world as follows: USA is on top ($65k), then Russia ($28k), Mexico ($20k), China ($18k), Brazil ($15k), Indonesia ($12k), India ($7k), Bangladesh ($6k), Pakistan ($5k), and Nigeria (also $5k). Any of the countries on the lower end of the spectrum could have rapid economic growth in the coming decades, and indeed some of them seem to be following the growth path of China, as shown in the World Bank chart below for India and Bangladesh (but less so for Pakistan and Nigeria).
That could well translate into rapid growth in CO2 emissions. To see how, let’s look at what’s happened in the past decades. Start with CO2 emissions through 2021 (from Our World in Data) for Indonesia, Pakistan, and Bangladesh:
Looks like exponential growth! But of course these three countries only account for a small share of global CO2 emissions from fossil fuel and industry: about 1b tons CO2 in 2021 out of a global total of about 40b tons. So let’s add India, bringing the 4-country total to about 4b tons CO2, about 10% of global emissions:
Looks like exponential growth! And India is a prime candidate for rapid emissions growth. The charts above show that India’s GDP per capita is roughly where China’s was 20 years ago and that India’s carbon emissions are roughly where China’s were 30 years ago. The Economist says it has a “strong, pro-business government”; projections from the EIA’s IEO 2023 and the IEA’s WEO 2023 are that CO2 emissions from India (and from the rest of developing Asia, excluding China and India) will roughly double by 2050, but the first chart above, from IEO 1994, suggests that much more rapid growth is possible.
3. China could be the next China.
Let’s continue exploring the graphical theme above by adding China, bringing the 5-country total to about 15b tons, or about 40% of global emissions:
Looks like exponential growth! What puzzles me is that I can’t figure out how to square this with other projections. For example, below is EIA’s IEO 2023 projection for China (with India and other developing Asia thrown in for good measure). It shows China’s emissions peaking pretty much now!
There’s a similar projection in this chart from the NY Times:
Look at that chart and ask yourself, “Is this projection really believable given the historical data?”
We will find out in the years ahead, but note that GDP per capita in 2022 was $18k in China, compared to $65k in the USA, that electricity production in China was only about double electricity production in the USA even though China has 4 times as many people, and that electricity generation in China is still dominated by coal:
4. Solar and wind have yet to make a strong showing.
The good news is that the USA shows clear evidence of an energy transition in the electricity sector:
The bad news is that so far it’s mostly been a transition from coal to natural gas. That’s still a win for the climate, but a transition from fossil fuels to non-carbon sources would be much better.
Even worse is that the USA (and, to a lesser extent, the EU) are pretty much the only places showing much evidence of an energy transition in the electricity sector. Three of the top ten most populous countries continue to be dominated by coal: China (shown above), India, and Indonesia. They all look quite similar; here’s India:
Of the remaining 10 most populous countries in the world, Brazil has always been hydropower-dominant, and the other five (Pakistan, Nigeria, Bangladesh, Russia, and Mexico) continue to be dominated by natural gas. For example, here’s Bangladesh:
The view looks pretty similar if you look at all of Africa:
Put that all together and it’s a whole lot of fossil fuels! Here’s the global total:
The counter-argument I often hear is that solar and wind are growing exponentially and are unstoppable, etc… but here are three responses:
- IEA’s WEO 2023 notes the disproportionate impact on renewables of rising interest rates: “Financing costs for clean energy projects have recently been driven up significantly by rising interest rates in markets around the world, in particular in emerging market and developing economies… Increases in financing costs have the biggest impact on large-scale projects involving capital-intensive technologies such as offshore wind, grids or new nuclear power plants, but rising interest rates also affect consumers that rely on credit to finance an EV or the installation of a heat pump.” (See also this piece, referenced by Paul Krugman.)
- IEA’s WEO 2023 also notes that China is still building lots of coal-fired power plants: “Increasing demand for electricity has nevertheless led to China completing almost 40 GW of new coal capacity on average per year over the past five years, more than the rest of the world combined in the same period. In 2022, close to 90 GW of new coal-fired capacity was approved to start construction plus another 50 GW planned, adding to the nearly 100 GW under construction as of the end of the year, indicating that total coal capacity in China will continue to increase to 2030.” Keep an eye on India, too; IEA also notes that China and India “together accounted for around 80% of global investment in coal-fired power plants and supply in 2022.” Despite all this, IEA’s WEO 2023 nonetheless claims that these new coal plants will be offset by lower coal plant usage (e.g., “China will gradually use its coal-fired power more to provide flexibility and less to deliver bulk energy”) but…
- Will it really make sense for China and India to make big investments, at high interest rates, in solar and batteries and whatnot when they could just run their coal plants more? And remember that they’ll need more electricity to accommodate economic growth, plus even more to power EVs and other efforts to “electrify everything”. In my opinion those coal plants will be awfully tempting, especially since they may also help solve domestic issues like coal-industry employment.
5. It’s questionable that “electrify everything” will replace oil and gas.
I don’t know how the numbers work out if you electrify everything but still rely on lots of fossil fuels for electricity. That’s an important topic, but even more important is that demand for oil and gas still looks strong. EIA’s IEO 2023 (which—somewhat important caveat—doesn’t consider the impacts of the Inflation Reduction Act) shows global emissions from oil and gas rising through 2050, both in the Reference case (blue) and the High Economic Growth case (brown):
Heck, EIA even has emissions from coal as flat in the Reference case and rising in the High Economic Growth case:
By contrast, IEA’s WEO 2023 has all three fossil fuels peaking before 2030 even in their baseline case (called STEPS). I hope I’m wrong in saying this, but these all look like wishful thinking to me, with too much of an “and then a miracle occurs” feeling during the 2020s:
Prospects for a 4C global temperature rise by 2100
Let’s return to our original question by first noting that a 4C temperature increase (relative to the 1850-1900 baseline used by the IPCC) basically corresponds to IPCC scenario SSP3-7.0. Table B.1.2 from IPCC AR6 WG1 SPM shows SSP3-7.0 leading to a 3.6C temperature increase by 2081-2100, i.e., by about 2090, so an addition decade of temperature increases will lead to about 4C by 2100.
Second, note that SSP3-7.0 basically corresponds to a doubling of CO2 emissions between 2015 and 2100, from 40 GtCO2 per year to 80 GtCO2 per year, as shown in Figure SPM.4(a), again from IPCC AR6 WG1 SPM:
Third, Global Carbon Project 2022 reports that emissions have been “approximately flat since 2015”. So in order to double between 2023 and 2100, emissions have to increase by a bit less than 1% per year for the rest of this century. (The Rule of 70 says that growth at x% will produce a doubling in about 70/x years.)
Now, EIA’s IEO 2023 projects out to 2050 and has world energy-related CO2 emissions roughly flat in their Reference case but rising by an average of 1.1% per year in their High Economic Growth case. (Note again the somewhat important caveat that IOE 2023 doesn’t consider the impacts of the Inflation Reduction Act.) In contrast, IEA’s WEO 2023 projects all three fuels to peak by 2030, as discussed above.
Look at emissions other than fossil CO2 and there are at least two possible wild cards that could increase global temperatures. One is CO2 from deforestation: a die-back in the Amazon could, by itself, contribute ten years’ worth of emissions, or about 400 GtCO2. The second is methane, the main component of natural gas: increases in natural gas usage could produce more methane leaks, and on the timescale of 100 years methane causes almost 30 times more warming than an equivalent amount of CO2.
There could be other wild cards too. What if temperatures increase faster than we thought? What if interest rates stay high? What if there’s a policy disaster relating to offsets? What if there’s higher-than-expected economic growth as in the China chart at the very top of this post?
All this makes me think that SSP3-7.0 is still very much within the realm of possibilities, and that it is asking for trouble to call SSP3-7.0 “the world avoided”, as Zeke Hausfather and co-authors do here. (I should be clear that I greatly respect Zeke et al., but also that he and others have been overly optimistic on at least one previous occasion, in arguing that global CO2 emissions from fossil fuels and industry had perhaps peaked in 2019.)
I will give Zeke the last word; you can decide for yourself if you find it convincing: