Can Renewables Grow as Fast as in Climate Scenarios?

Every year, more wind and solar power are added than the year before but can renewables be expanded sufficiently fast to keep global warming below 1.5°C or at least 2°C? Most of the scenarios for achieving these climate targets envision very rapid growth of renewables, but many also see parallel expansion of other low-carbon technologies such as nuclear and carbon capture and storage (CCS). There is however a growing criticism that the models used to construct these scenarios underestimate the true potential of renewables^{Creutzig, F., Agoston, P., Goldschmidt, J. C., Luderer, G., Nemet, G., & Pietzcker, R. C. (2017). The underestimated potential of solar energy to mitigate climate change. Nature Energy, 2(9), 17140. https://doi.org/10.1038/nenergy.2017.140Jaxa-Rozen, M., & Trutnevyte, E. (2021). Sources of uncertainty in long-term global scenarios of solar photovoltaic technology. Nature Climate Change, 11(3), 266–273. https://doi.org/10.1038/s41558-021-00998-8Victoria, M., Haegel, N., Peters, I. M., Sinton, R., Jäger-Waldau, A., Cañizo, C. del, Breyer, C., Stocks, M., Blakers, A., Kaizuka, I., Komoto, K., & Smets, A. (2021). Solar photovoltaics is ready to power a sustainable future. Joule. https://doi.org/10.1016/j.joule.2021.03.005}given the recent falling cost of solar and wind power. Some scholars even argue that pursuing other technologies is distracting^{Stoddard, I., Anderson, K., Capstick, S., Carton, W., Depledge, J., Facer, K., Gough, C., Hache, F., Hoolohan, C., Hultman, M., Hällström, N., Kartha, S., Klinsky, S., Kuchler, M., Lövbrand, E., Nasiritousi, N., Newell, P., Peters, G. P., Sokona, Y., … Williams, M. (2021). Three Decades of Climate Mitigation: Why Haven’t We Bent the Global Emissions Curve? Annual Review of Environment and Resources, 46(1), 1–37. https://doi.org/10.1146/annurev-environ-012220-011104}particularly since "granular" technologies like renewables grow faster than "lumpy" technologies like nuclear^{Wilson, C., Grubler, A., Bento,N., Healey, S., De Stercke, S., and C. Zimm. 2020. “Granular Technologies to Accelerate Decarbonization.” Science, 368, no. 6486: 36–39.}and since developing countries — where the bulk of renewable growth is supposed to occur — may be able to learn from the experience of developed countries^{Grubler, A., Wilson, C. & Nemet, G. Apples, oranges, and consistent comparisons of the temporal dynamics of energy transitions. Energy Res Soc Sci 22, 18–25 (2016).}and grow renewables faster. Is such criticism justified: can renewables really grow faster or at least as fast as in main climate scenarios?

In a recent Nature Energy article^{Cherp, A., Vinichenko, V., Tosun, J., Gordon, J. & Jewell, J. National growth dynamics of wind and solar power compared to the growth required for global climate targets. Nature Energy 6, 742–754 (2021). https://doi.org/10.1038/s41560... (see https://rdcu.be/cptRl for free view-only version)}, we analyze the historical growth of solar and wind power in the 60 largest electricity markets which account for 95% of electricity worldwide and show that climate scenarios over- rather than under-estimate feasible rates of global wind and solar expansion. We also don't find that in terms of electricity generation the more granular solar grows faster than "lumpier" wind. Our third surprising finding is that solar and wind do not grow faster in developing countries, which introduce renewables later. This means that other low-carbon technologies and climate solutions might need to receive more, not less, attention from policymakers.

Our analysis proceeds from the premise that in each country solar and wind power grow along a so-called S-curve of technology expansion. This means that their growth is initially slow and erratic, then 'takes off' in nearly exponential acceleration, after which it achieves its maximum rate and stabilizes at the so-called ‘inflection point', where the slope of the S-curve is steepest, before gradually slowing down as the technology achieves its final market saturation.

In more than half of our sample countries, wind and solar power have already taken off (which we define as achieving 1% of electricity production). The take-off first occurred in the EU and OECD followed by the largest emerging economies (India, China, Brazil, Mexico and Turkey), which offer the greatest potential for investors. Non-OECD energy exporters such as Russia and the Gulf states with their largest fossil fuel subsidies^{Jewell, J. et al. Limited emission reductions from fuel subsidy removal except in energy-exporting regions. Nature 554, 229–233 (2018).}are still below this threshold.

For countries, where wind and solar power are already growing steadily, we fit growth models to approximate the empirical growth observations with idealized S-curves. To begin, we check whether a country has already achieved the inflection point, where the growth rate reaches its maximum. To our surprise, we find that most countries are already past this point, which means wind and solar is no longer accelerating. For all such countries, we measure their maximum growth rates which can be accessed through our interactive visualizations and replicated with our publicly available code. On average, the maximum growth rate is 0.8% of the total electricity supply per year for onshore wind and 0.6% for solar. Of course, some countries achieve higher growth rates than these averages, for example, the maximum rate of wind expansion in Ireland is 2.6% and 1.8% of solar in Chile. However, faster rates are only observed in relatively small countries, while in large countries maximum growth rarely exceeds 1.1% of electricity supply for solar (e.g. in Japan) and 1.5% for wind (e.g. in the UK).

This is of course not the whole story, because we can't reliably measure maximum growth rates in all countries: in some, growth is still accelerating while in others, solar and wind are still at the erratic pre-take-off phase. It is conceivable these late-comers would learn from the pioneering countries' experience and reach higher growth rates in the future. To test this widely-held hypothesis, we check whether growth of wind and solar in countries which introduce these technologies later is faster than in those introducing them earlier. Once again, to our surprise we find no such relationship. In fact, it seems solar power grows slower, not faster, in countries that introduce it later. This holds true whether or not we control for a variety of factors such as solar irradiation, GDP per capita and fossil fuel exports. The explanation for this paradoxical observation is that the positive effects of global technological learning are cancelled out by less favorable socio-economic and political circumstances in late-comers. In other words, the same factors that delay the introduction of wind and solar power in some countries also depress their growth rates.

In the final step of our research, we compare the maximum growth rates which we measured in individual countries to those required globally or for continental-size regions in the 1.5°C and 2°C scenarios. Due to different assumptions about population, economy, land use and technology development, the scenarios envision a relatively wide range of solar and wind power growth. Still in at least one-half of 1.5°C and 2°C climate scenarios, global or regional growth of wind or solar power exceeds the maximum growth rate observed in any large country so far. In about one-quarter of the scenarios the global growth of solar power is above 3.3% per year, which is three times faster than in the most successful large country (Japan) and twice as fast as in the fastest country (Chile).

To summarize, our research challenges several dominant views about the growth of solar and wind power and their potential role in mitigating climate change. We show the growth in most countries is no longer exponential – rather it stops accelerating at about 1% of total electricity supply per year. While maximum growth differs from one country to another, only in leading countries, like Germany, does it approach the speed that we would need in the world as a whole to reach climate targets. Furthermore, our results show that developing countries which introduce wind and solar power later do not develop them faster, probably because of adverse socio-economic and political circumstances. This makes it especially difficult to replicate or exceed the growth rates from the national leaders to the continental or global scale. Overall, the paper articulates the enormous scale of the challenge of replacing traditional energy sources with renewables and the need to explore diverse climate solutions and scenarios.