(& here’s a smoothed version of the same thing, done for visual appeal rather than analytical rigour!)
Each year the IEA (International Energy Agency) produces one of its flagship reports, called “World Energy Outlook” (WEO). And each year it gets roundly criticised for underestimating its future ‘predictions’ of solar PV growth, exemplified through charts like the one below, which you can find here, on the excellent Carbon Brief website (near the bottom of the article):
Firstly, let me stress that there is absolutely nothing wrong with this last chart! It correctly plots the IEA’s own data for annual solar capacity growth; data which does seem to suggest an inexplicable freezing of future solar growth rates, flying in the face of the recent trend (the red line).
However as with anything, it’s important to read the small print – in this case the line just below the title. Here you’ll see mention of the WEO’s “stated policies scenario”.
At which point it becomes important to make the distinction between a scenario and a prediction/forecast. (Which is something you’ll find many exasperated IEA representatives trying to emphasize at this time of year!)
I’m certainly not here to defend the IEA, their assumptions or their modelling, but I am always keen to investigate a bit deeper into such debates. So I took it upon myself to examine the full dataset within WEO2019.
The IEA actually include within WEO2019 not just a “stated policies scenario”, but also a “current policies scenario” and a “sustainable development scenario”. So, for this week’s chart, I plotted the solar growth rate data provided in all three of these scenarios against the historic trend, producing the top chart.
The weird stepped nature of the chart is simply because the IEA only give future total solar capacities for each scenario in 5 year increments (2025, 2030, 2035 and 2040), so those stepped growth rates reflect averages over those periods. Since that’s clearly unrealistic – but mainly to produce a more visually accessible chart – I applied a quick-but-rather-unscientific smoothing to produce the second version from the exact same data.
The scenario vs. prediction discussion is a much longer one, along with what’s behind the different scenarios: for another time and a much wordier article.
But let’s at least leave with a positive thought. Comparing with what’s actually been happening, I would suggest we consider the “sustainable development scenario” as representing a much more likely future than the “stated policies” one that the IEA and others often seem to focus on. And, given the other outcomes that emerge from this scenario (in terms of emissions impacts and so on), let’s hope I’m right!