The sun is constant. The sky is not.
The solar output reaching the top of the atmosphere varies by about a tenth of a percent over a solar cycle: for planning purposes, the sun is a constant. What varies is the atmosphere between it and the panel, chiefly aerosols and the clouds they help form. The scientific record calls the resulting swings global dimming and brightening: measured surface sunlight fell several percent across the industrialised world from the 1950s to the 1980s as pollution rose, then recovered as clean-air laws took hold, a history documented in surface-radiation networks and reviewed by Wild and colleagues. The last two decades are the part of that history a solar buyer owns, and it is now measurable market by market from space.
Two instruments, one question
Our yield baseline carries two independent readings of surface irradiance for every market: NASA's MERRA-2 reanalysis, a physics-model reconstruction spanning 1994 to 2024, and NASA's CERES satellite radiometers, a direct measurement record from 2000. Comparing each instrument's early period with its late period asks the same question two different ways. MERRA-2, decade against decade:
And the same markets through CERES:
| Market | GHI 1994-2003 kWh/m²/day | GHI 2014-2024 | MERRA-2 change | CERES change | Agree |
|---|---|---|---|---|---|
| Berlin, Germany | 2.72 | 2.91 | +6.8% | +4.0% | yes |
| London, United Kingdom | 2.89 | 3.00 | +3.6% | +3.8% | yes |
| Santiago, Chile | 5.34 | 5.53 | +3.5% | +6.0% | yes |
| Athens (Attica), Greece | 4.74 | 4.89 | +3.3% | +2.4% | yes |
| Perth, Australia | 5.56 | 5.61 | +1.0% | +0.7% | yes |
| Riyadh, Saudi Arabia | 6.21 | 6.11 | -1.5% | -1.5% | yes |
| Shanghai, China | 3.86 | 3.76 | -2.6% | +0.5% | no |
| Jakarta, Indonesia | 4.89 | 4.75 | -2.9% | -3.8% | yes |
| Reykjavik, Iceland | 2.33 | 2.24 | -4.1% | -4.2% | yes |
| Jodhpur (Rajasthan), India | 5.82 | 5.47 | -6.1% | -4.0% | yes |
Solar Analytica yield baseline: MERRA-2 decade means (1994-2003 vs 2014-2024) and CERES period means (2000-2009 vs 2020-2024), per market. Full 40-market table in the public dataset. Set averages: +0.4% (MERRA-2), +0.5% (CERES); 26 markets brightening, 14 dimming; direction agreement in 29 of 40.
The pattern is not random. The strongest brightening sits over Europe (Berlin +6.8%, London +3.6%, Paris +3.5%), exactly where aerosol emissions have fallen hardest since the 1990s, with Santiago beside them as Chilean air quality improved. The strongest dimming sits over South Asia (Jodhpur -6.1%, the same market whose local temperature trend ran negative in part 01: hazier skies are both dimmer and cooler, and the two instruments and two variables tell one coherent story). Reykjavik and Jakarta dimmed on both instruments. Where the instruments disagree on direction (Shanghai most notably, with Antofagasta and Lhasa marginal), this report treats the trend as unresolved rather than picking the convenient reading.
What a percent of sky is worth
Irradiance converts to output almost one for one: a panel in a sky delivering 5% more light produces close to 5% more energy, before any other term in the yield model moves. That makes these trends directly comparable with part 01's warming penalty, and the comparison is lopsided. Berlin's observed two-decade brightening (+6.8%) is roughly eight times the heat loss that warming is modelled to add there by year 25 (-0.83%), the largest such penalty in the set. In Jodhpur the sign flips: the dimming (-6.1% over two decades) is forty times its modest heat story, and it, not temperature, is the climate variable a Rajasthan yield model should worry about. Perth's +1.0% sits with the south-west WA drying trend documented in the series' rainfall work: fewer clouds mean drier panels and brighter skies, the same shift read through two variables.
The buyer action is about data vintage, not technology. Ask which years your yield estimate is built on. An estimate resting on a 1990s-era typical-year dataset understates Berlin by several percent and overstates Jodhpur by more; a recent-decade basis (2014-2024) prices the sky you are actually buying under. In brightening markets an older estimate is quietly conservative, which is a pleasant surprise to receive but a poor way to plan; in dimming markets it is optimistic, which is worse. And in the three disagreement markets, treat any single-source trend claim with suspicion, because the two best instruments in the world do not currently agree there.
Method and limits
The figures compare period means within each instrument separately: MERRA-2's 1994-2003 decade against its 2014-2024 decade, and CERES's 2000-2009 against its 2020-2024. Four limits matter. The CERES late window is five years against ten, so it is noisier. A two-decade change is an observation, not a forecast; aerosol policy, which drives much of it, can move in either direction, and Asian clean-air programs could flip today's dimming markets the way Europe's did. Percent changes of a few points sit within the reach of cloudy-decade chance in some markets, which is exactly why two independent instruments are shown and sign-disagreements are flagged rather than resolved. And these are horizontal-irradiance trends: tilt, temperature, soiling and smoke (parts 01, 05 and 06) sit on top. Every input series is public, and the per-market values can be recomputed from the named NASA products.