The moment most countries in Europe went into lockdown, marked the beginning of what was the sunniest spring on record in western Europe. In the Netherlands, spring 2020 was significantly sunnier than any year since the start of the measurements in 1928. The restrictions, put in place to stop the spread of COVID-19, put industry and traffic to a near standstill. With the human-induced air pollution drastically reduced, deep blue skies ensued. You can see observations of this in the figure below. Naturally, this raises the question: Did the lockdown cause the sunny weather?
Our Research Method
The answer to this question has implications for our understanding of climate change and for the growing solar energy sensor. To find an answer, we look at historical measurements of sunlight, air pollution, weather patterns and clouds. How exceptional were the atmospheric conditions really? Next, we run a model that calculates the amount of sunlight for March, April and May for different years, using the same observations. With the model, we can, for example, switch clouds on and off, and compare it to the effect of air pollution. This helps to put a number on the effect of the lockdown to the increase in sunlight.
We find that the weather patterns during spring 2020 were consistently favorable for clear and clean skies. The atmosphere was often too dry and stable for clouds to form, and common northerly winds brought in air with little pollution. The air on average was clean, but not much cleaner than in recent years. This is surprising, given how much the industry was impacted, but only further stresses the dominant role of the weather patterns.
Conditions for aircraft induced contrails to form were unfavorable, the air was often too dry at flight level. Even with that in mind, contrails were relatively rare compared to recent years, but we find that their influence in the total sunlight is insignificant. Their absence likely reduced the amount of scattered light, though, contributing a bit to the overall blueness of the sky.
Results from the modelling study indicate that the effect of the exceptional weather towards to record sunlight was more than 10 times as strong as the reduced air pollution. Our conclusion is therefore, that spring 2020 being exceptionally sunny is due to an unusual weather event, and not driven by a lack of contrails or air pollution. With or without a lockdown, spring 2020 shattered the records.
van Heerwaarden, C.C., Mol, W.B., Veerman, M.A. et al. Record high solar irradiance in Western Europe during first COVID-19 lockdown largely due to unusual weather. Commun Earth Environ 2, 37 (2021). https://doi.org/10.1038/s43247-021-00110-0