The Intergovernmental Panel on Climate Change (IPCC, 2021) concluded that global solar radiation decreased from around 1960 to the late 1980s before increasing rapidly through the 1990s.
Observations in Japan show a similar pattern. Global solar radiation declined from the late 1970s to around 1990, increased markedly from around 1990 to the early 2000s, and has shown no clear changes since then. The averages of four JMA observation stations also designated as Baseline Surface Radiation Network (BSRN) stations show the same long-term characteristics, including a pronounced increase beginning around 1990. These variations are consistent with the global trends reported by IPCC.
Time-series representations of annual means of global solar radiation at four stations in Japan (Abashiri, Tsukuba, Ishigakijima and Minamitorishima), together with four-station averages (black line) and related five-year running means (red line)
Annual means are defined as the average of monthly means, calculated only for months with 20 days or more of daily total observations. Before March 2010 (February 2021 at Abashiri; December 1987 at Tsukuba), observations were global pyranometer-based, while values have since been derived from the sum of direct solar irradiance measured with a pyrheliometer and diffuse solar irradiance measured with a pyranometer (since April 2010; March 2021 at Abashiri; January 1988 at Tsukuba).
Downward infrared radiation has been monitored since the early 1990s at Tsukuba. Analysis of observations from 1993 to 2025 indicates an increase of around 0.3 W/m2 per year (statistically significant at a 99% confidence level). This trend aligns with global results reported by IPCC (2021) and Wild (2016), as well as earlier analyses based on worldwide BSRN stations.
Similar increases are found at domestic stations (Ishigakijima and Minamitorishima) where measurements have been conducted since 2010. The average trend of the three stations above from 2011 to 2025 also indicates an increase of around 0.4 W/m2 per year (also significant at a 99% confidence level).
As the Sapporo station was relocated to Abashiri in 2021, the series is not continuous and is excluded from multi station averaging.
Time-series representations of annual means of downward infrared radiation at four stations in Japan (Sapporo (prior to Abashiri), Tsukuba, Ishigakijima and Minamitorishima)
As reference, the annual means at Sapporo – which operated prior to Abashiri – are plotted as a dotted line for 2011–2019. The thick orange line denotes five-year running means computed from the Tsukuba series since 1993, the thick black line denotes annual means averaged over three stations (Tsukuba, Ishigakijima and Minamitorishima) since 2011, and the thick red line denotes five-year running means derived from this series of annual means.