The SOL-PHENO project (Quantification of Solar Irradiance Forcing by Combining Climate and Phenological Models) aims to use the dates of cherry blossom in Kyoto and Edo in Japan as a historical climate proxy to help modelling solar irradiance back to about the year 1500 in order to predict future solar irradiance forcing.
A grand solar minimum is predicted from statistical evaluations to occur within the next 50 to 100 years. There is no consensus on the magnitude of this natural influence on climate warming, but it can be expected that there will be a similar influence in the future as in the past during the Little Ice Age (coinciding with the last grand solar inactivity phase, the Maunder Minimum). Unfortunately, there is also no commonly agreed assessment of the global temperature reduction during the Maunder or Dalton minima.
For assessing the past minima with climate simulations, we know the relative solar irradiance variations from the solar activity proxies derived from the abundance of cosmogenic isotopes in ice cores or tree rings. However, there is an apparent lack of knowledge about the absolute value of the solar forcing as different hypotheses on the underlying solar physics processes lead to vastly different estimates of this crucial value. Local historical climate variations can be reconstructed from climate proxies, which in several cases indicate clear cold periods within the Little Ice Age.
The most attractive historical climate proxy for assessing the solar influence on climate is the date of cherry blossom in Kyoto and Edo in Japan. This proxy has an excellent time coverage back to 1500, can be modeled reliably from climate data using a biogenic phenology model, and shows very high correlation with the relative variations of the solar activity. Thus, the timing of cherry blossom in the Kyoto and Edo areas is excellent for assessing the historical solar influence on climate.
Figure 1. TSI variability over the last 500 years is illustrated in comparison with average March temperatures in Kyoto and Edo (Tokyo). The comparison reveals a remarkably similar overall pattern and is highly indicative that historic terrestrial temperatures are influenced by variability in solar irradiance.