More clouds and less sun in winter


A warmer climate will make winters cloudier and the sun will be seen more rarely. Changes will remain minor in summertime.

Winters become darker

The maps in figure 1 show changes in the amount of solar radiation for different seasons. In wintertime, both Finland and other parts of Northern Europe will be cloudier, and therefore less solar radiation will reach the surface of the earth. The drop would be around 10-15%.[1] The greyness of winters will be further emphasised by the fact that snow cover, that reflects light, becomes increasingly rare.

In unison with changes in radiation, the models also predict a distinct increase in wintertime cloudiness (no figure). Already at present, the cloud cover is around 80% in early winter. The predicted increase in cloud cover, by up to five percentage points, would thus decrease the share of clear sky by a quarter.

In summertime, Finland will probably have approximately as much sunshine as at present. However, the quantity of radiation would increase by 5-10% in large areas in Central Europe.

Results vary by model

Results from different models differ somewhat from one another. Figure 2 demonstrates differences between results from models. For instance in February, the best estimate, i.e. the median of different results, would result in a 13 per cent reduction in the quantity of radiation, and with 90% probability, the change will fall within the range -32% ... +5%.

In winter months, the models are reasonably unanimous on the reduction of sunshine — radiation will drop with the probability of 80-90%. For summer months, in turn, almost one half of the models predict a slight increase in the amount of radiation, while the rest indicate a drop. In September-October, the quantity of radiation would increase with the probability of 70-80%.

Figure 1. Projected percentual change in the quantity of solar radiation reaching the earth's surface (1971–2000 → 2070–2099) during the entire year on average (upper picture), in December-February (middle picture) and in June-August (lower picture) in the area of Europe and North Atlantic. The changes are calculated as a mean value of 18 global climate change model scenario A1B runs (fairly pessimistic greenhouse gas scenario).

Figure 2. Change in the quantity of solar radiation reaching the earth's surface (in per cent) in southwestern Finland for different months of the year, difference between the periods 2070-2099 and 1971-2000. The curve shows the median for changes projected by 18 global climate change models, the vertical beams the 90% probability interval calculated for the change. The calculation is based on greenhouse gas emission scenario A1B.