Lövgrund

Celsius decided to look at rocks that had been documented to host seals, and therefore had a value for seal hunters. Seals love to get on rocks to bask under the sun, but given their mass, they can only access rocks that are flat and very close to the water level. Going along the shore of Sweden, he documented such seal rocks and questioned the local population about how long these rocks were used by seals. By analysing these data, he could figure out a rough rate of decrease, which he estimated to be 1.4 cm per year. This was a bit overestimated as the rate of decrease is between 0.8 cm and 1 cm along the sea of Bothnia.

Below is the rock with annotations from the marks found on it. The distance from the 1731 and the 1831 marks is 78 cm, corresponding to a rate of change of 7.8 mm, close to the current estimations. However, the mark between 1831 and 1931 is close to 10 cm less than the previous 100 years, questioning whether the rate of sea level decrease is changing over the years. However, thinking twice, it is known that the level of the sea in the Baltic is dependant upon other factors, in particular related to atmospheric pressure and strength and direction of the winds in the previous weeks. The difference due to these additional factors can account for several dozen centimetres, and therefore could explain that marks made 100 years apart may have been affected by these factors.

The rock, once a favourite place for seals, is now accessible from the shore.