Trends in soil temperature in the Icelandic highlands from 1977 to 2019

The soil temperature trend at Hveravellir, a weather station in the Icelandic highlands, is investigated. Hveravellir is stationed in a barren landscape but the site itself is grass covered. The maritime climate of Iceland is warmer than expected for the latitude and the annual temperature at Hveravellir, at 641 m a.s.l., is 0.3 degrees C. Soil temperature has been measured at the site since 1965 at 10, 20, 50 and 100 cm depth but the data is not fully digitized. Here, the time series used span 42 years, from 1977 to 2019. Annual and seasonal trends are calculated as well as the annual number of thawing degree days (TDD) and freezing degree days (FDD). During this period, there has been a considerable warming in Iceland, not only because of global warming but also because of its start coinciding with the end of a local cold period. In the soil, warming is detected in all seasons except May to June, which is the melting season, the end of which varies greatly from year to year. The most warming is found during the autumn and winter months. These results contrast with those from continental cool sites where the largest warming has been seen in the spring months. The autumn cooling starts 2-3 weeks later now than at the start of the time series, resulting in the soil experiencing a longer summer. The number of TDD has increased and the number of FDD has decreased, both in air and in soil. At 100 cm depth TDD has increased by 80 degrees C day dec(-1) and FDD decreased by 38 degrees C day dec(-1). There is a correlation between the increase in air temperature and soil temperature, for every 1 degrees C increase in 2 m temperature the soil temperature at 100 cm has increased by 0.6 degrees C.