In the second half of the 20th century the process of global climate change has begun on the Earth, the atmosphere and the ocean have warmed up, snow and ice reserves decreased, the sea level rose, concentrations of greenhouse gases increased. An ongoing warming response has been recorded also in most of Northeast Asia. Climate and its change affect the life of the biota. The aim of this study is to reveal ongoing changes in the thermal regime in and around Magadansky Strict Nature Reserve, which is located in North-East Russia, in the Magadan Region. The nature reserve has four separate areas with maritime, sharply continental and transitional climates. Weather stations Magadan, Talon, Seimchan provide data that characterize the climate in the nature reserve. The annual duration of periods with mean daily temperatures above 5°C (growing season) were calculated for years 1953-2013 using long-term air temperature data series from the above weather stations. 30-year moving averages (tentative normals) were calculated from these series. Tentative winter season (with mean daily air temperatures steadily below 0°C) normals were determined similarly. The calculations and plotting were performed using Microsoft Excel spreadsheet software. Long-term series of mean annual air temperatures demonstrated statistically significant upward trends. Annual air temperatures averaged over 1984-2013 were 1.1-1.2 °C higher compared to the preceding 30-year period. This resulted in a shift of the onset and end of the growing season and winter season. Their tentative normal durations trended in the opposite directions. The average duration of the growing season increased by 4-6 days, whereas the average duration of the winter season decreased by 4-8 days. Tentative normal durations of the growing season and winter season correlate well with mean annual air temperature. Formulas were obtained for calculating normal durations of these seasons under different scenarios of mean annual air temperature rise in the 21st century.