Palsas are permafrost mounds in mires with a core of ice, widespread situated in the sporadic permafrost zone. A tendency towards decay of palsa mires since the second half of the 20th century has been observed in Fennoscandia. This thesis is investigating the lateral changes and the distribution of palsas in Finnmark by utilizing multiple aerial images from 1950s onwards and by Geomorphological Distribution Modelling. Aerial images in a north-south transect from Lakselv, Suossjavri and Goatheluoppal reveal a total decrease in areas of palsas by 48 %, 33 % and 71 %, respectively, wherease the rate of degradation has increased since the start of 2000. Signs of degradation on aerial images from the 1950s suggest that the tendency of decay started at latest in the 1950s, and probably already from the warming period in the 1920s-1930s. The most important factors for the increase in rate of degradation are most likely the increase in both temperature and precipitation observed in the last few decades. By utilizing Generalized Linear Model, the probability of presence of palsas increase with 1) decreasing freezing degree days, 2) a humped (nonlinear) curve of thawing degree days, 3) decreasing mean annual precipitation, 4) increasing mean summer precipitation, 5) increasing area of mire and 6) a humped (nonlinear) curve of area of water. Hierarchical Partitioning indicates that the climate variables are the most important group of variables to independently explain the distribution of palsas. The total area of palsas in Finnmark in 2010 based on GDM and aerial images is estimated to be roughly 0.3 % of the total area of Finnmark. By utilizing the degradation rate and the total area of palsas in Finnmark, the total amount of potential carbon gas release in form of CH4 from decay of palsas from 1960 to 2010 was estimated to be less than three times as much as the human emissions of CH4 in Finnmark for one year (2010), and thus of rather minor importance in the global carbon cycle.