Predicted global warming is expected to raise the water temperatures 2–6° C after doubling of atmospheric CO2. As an extreme coldwater fish species the Arctic charr appear to avoid water temperatures above 16–18° C. Arctic charr lives in Finland only in the mountain lakes of northernmost parts of the country and in some deep lakes of eastern Finland. Charr is most abundant at high altitudes. The global warming is expected to threaten first populations living in low altitude shallow lakes in which no thermal summer stratification occurs. Climatic warming will increase duration of high temperatures and thermal stratification in deep lakes. This indicates that Arctic charr will spend longer periods in minimum thermal habitats. Assuming that no factors other than temperature will change in future climates, reductions in summer habitat will probably lead to noticeable reductions in many charr populations. Populations living in deep lakes will survive better than those living in shallow lakes provided that the water quality of these lakes does not deteriorate.