ABSTRACTThis study begins with a short review of the climato-logical evidence which has been presented of a warming of the Northern Hemisphere climate during the period 1880 to 1950. It is shown that winter temperatures in general increased continously from 1880 to 1940 over most of the hemisphere and particularly in high latitudes. There was however a period of slow or no increase in temperatures from 1910 to 1920. Summer temperatures showed a slight increase from 1880 to 1910, a decrease from 1910 to 1920 and then a continous increase until 1950. Key areas of strong temperature increase in winter have proved to be the Arctic, northern Scandinavia and USSR, southern Canada and most of the USA. Similar areas of strong warming in summer have proved to exist in Scandinavia, USSR, northern USA and southern Canada. Hence in many areas there was considerable warming both in winter and summer.For the whole Northern Hemisphere, the annual mean temperature increase from 1880 to 1950 was about 0.6°C while mean temperature in January rose from the coldest to the warmest decade by as much as 6°C in the Arctic and in parts of southern Canada and northern USA. July temperatures similarly rose 3°C in northern Scandinavia and in the Great Plains of the USA and Canada.In the subsequent part, it is demonstrated that after 1950 temperatures have shown much less of consistent trends. The 1950's were still reasonably warm both in summer and winter but the 1960's showed a definite cooling in both seasons. The 1970's again were in general warmer than the 1960's but the temperature level of the first half of the century was not reached.The general circulation conditions which lead to the warming of the Northern Hemisphere in the period 1880–1950 are also summarized. It is shown that from 1880 to 1920 the main cause was an increase in the temperature latitude zonal westerlies while from 1920 to 1940 the general circulation was more meridional with dominating southwesterly to south-easterly wind components, whereby the carrying of warm air into high latitudes was further enhanced.In the second main part of the study the impact of the Northern Hemisphere warming is discussed with particular reference to three key areas, the Arctic, northern Scandinavia and the Great Plains in northern America. It is shown that the warming had considerable consequences for sea temperatures in the northern Atlantic and Arctic waters which in turn led to impact on deep-sea fishing and sea transport in northern latitude areas. Cod catching and industry spred from Iceland waters all the way to Spitsbergen, along the east coast of Greenland and to Jakobs- havn on the Greenland west coast. Sea transport, north of Siberia, was normal throughout the winter in the late 1930's.The warming had a considerable impact on agriculture in northern Finland and in Canada due to the lengthening of the vegetation season. In the 1930's and 1940's settlers in both countries moved northwards into areas not hitherto used for agriculture. In Finland, these settlers experienced a catastrophic setback in the cold 1960's and had to abandon their homes. In Canada, the gains from the move towards the north in the 1930's were more than counterbalanced by the droughts that hit agricultural production in the more central parts of the Great Plains in the same period. Various socio-economic efforts to mitigate such adverse consequences of climate impact on agriculture have been undertaken in many areas situated on the climatic margin in the Northern Hemisphere during the last decades. It is concluded that many areas in the developing countries, where such efforts are not yet undertaken are still suffering from impact of climate fluctuations in a similar way that areas on the margin in the Northern Hemisphere suffered earlier in this century.