Лëд и снег (Sep 2022)
The role of natural fluctuations and factors of external forcing in the Early 20<sup>th</sup> Century Warming in Northern Hemisphere
Abstract
The warming in the first half of the 20th century in the Northern Hemisphere, the Early Twentieth Century Warming was comparable in magnitude to the current warming but occurred at a time when the growth rate of the greenhouse gases concentration in the atmosphere was 4–5 times slower than in recent decades. The mechanisms of the early warming are still a subject of discussion. The Early Twentieth Century Warming was most pronounced in the high latitudes of the Northern Hemisphere, and the recent reconstructions consistently indicate a significant negative anomaly of the Arctic sea ice area during early warming period linked with enhanced Atlantic water inflow to the Arctic and amplified warming in high latitudes of the Northern Hemisphere. Assessment of contributions of internal variability and external natural and anthropogenic factors to this climatic anomaly is a key issue for understanding historical and modern climate dynamics. The paper overviews mechanisms of the Early Twentieth Century Warming associated with various internal variability and external anthropogenic and natural factors. An analysis of the findings on the topic of long-term studies of climate variations in the Northern Hemisphere during the period of instrumental observations does not allow one to attribute the Early Twentieth Century Warming to one particular mechanism of internal climate variability or external forcing of the climate. It is most likely, that this event was caused by a combined effect of long-term climatic fluctuations in the North Atlantic and the North Pacific Ocean with a noticeable contribution of external radiative forcing associated with a decrease in volcanic activity, changes in solar activity, and an increase of greenhouse gases concentration in the atmosphere due to anthropogenic emissions. Furthermore, this climate variation in high latitudes of the Northern Hemisphere was enhanced by a number of positive feedbacks.
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