The Role of Natural Fluctuations and Factors of External Forcing in Early 20th Century Warming in the Northern Hemisphere.

The warming in the first half of the 20th century in the Northern Hemisphere (NH) (early 20th century warming (ETCW)) was comparable in magnitude to the current warming, but occurred at a time when the growth rate of the greenhouse gas (GG) 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 ETCW was most pronounced in the high latitudes of the NH, 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 NH. Assessing the contributions of internal variability and external natural and anthropogenic factors to this climatic anomaly is key for understanding historical and modern climate dynamics. This paper considers mechanisms of ETCW 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 NH during the period of instrumental observations does not allow one to attribute the ETCW to one particular mechanism of internal climate variability or external forcing of the climate. Most likely, this event was caused by a combined effect of long-term climatic fluctuations in the North Atlantic and the North Pacific with a noticeable contribution of external radiative forcing associated with a decrease in volcanic activity, changes in solar activity, and an increase in GG concentration in the atmosphere due to anthropogenic emissions. Furthermore, this climate variation in high latitudes of the NH has been enhanced by a number of positive feedbacks. An overview of existing research is given, as are the main mechanisms of internal and external climate variability in the NH in the early 20th century. Despite the fact that the internal variability of the climate system is apparently the main mechanism that explains the ETCW, the quantitative assessment of the contribution of each factor remains uncertain, since it significantly depends on the initial conditions in the models and the lack of instrumental data in the early 20th century, especially in polar latitudes. [ABSTRACT FROM AUTHOR]