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Arctic Sea Ice and the Mean Temperature of the Northern Hemisphere

The importance of snow cover and ice extent in the Northern Hemisphere was recognized by various authors leading to a positive feedback of surface reflectivity on climate. In fact, the retreat of Arctic sea ice is accompanied by enhanced solar input in the Arctic region, i.e. a decrease of the terrestrial albedo. We have studied this effect for the past six decades and estimate the corresponding global warming in the northern hemisphere. A simple 1-dimensional model is used that includes the simultaneous increase of the greenhouse gases. Our results indicate that the latter directly cause a temperature rise of only 0.2 K in 1955 to 2015, while a notably larger effect 0.7 +/- 0.2 K is found for the loss of Arctic sea ice in the same time. These numbers comprise most of the reported mean temperature rise of 1.2 +/- 0.2 K of the northern hemisphere. The origin of the sea-ice retreat is discussed, e.g. internal variability or feedback by the CO2 concentration increase. Our data also suggest a delayed response of the global surface temperature rise to the loss of sea ice with a time constant of approximately 10 to 20 years.

sea-ice retreat is discussed, e.g. internal variability or feedback by the CO 2 concentration increase.
Our data also suggest a delayed response of the global surface temperature rise to the loss of sea ice with a time constant of approximately 10 to 20 years.
KEYWORDS Global warming, Arctic sea ice, albedo change INTRODUCTION: In the Arctic a tremendous loss of sea ice occurred in the past decades accompanied by a warming larger than the global average known as Arctic amplification (1)(2)(3)(4)(5)(6). The trend is continuing and may lead to an ice-free Arctic within the next few decades. The prospect has received great scientific attention and public interest (7)(8)(9). Various publications arrived at the conclusion that the ice loss originates only in part from man-made factors. A contribution of 30 to 50 percent was estimated for internal variability (3). The surface temperature of the earth, on the other hand, depends on various natural and anthropogenic factors that affect the solar input and the infrared reemission of the earth. Examples are volcanic activities, oceanic temperature profiles and the concentration of the greenhouse gases (GHG) (10)(11)(12)(13)(14). An interesting question in this context is the causal relationship between ice-loss and global warming. In the following we elucidate the role of the polar ice cover for the mean surface temperature T surf of the Northern Hemisphere (NH). We present arguments that the global temperature rise is to a larger extent the direct consequence of the sea-ice retreat than vice versa.
Our discussion is based on reported data of Arctic sea ice as empirical facts. Since 1979 results on the maximum and minimum ice areas are available from satellite observations including the phase shift relative to the seasonal solar input. The data are depicted in Fig. 1 supplemented by linear fits (red points and straight solid lines) (15)(16)(17). The ice maxima in winter decline from 14.90 to 13.47·10 12 m 2 while the September minima drop from 5.59 to 3.07·10 12 m 2 in the years 1979 to 2015. The loss of the sea ice area is noteworthy. The data of the ice areas are extended back to 1955 (note fitted red lines) by the help of auxiliary information on the sea ice extent (18,19) that are available also for earlier years. The reported annual average sea ice extent is depicted in the i.e. practically ice-free Arctic (20). Taking into account the smaller backscattering of sea water compared to snow-covered ice for incident sun light (factor of 0.5) and averaging over the daily and seasonal variation of the solar input, the net input of solar radiation is estimated to grow by 3.22·10 14    10 to 20 years is reasonable for the equilibration by heat transfer between the pole area and total NH system. We mention that the warming of the Arctic is reported to be twice the global average during the past three decades. The difference presents evidence for more direct warming of the pole region and delayed equilibration with the NH-system.
The relationship between Arctic warming and sea-ice loss is not well understood (28).
Some authors point out that it is partly due to natural reasons (29,30). There is evidence for climate changes in past centuries evolving on time scales of hundreds of years, accompanied by distinct changes of the Arctic sea ice (31)(32)(33). Climate-model simulations of the sea-ice retreat differ substantially (34). The loss of sea ice was related to the North Atlantic Oscillation Index (35,36) and the Atlantic Driver (37).
It was also argued that ocean currents appear as the only possible source for the delivering of thermal energy to the Arctic, since the warming started suddenly at the end of a two thousand years cooling period so that an equally sudden cause should be required -not supplied by the greenhouse effect (38). On the other hand, evidence was reported very recently for a 60% contribution of summertime atmospheric circulation to September sea ice loss (3). The authors conclude that 30-50% of the ice retreat may be due to internal variability. Notz and Stroeve reported that approximately 60% of the observed decline from 1979-2011 is externally forced and directly follows anthropogenic CO 2 emissions (34,39,). The finding may suggest that the temperature rise attributed in Fig. 2 to the ice retreat partially results from the GHG effect. As to further anthropogenic factors, the present authors wish to point to the precipitation of air pollution that is likely to enhance solar input on sea ice and glaciers (28,40,41).
Several authors discussed the feedback of surface reflectivity on climate (2,(42)(43)(44)(45). In this context, it is interesting to compare our results with data of Flanner et al. The difference may be due to the fact that the NH sea ice is declining faster than models simulate (2).
In conclusion, we wish to say that we have studied the causal influence of the shrinking Arctic sea ice and of the growing greenhouse gases on the global warming in the northern hemisphere. Our