Climate Oscillations 3: Northern Hemisphere Sea Ice Area
20 hours ago Andy May 34 Comments
By Andy May
Northern Hemisphere sea ice area is an important climatic indicator because it determines how much of the Arctic Ocean and surrounding seas are open to the atmosphere. Ice is a good insulator and traps heat in the water below it (Peixoto & Oort, 1992, p. 361). Ice is also a good reflector of sunlight (high albedo), whereas water is a good absorber (low albedo). While we have no accurate data on Northern Hemisphere sea ice area (called NH_ice here) before 1978, the first year of good satellite data, it does appear to follow the global 60-70-year global climate oscillation (Wyatt, 2020). This may be because the closely related AMO affects the sea ice area as it warms and cools, of course the reverse could also be true.
Prior to 1978 we have reports on ice extent, which is weakly related to sea ice area, from ship observations. Sea ice extent was very low in the early 20th century warm period from about 1920 to 1945, this was also within the early 20th century rise in the AMO, which is analogous to the AMO rise that began in 1977, see here and here. Ship observations frequently mention an ice-free Arctic Ocean from the 1850s through the 1870s, another time of a warming AMO. Silas Bent proposed that the Atlantic Gulf Stream met the warm Pacific Kuroshio current in the Arctic and kept it ice free in 1872 as shown in figure 1 (Luedtke, 2015).
Systematic observations of Arctic ice did not begin until 1885 when the Danish Meteorological Institute (DMI) began to systematically make maps of ice extent using reports from “explorers, whalers, and other navigators of the polar seas.” These reports were published in both Danish and English (Luedtke, 2015). However, by then the AMO was entering a negative state.
Figure 1. Silas Bent’s map of a supposed open Arctic Ocean in 1868. Source: (Luedtke, 2015)
By the mid-thirties, during the early 20th century warm period, which was also a prominent AMO warm period, the Arctic Northeast Passage was unusually open for ordinary steamships (Luedtke, 2015). This part of the Arctic Ocean, around Svalbard, Novaya Zemlya, and Franz Josef Land (see figure 2) is usually closed by ice most of the year.
https://wattsupwiththat.com/2025/06/24/climate-oscillations-3-northern-hemisphere-sea-ice-area/
