Special issue statement. This article is part of a special issue published on the occasion of the 70th anniversary of E&G Quaternary Science Journal (EGQSJ). The special issue celebrates the journal's notable contribution to Quaternary research by revisiting selected milestone articles published in the long history of EGQSJ. The German Quaternary Association (DEUQUA) presents translations of the originals and critical appraisals of their impact in tandem anniversary issues of DEUQUASP and EGQSJ, respectively.

Original article: https://doi.org/10.3285/eg.14.1.17

Translation: https://doi.org/10.5194/deuquasp-3-61-2021

At the beginning, Flohn's paper discusses the interaction of the meteorological flow regimes with the glaciation of the Northern Hemisphere, as well as the contribution of these flow regimes to the buildup of the glaciers and their the melting. The general assumption in 1963 was that during the glacials and interglacials the jet stream moves south- and northwards, respectively. The climate zones are shifted synchronously with the glaciation events, linked with a shift of pluvials. However, as Flohn pointed out, geological evidence indicates that this is not consistently the case. He raises the question of whether these inconsistencies are caused by uncertainties in the age determination or if a mechanism has been overlooked that causes regional pluvials to develop independently from the glaciation state. Nowadays changes in the large-scale precipitation pattern are attributed mainly to a shift in the Intertropical Convergence Zone (ITCZ), which follows the changes in the solar insolation induced by variations in the orbital parameters (Milankovic-cycles; Berger, 1978). These insolation changes are also seen as the main driver leading to glaciations (Kaspar and Cubasch, 2007; Wanner et al., 2008). The mechanisms Flohn had found missing are feedback processes, which according to current knowledge influence the formation of a regional wet climate. One feedback mechanism that has been known for a long time in agricultural meteorology consists of the feedback loop in which enhanced precipitation leads to more vegetation, which in turn reinforces the evaporation, which then leads to more precipitation. Its importance for the large-scale climate, however, was only discovered more than 30 years later by Claussen et al. (1999). He proved in model experiments that the vegetation cover of the Sahara is strongly dependent on this vegetation feedback. Flohn assumes that during some of the interglacials the Arctic was free of sea ice. He deduces that this reduces the meridional temperature gradient in the Northern Hemisphere and that this would lead to a stronger meridionalization of the airflow. This assumption is still followed today: with enhanced global warming, the Arctic ice cover is dwindling in the present. The IPCC projections for the high climate change scenarios indicate that the Arctic ice will almost completely disappear within the next 100 years (IPCC, 2013). It has been found by Francis und Vavrus (2015) that already now the decrease in the Arctic ice coverage leads to a diminished Equator–North pole temperature contrast, which induces a wavier jet stream. Flohn calculated that the changes in the radiative balance alone would not have been effective enough for the glacier melt and deduced that during the late- and post-glacials, the changes in the large-scale circulation must have enhanced the advection of warm air. He estimated that, in general, the available heat within the climate system is sufficient to explain the melting of the glaciers and the induced sea level rise. At a first glance, it is a bit surprising that Flohn focused on the atmospheric circulation and does not mention at all the contribution of the heat advection by oceanic currents. However, if one takes the standard German textbook on Oceanography (Dietrich et al., 1975) as an indicator of the state of knowledge of that time (12 years after Flohn's paper was published), only a very limited number of locally confined estimates of the heat transport by the Gulf Stream (some of them estimated during the 1930s) can be found but no comprehensive portrayal. It took more than a quarter of a century after Flohn's work until Broecker (1991) summarized the structure of three-dimensional oceanic circulation and its heat and freshwater transport in terms of a “great conveyor belt circulation”. Flohn presents quantitative estimates of the amount of freshwater melting (freezing) during a deglaciation (glaciation). The order of magnitude of the glacier melting rates and the sea level change derived from the mass balance agree with the present-day estimate published by the IPCC (2013). Flohn points to the fact that large quantities of meltwater have to be transported in huge rivers. However, he does not go so far as to speculate what impact these freshwater masses would have on the oceanic circulation since, as mentioned before, little was known about the oceans in 1963. The idea that the amount of freshwater flowing into the North Atlantic ocean could lead to a change in the Gulf Stream circulation emerged about 20 years later after the analyses of sediment and ice cores (Broecker et al., 1985). It was later verified in model studies by Stocker und Wright (1991) and Rahmstorf (1995).

The article processing charge was funded by the Quaternary scientific community, as represented by the host institution of EGQSJ, the German Quaternary Association (DEUQUA).

This paper was edited by Christine Thiel and reviewed by one anonymous referee.