{'fraction_non_alphanumeric': 0.02173069460613116, 'fraction_numerical': 0.0038804811796662787, 'mean_word_length': 5.78421052631579, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
On the Origin of the Surface Air Temperature Difference between the Hemispheres in Earth's Present-Day Climate

In today’s climate, the annually averaged surface air temperature in the Northern Hemisphere (NH) is 18‐28C higher than in the Southern Hemisphere (SH). Historically, this interhemispheric temperature differencehas beenattributedtoanumberoffactors,includingseasonaldifferencesin insolation,the largerarea of (tropical) land in the NH, the particularities of the Antarctic in terms of albedo and temperature, and northwardheat transportby ocean circulation. A detailedinvestigation ofthese factors andtheir contribution to the temperature difference, however, has to the authors’ knowledge not been performed so far. Here the origin of the interhemispheric temperature difference is traced using an assessment of climatological data and the observedenergybudget ofEarth as well as modelsimulations. It is found that for thepreindustrialclimate the temperature difference is predominantly due to meridional heat transport in the oceans, with an additional contribution from the albedo differences between the polar regions. The combination of these factors (that are to some extent coupled) governs the evolution of the temperature difference over the past millennium. Since the beginning of industrialization the interhemispheric temperature difference has increased due to melting of sea ice and snow in the NH. Furthermore, the predicted higher rate of warming over land as compared to the oceans contributes to this increase. Simulations for the twenty-first century show that the interhemispheric temperature difference continues to grow for the highest greenhouse gas emission scenarios due to the land‐ocean warming contrast and the strong loss of Arctic sea ice, whereas the decrease in overturning strength dominates for the more moderate scenarios. ‘‘In few departments of Natural Philosophy have Philosophers differed more widely in opinion, than in the comparison of the temperatures of the two hemispheres.’’ (Harvey 1834, p. 29). Indeed, the scientific debate on the surface air temperature difference between Earth’s hemispheres and its causes has a long and rich history that can be traced back to the beginning of the sixteenth century. When the early navigators explored the higher latitudes of the Southern Ocean, their reports suggested cooler conditions than the ones found at equivalent latitudes in the Northern Hemisphere (NH). Therefore, the notion of the Southern Hemisphere (SH) being much colder than the Northern Hemisphere on annual aver