Article Atmospheric Temperature and CO2: Hen-Or-Egg Causality? Demetris Koutsoyiannis 1,* and Zbigniew W. Kundzewicz 2 1 Department of Water Resources and Environmental Engineering, School of Civil Engineering, National Technical University of Athens, 157 80 Athens, Greece 2 Institute for Agricultural and Forest Environment, Polish Academy of Sciences, 60-809 Pozna´n,Poland; [email protected] * Correspondence: [email protected] Received: 7 September 2020; Accepted: 16 November 2020; Published: 25 November 2020 Abstract: It is common knowledge that increasing CO2 concentration plays a major role in enhancement of the greenhouse effect and contributes to global warming. The purpose of this study is to complement the conventional and established theory, that increased CO2 concentration due to human emissions causes an increase in temperature, by considering the reverse causality. Since increased temperature causes an increase in CO2 concentration, the relationship of atmospheric CO2 and temperature may qualify as belonging to the category of “hen-or-egg” problems, where it is not always clear which of two interrelated events is the cause and which the effect. We examine the relationship of global temperature and atmospheric carbon dioxide concentration in monthly time steps, covering the time interval 1980–2019 during which reliable instrumental measurements are available. While both causality directions exist, the results of our study support the hypothesis that the dominant direction is T CO . Changes in CO follow changes in T by about six months on ! 2 2 a monthly scale, or about one year on an annual scale. We attempt to interpret this mechanism by involving biochemical reactions as at higher temperatures, soil respiration and, hence, CO2 emissions, are increasing. Keywords: temperature; global warming; greenhouse gases; atmospheric CO2 concentration Póτερoν ἡ ὄρνις πρóτερoν « τὸ ᾠὸν ἐγèνετo (Which of the two came first, the hen or the egg?). Pλoύταρχo&,Hθικά, Sυµπoσιακὰ B, Pρóβληµα G (Plutarch, Moralia, Quaestiones convivales, B, Question III). 1. Introduction The phrase “hen-or-egg” is a metaphor describing situations where it is not clear which of two interrelated events or processes is the cause and which the effect. Plutarch was the first to pose this type of causality as a philosophical problem using the example of the hen and the egg, as indicated in the motto above. We note that in the original Greek text, “ἡ ὄρνι&” is feminine (article and noun), meaning the hen rather than the chicken. Therefore, here, we preferred the form “hen-or-egg” over “chicken-or-egg”, which is more common in English (Very often, in online Greek texts, e.g., [1,2], “ἡ ὄρνι&” appears as “ἡ ἄρνι&”. After extended searching, we contend that this must be an error, either an old one in copying of manuscripts, e.g., by monks in monasteries, or a modern one, e.g., in OCR. We are confident that the correct word is “ὄρνι&”). The objective of the paper is to demonstrate that the relationship of atmospheric CO2 and temperature may qualify as belonging to the category of “hen-or-egg” problems. First, we discuss the relationship between temperature and CO2 concentration by revisiting intriguing results from Sci 2020, 2, 83; doi:10.3390/sci2040083 www.mdpi.com/journal/sci SciSci2020 2020, ,2 3,, 83 x FOR PEER REVIEW 22 of of 33 33 data-based palaeoclimatic studies, where the change in temperature leads and the change in CO₂ proxyconcentration data-based follows. palaeoclimatic Next, we studies, discuss where the data thebases change of in modern temperature (instrumental) leads and measurements the change in COrelated2 concentration to global temperature follows. Next, and we atmospheric discuss the CO databases₂ concentration of modern and (instrumental) introduce a methodology measurements to relatedanalyse to them. global We temperature develop a stochastic and atmospheric framework, CO2 introducingconcentration useful and notions introduce of time a methodology irreversibility to analyseand system them. causality We develop while a stochastic we discuss framework, the logical introducing and technical useful notionscomplications of time irreversibilityin identifying andcausality, system which causality prompt while us we to discussseek just the necessary, logical and rather technical than complicationssufficient, causality in identifying conditions. causality, In the whichResults prompt section, us we to examine seek just the necessary, relationship rather of th thanese sutwoffi cient,quantities causality using conditions.the modern Indata, the available Results section,at the monthly we examine time the step. relationship We juxtapose of these time two se quantitiesries of global using temperature the modern and data, atmospheric available at CO the₂ monthlyconcentration time step. from We several juxtapose sources, time series covering of global the temperature common andtime atmospheric interval 1980–2019. CO2 concentration In our frommethodology, several sources, it is the covering timing therather common than the time ma intervalgnitude 1980–2019. of changes In that our is methodology, important, being it is thethe timingdeterminant rather thanof causality. the magnitude While oflogical, changes physic thatally is important, based arguments being the support determinant the “hen-or-egg” of causality. Whilehypothesis, logical, indicating physically that based both arguments causality directions support the exist, “hen-or-egg” interpretation hypothesis, of cross-correlations indicating that of both time causalityseries of directionsglobal temperature exist, interpretation and atmospheric of cross-correlations CO₂ suggests ofthat time the series dominant of global direction temperature is T → CO and₂, atmospheric CO suggests that the dominant direction is T CO , i.e., the change in temperature leads i.e., the change2 in temperature leads and the change in! CO₂ concentration2 follows. We attempt to andinterpret the change this latter in CO mechanism2 concentration by noting follows. the We posi attempttive feedback to interpret loop—higher this latter temperatures mechanism by increase noting thesoil positive respiration feedback and, hence, loop—higher CO₂ emissions. temperatures increase soil respiration and, hence, CO2 emissions. 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