Analyzing changes in the complexity of climate in the last four decades using MERRA-2 radiation data

Abstract

the energy balance of the earth is controlled by the shortwave and longwave radiation emitted to space. Changes in the thermodynamic state of the system over time affect climate and are noticeable when viewing the system as a whole. In this paper, we study the changes in the complexity of climate in the last four decades using data from the Modern-era Retrospective analysis for Research and Applications, Version 2 (MERRA-2). First, we study the complexity of the shortwave and longwave radiation fields independently using Approximate Entropy and Sample Entropy, observing that the rate of complexity change is faster for shortwave radiation. Then, we study the causality of those changes using Transfer Entropy to capture the non-linear dynamics of climate, showing that the changes are mainly driven by the variations in shortwave radiation. the observed behavior of climatic complexity could be explained by the changes in cloud amount, and we research that possibility by investigating its evolution from a complexity perspective using data from the international Satellite cloud climatology Project (ISCCP). Climate is a complex system 1 composed of many different processes operating at all spatiotemporal scales, showing non-linear relations between its variables in long time spans 2 or its acceleration in the last four decades 3. Understanding the changes in these non-linear relations is of the utmost importance for understanding current climate change and its implications for the future 4,5. Great efforts have been made in the last years to characterize those variations due to its importance for future abrupt changes in the structure of climate, being it potentially harmful to biodiversity and society 6,7. The atmosphere of the Earth can be considered as a closed non-linear thermodynamic system which basically only exchanges radiation with its environment 8. The solar radiation flux maintains the atmosphere in a non-equilibrium thermodynamics situation, generating a climate dynamics which has an influence on the temperature of the planet. The energy balance is modulated by the incoming solar radiation and the outgoing shortwave and longwave Earth's radiation. Changes in the energy balance of the atmosphere are the drivers of climate change. Nowadays, it is believed that the Earth absorbs more energy from the Sun than it is emitted to the space, creating a situation of energy imbalance 9,10. Although those differences can be monitored from the ground, the use of satellite sensors to provide measures of the fluxes of energy to and from Earth offers a more precise view of the system 11. The evolution of the energy imbalance has been investigated using satellite data and models, and it seems to be increasing in the last decades 12,13. While those changes have been argued to be due to human activities 14 , the questions of which of the parameters are changing faster and if that is affecting the dynamics of the system are still open. When reaching the Earth, part of the incoming solar radiation is reflected off clouds and the...

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