Towards an Improved Understanding of the Global Energy Balance: Temporal Variations of Solar Radiation in the Climate System

Abstract

The spatiotemporal distribution of absorbed solar radiation in the Earth’s climate system is a fundamental driver of the Earth’s climate. Today, the global long-term annual mean energy fluxes of the global energy balance are reasonably well known. In this thesis, we go beyond the long-term annual mean perspective and focus on temporal variations in the energy balance. Specifically, we study annual cycles and multidecadal changes of the shortwave fluxes of the energy balance in an observation-based framework. To this end, we combine observations of top-of-the-atmosphere (TOA) radiative fluxes and surface albedo, both obtained from satellites, with surface flux observations. The latter are available from in-situ or satellite observations. In-situ observations are the most robust and accurate way to determine surface solar radiation but their spatial availability is limited. Satellite-derived surface radiation is less accurate but has greater spatial coverage. In this thesis, we use both data sources depending on whether accuracy or spatial coverage is needed. Using in-situ data – which from a spatial perspective is point data – in combination with other gridded data products requires a thorough assessment of potential methodological uncertainties which arise when combining those two data sets. In Chapter 2 and Chapter 3 of this thesis, different aspects of spatiotemporal representativeness of monthly mean records of surface solar radiation point observations are assessed globally. Although large regional differences in the representativeness of point measurements occur, in most regions they can be considered representative of a larger surrounding. Therefore, examining temporal variations in the shortwave energy fluxes by combining in-situ and colocated gridded flux data is feasible. However, additional uncertainties must be taken into account. Due to the Earth’s celestial movement around the Sun, large seasonal cycles in the amount and geographic distribution of solar insolation occur. These variations play a key role in the distribution of energy in the climate system and ultimately drive the seasonality of Earth’s general circulation systems, climate, and weather. Nevertheless, as recently shown by Hakuba et al. (2014b), the fraction of the incoming shortwave radiation at the top-of-the-atmosphere which is absorbed within the atmosphere does not substantially vary with season or latitude in Europe. An astonishing result, when considering that various climate elements which potentially influence the shortwave radiative fluxes have notable annual cycles. In Chapter 4 we go beyond Europe and identify several regions with large annual cycles in fractional atmospheric absorption. We find that they are driven mainly by annual cycles of water vapor and different aerosol species. The largest annual cycles in fractional atmospheric absorption are apparent in regions with large annual cycles in aerosol loading related to seasonal biomass burning, which is a strong indication that these annual cycles are anthropogenically intensified or even entirely forced. We also show that the skill of global climate models to simulate the observed patterns in the annual cycles of atmospheric shortwave absorption is limited. It is well known from observations that significant widespread variations in the amount of solar radiation reaching the Earth’s surface occurred on decadal time scales, known as global dimming and global brightening. According to the principle of energy conservation – the foundation of the energy balance – such changes in surface solar radiation must go along with a change in surface absorption, the TOA net flux, or the atmospheric absorption. In the observation-based energy balance framework, we find that changing atmospheric shortwave absorption is a major cause for the observed trends in Europe and China. The simultaneous trend analysis for surface, atmospheric, and TOA fluxes suggests that changes in aerosols – and in particular absorbing ones – are responsible for the observed trends. Taken together, this thesis demonstrates that an integrative approach to the shortwave energy balance, in which TOA, atmospheric and surface fluxes are simultaneously analyzed, can lead to valuable insights into key aspects of the climate system.