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1.3 The Sun-Earth connection, on scales from minutes to millennia

The Sun affects the terrestrial environment in many different ways, and is its main energy source, continuously providing 2 · 10 W. The main mechanisms by which this energy enters the atmosphere are illustrated in Figure 1. Energy-wise, solar radiative forcing in the visible, near-ultraviolet and infrared bands constitutes by far the dominant input into the Earth’s environment, and represents over 99% of all the natural forcings. However, if we consider instead the variable contribution of each input, then a completely different picture emerges, in which several seemingly weak mechanisms end up having significant leverage too. For example, the amplitude of the variations observed in the ultraviolet band becomes comparable to that observed in the visible band. Likewise, the energy input associated with energetic electrons that precipitate intermittently into the upper atmosphere cannot be neglected anymore. However, the impact of each of these solar inputs on the terrestrial environment also strongly depend on what time-scale they are operating. As it turns out, these time scales cover a wide range, and our simple energy budget reveals one side only of the full story. Most Sun-climate investigations proceed by comparing a solar driver at one particular time scale, and looking for a possible climate response at that same time scale, with the hope of using their possible correlation to disentangle the multiple mechanisms that could lead to such a response. Scientists have been hunting down for cycles, because these can be more easily identified in noisy observations. This systemic approach wherein the response of a system (the Earth’s environment) to a small perturbation by an external driver (solar variability) is studied, is widely used in many disciplines. However, for a system that is as complex and nonlinear as the Sun-Earth system, unexpected results may show up.