How constant is the “solar constant?”

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Well…it’s complicated. The available data is less than perfect, but the ‘natural variation option’ (so to speak) is still on the table.

The IPCC lowered their estimate of the impact of solar variability on the Earth’s climate from the already low value of 0.12 W/m² (Watts per square-meter) given in their fourth report (AR4), to a still lower value of 0.05 W/m² in the 2013 fifth report (AR5), the new value is illustrated in Figure 1. These are long term values, estimated for the 261-year period 1750-2011 and they apply to the “baseline” of the Schwabe ~11-year solar (or sunspot) cycle, which we will simply call the “solar cycle” in this post. The baseline of the solar cycle is the issue since the peaks are known to vary. The Sun’s output (total solar irradiance or “TSI”) is known to vary at all time scales (Kopp 2016), the question is by how much. The magnitude of short-term changes, less than 11 years, in solar output are known relatively accurately, to better…