Can solar activity or other natural processes explain global warming?
No. The incoming solar radiation has been almost constant over the past 50 years, apart from the well-known 11-year solar cycle (Figure 5). In fact it has slightly decreased over this period. In addition, over the past three years the brightness of the sun has reached an all-time low since the beginning of satellite measurements in the 1970s (Lockwood and Fröhlich 2007, 2008). But this natural cooling effect was more than a factor of ten smaller than the effect of increasing greenhouse gases, so it has not noticeably slowed down global warming. Also, winters are warming more rapidly than summers, and overnight minimum temperatures have warmed more rapidly than the daytime maxima – exactly the opposite of what would be the case if the sun were causing the warming. Other natural factors, like volcanic eruptions or El Niño events, have only caused short-term temperature variations over time spans of a few years, but cannot explain any longer-term climatic trends (e.g., Lean and Rind 2008)