Warming by the Sun and other variations in natural systems cannot explain global warming. Experiments using computer models confirm the importance of human-produced emissions in the temperature trends of recent decades. This graphic depicts global average temperature since 1890 as reproduced by the NCAR/DOE Parallel Climate Model.
The simulations that include only natural variability, including changes in the Sun and eruptions of volcanoes, show that we should have seen a decrease in the global average temperature in the last several decades.
The simulations that most closely resemble the observed record are the ones that take the cooling effect of air pollution and the warming effect of greenhouse gases into account.
Learn more about climate (NCAR)
What tools are used to study climate? (video explaining the complexity and reliability of the observations, prehistoric records, and climate models used to study past, present, and future climate; part 2 of Climate Future: Voices of Science)