The United Nations International Panel on Climate Change (IPCC) produces a major scientic report involving up to 2500 scientists in the writing and reviewing process every 5th year. The IPCC Third Assessment Report (TAR) states: A climate model can be used to simulate the temperature changes that occur from both natural and anthropogenic causes. The simulations in a) were done with only natural forcings: solar variation and volcanic activity. In b) only anthropogenic forcings are included: greenhouse gases and sulfate aerosols. In c) both natural and anthropogenic forcings are included. The best match is obtained when both forcings are combined, as in c). Natural forcing alone cannot explain the global warming over the last 50 years.
Source: "Climate Change 2001. Synthesis Report" p.7