Including the effects of natural external forcing factors has a relatively small impact on our 1950 — 2005 results, but improves the correspondence between model and observations over 1900 — 2005. (link.springer.com)
The warming occurred in both the ocean and the atmosphere and took place at a time when natural external forcing factors would likely have produced cooling. (dotearth.blogs.nytimes.com)
Natural external forcing also results from explosive volcanism that introduces aerosols into the stratosphere (Section 2.7.2), leading to a global negative forcing during the year following the eruption. (ipcc.ch)