Natural climate variability is unlikely to have contributed more than about one-quarter of the temperature rise observed in the past 60 years. At least 74 % of the observed warming is due to human activity.
Huber, M & Knutti, R. (2011). Anthropogenic and natural warming inferred from changes in Earth’s energy balance. Nature Geoscience. Available www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1327.html