Dear Mr Guignard, thank you for your question posed to Ms Valery Detemmerman from the World Climate Research Programme, World Meteorological Organization.
Global warming has both anthropogenic and natural drivers (or 'forcings'). Anthropogenic forcings arise from the emission of greenhouse gases from fossil fuel burning and land-use change; natural forcings result from solar changes and explosive volcanic eruptions.
Solar output has increased gradually in the industrial era, causing a small positive radiative forcing of +0.12 watts per square meter between 1750 and 2005. This is in addition to the cyclic changes in solar radiation that follow an 11-year cycle. Solar energy directly heats the climate system and can also affect the atmospheric abundance of some greenhouse gases, such as ozone in the stratosphere (through the creation of the 'ozone hole'). The difference in radiative forcing estimates between the present day and the start of the industrial era for solar irradiance changes are very small compared to the differences in radiative forcings estimated to have resulted from human activities. In fact, the total net anthropogenic radiative forcing for the past 150 years has with +1.6 watts per square meter a 13-times stronger impact on global warming compared to solar activity. As a result, in today's atmosphere, the radiative forcing from human activities is much more important for current and future climate change than the estimated radiative forcing from changes in natural (solar!) processes.
For more information please consult the technical documents of the Intergovernmental Panel on Climate Change (IPCC, le Groupe d'experts intergouvernemental sur l'évolution du climat (GIEC): http://www.ipcc.ch/languages/french.htm#21.