Abstract: "Climate Change 2007? The Physical Science Basis", the Working Group I contribution to the IPCC Fourth Assessment Report (AR4) was launched on February 2, 2007. The report clearly indicates that the updated 100-year (1906-2005) linear trend of global surface temperature is 0.74℃. The linear warming trend over the last 50 years is 0.13℃per decade. Eleven of the last twelve years (1995-2006) rank among the 12 warmest years since 1850. Warming of the climate system is unequivocal. The report also indicates that most of the observed warming of the climate system over the last 50 years is very likely due to human activities. IPCC assessment reports summarize peer-reviewed and comprehensive findings from international scientific communities, represent the current level of scientific understanding of global climate change research, and provide an important basis for international climate regime and relevant national policies.