The purposes of the research project on forcing and impacts of warm periods in the past 2000 years, which was funded by the China Global Change Research Program, are summarized as follows: to reconstruct past climate changes in different regions and the pattern of warm periods over China, to analyze climate forcing and diagnose the mechanisms in warm periods by climate change simulations, to assess the impacts of climate change on socio-economic systems and human adaptation to warm periods in China, and to investigate Northern Hemisphere (NH) temperature changes and the coherences/differences in regional temperature anomalies during warm periods across the NH. The new findings are highlighted as follows: (1) Warming in the 20th century was not unprecedented in the past 2000 years. However, warming in the 20th century was almost simultaneous over the entire NH, whereas warming in the Medieval Warm Period was heterogeneous in amplitude and phase at regional and continental scales. The difference could be mainly attributed to the mechanism driven by different external forcings. Warming in the 20th century was mainly driven by a stabilization mechanism, forced by increased greenhouse gases, whereas the Medieval Warm Period was mainly driven by a thermostat mechanism, forced by increased solar radiation. (2) Extremely cold winter events, such as the snowstorm in southern China in early 2008, was the early warning of decadal cooling in historical terms. It suggests that the recent global-warming hiatus might be an early signal of cooling induced by natural variability. (3) In Chinese history, warm periods usually corresponded to prosperous times for the socio-economy, whereas it was contrary in cold periods. However, socio-economic development and population growth in warm periods may increase pressures on the natural environment system because of increased resource utilization. Such pressure could lead to disequilibria in the human-environment system and trigger social crises when abrupt climate change (such as cooling and precipitation decrease) occurs. These results can help in improving our understanding on the characteristics and mechanisms of decadal to centennial climate change as well as provide insight for successful adaptation to climate change in the future.