Abstract: The working group 1 of IPCC has published the fifth science assessment report in 2013. Based on lots of evidences of observations and simulations by climate models, it continues to emphasize the key point that the global is warming at the present time and continues to warm in the future due to increasing human emissions. Several scientific issues of global warming in this paper will be summarized, such as differences among several datasets of observed global surface air temperature, effects of different basic climate periods, detection and attribution of global warming during the 20th century, and the trends of global temperature change in the 21st century. It raises the further research issues of global warming in the future. The key points are: the qualities of the observed data should be raised; it should be noticed that the different basic climate periods have various values; the capabilities of both decadal and interdecadal variabilities as simulated by the climate models would be improved, and the reasons of the hiatus in global mean surface warming of the past 15 years have to be studied, therefore to revise the climate models; the uncertainties of projected the global climate change in future are from climate model differences, and various anthropogenic emission scenarios, as well as impacts of internal variability of climate system and actions of the natural external forcing.