There is compelling scientific evidence that recent changes in the global climate are due to human activity and that catastrophic consequences will ensue if atmospheric carbon levels continue to rise. The burning of fossil fuels and deforestation are the main causes of this. CO2 concentrations in the atmosphere are often used as an indicator because they have been tracking with the mean average temperature of the earth for thousands of years, as demonstrated by climate paleontologists, and recent levels are unprecedented in human history as shown in this Carbon Tracker video published by the National Oceanic and Atmospheric Administration (NOAA), USA. More infographics and videos for explaining the processes of global warming can be found at The Hadley Centre (UK Met Office) and The Environmental Protection Agency (USA).
The Intergovernmental Panel on Climate Change (IPCC) present the current state of knowledge on Climate change and its potential impacts. You can watch the video above or go to the website for the Physical Science Basis part of the latest IPCC report (IR5). The Synthesis Report is a summary of all the current findings, written for governments and policymakers. It is the final part of the IR5 assessment process and is due to be adopted and released on 31st October 2014. See the FAQ section for more about how the IPCC work.