Among opponents of the mainstream scientific assessment , some say that while there is agreement that humans do have an effect on climate, there is no universal agreement about the quantitative magnitude of anthropogenic global warming (AGW) relative to natural forcings and its harm to benefit ratio.  Other opponents assert that some kind of ill-defined "consensus argument" is being used, and then dismiss this by arguing that science is based on facts rather than consensus.  Some highlight the dangers of focusing on only one viewpoint in the context of what they say is unsettled science, or point out that science is based on facts and not on opinion polls or consensus.  
In May 2013, it was reported that readings for CO 2 taken at the world's primary benchmark site in Mauna Loa surpassed 400 ppm . According to professor Brian Hoskins , this is likely the first time CO 2 levels have been this high for about million years.   Monthly global CO 2 concentrations exceeded 400 ppm in March 2015, probably for the first time in several million years.  On 12 November 2015, NASA scientists reported that human-made carbon dioxide continues to increase above levels not seen in hundreds of thousands of years; currently, about half of the carbon dioxide released from the burning of fossil fuels is not absorbed by vegetation and the oceans and remains in the atmosphere.