 The study's lead author, David Ridley, said this could help explain why climate models did not predict slower surface warming.Additional proposed causes of the decreased rate of surface warming in about 1999-2014 include the emission of pine-smelling vapors from pine forests, which have been shown to turn into aerosols, and the ban on chlorofluorocarbons as a result of the Montreal Protocol, since they were potent greenhouse gases in addition to their ozone-depleting properties.Spurious differences in observed warming rates may also arise from the mathematics of trend analysis itself, particularly when the study period is brief and regression assumptions are violated.