Climate scientists have been warning of major climate consequences from man-made greenhouse gas emissions for almost three decades. Those predictions have not changed, indeed have only improved, as scientific capabilities grew. If humanity had responded earlier, the costs of action would have been lower or spread over longer times. In the absence of past action at the scale needed, now is the time to act.
Source: Intergovernmental Panel on Climate Change, Fifth Assessment Report, Working Group 1, Summary for Policymakers. http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf .