1) IPCC, 2007: Historical Overview of Climate Change. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. https://www.ipcc.ch/report/ar4/syr/
2) National Oceanic and Atmospheric Administration. 2007. Global Warming: Frequently Asked Questions. (Link no longer available, but find other information here: https://www.ncdc.noaa.gov/climate-information/climate-change-and-variability)