Most climatologists believe that human activities are now elevating the natural greenhouse effect by increasing concentrations of carbon dioxide from the burning of fossil fuels and changes in land use.
Greenhouse gases, such as carbon dioxide, act like a blanket, trapping heat in the earth’s atmosphere and therefore increasing the temperature. Observations of global average temperatures dating back to 1861 indicate a clear warming trend. In particular, since 1975, they show a sustained rise over the past three decades.