GEF

Unit 2  |  Humanity’s Ecological Footprint 45 The Earth has a naturally occurring greenhouse effect. 2.6  CLIMATE CHANGE AND ITS CAUSES Compared with other gases such as nitrogen and oxygen, CO 2 is found at very low concentrations in the atmosphere. But, small changes in CO 2 concentrations have large effects on the Earth’s climate. Towards the beginning of the Industrial Revolution, CO 2 was at about 280 parts per million (ppm) . As of 2017, Earth is at 400 ppm, representing a significant increase. During the past one hundred years, the average global temperature has warmed by 1.4 degrees Fahrenheit (0.8 degrees Celsius), with about two-thirds of the increase occurring since 1980. An important factor in climate change is the role of carbon sinks , land and sea areas that absorb CO 2 from the atmo- sphere. Oceans absorb about one quarter of all CO 2 generated by human activity. Without this vital carbon sink, climate change would be much more severe than it already is. PARTS PER MILLION (PPM) Ameasurement of concen- tration used for atmospheric gases such as carbon dioxide. CARBON SINK A forest, ocean, or other natural environment that absorbs carbon dioxide from the atmosphere.