Since about 1750, human activity has increased the concentration of carbon dioxide and other greenhouse gases. As of 2021, measured atmospheric concentrations of carbon dioxide were almost 50% higher than pre-industrial levels.[26] Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity,[27] but over periods longer than a few years natural sources are closely balanced by natural sinks, mainly photosynthesis of carbon compounds by plants and marine plankton. Absorption of terrestrial infrared radiation by longwave absorbing gases makes Earth a less efficient emitter.