Pieter Tans leads the Carbon Cycle Greenhouse Gases Group in the Global Monitoring Division of NOAA's Earth System Research Laboratory in Boulder, Colorado. He has been actively interested in mankind's influence on climate since he was a student. In 1972, when reading Inadvertent Climate Modification, a report of the study of man's impact on climate, he became convinced that manmade climate change was a problem that was certain to grow in importance over time. He decided to change direction from solid state physics to earth science. He obtained a PhD from the University of Groningen, The Netherlands, in 1978 on a study of carbon-14 and carbon-13 in tree rings, which were used to reconstruct the increase of atmospheric carbon dioxide (CO2) since the late 19th century. Since 1985 he has led the Carbon Cycle Greenhouse Gases group at NOAA's Earth System Research Laboratory. He discovered the existence of a very large "sink" (uptake) of CO2 by terrestrial ecosystems at mid-latitudes in the northern hemisphere, partially offsetting the emissions caused by the burning of coal, oil, and natural gas. For several decades, his group has maintained a cooperative global atmospheric observing network producing the most widely used data of atmospheric CO2, methane (CH4), carbon monoxide (CO), and several other greenhouse gases and supporting measurements.