Pieter Tans 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 man-made climate change was a problem that was certain to grow in importance over time. He decided to change direction from solid state physics to the earth sciences. 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 since the late 19th century. As part of this work he developed a new way to calculate the behavior of CO2 isotopic ratios, separating the influence of net exchange of total carbon between Earth reservoirs from pure isotopic exchange in which 12CO2 and 13CO2 molecules are exchanged between air and water, for example, without changing total carbon in either reservoir.
He led the Carbon Cycle Greenhouse Gases group from 1985 until 2019. 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. Over four decades the carbon cycle group has maintained what we now call NOAA’s Global Greenhouse Gas Reference Network, producing the most widely used data of atmospheric CO2, CH4, and several other greenhouse gases and supporting measurements.