climate sensitivity. Climate sensitivity may further change if tipping points are crossed. It is unlikely that tipping points will cause short-term changes in climate sensitivity. If a tipping point is crossed, climate sensitivity is expected to change at the time scale of the subsystem that hits its tipping point. Especially if there are multiple interacting tipping points, the transition of climate to a new state may be difficult to reverse. The two most common definitions of climate sensitivity specify the climate state: the ECS and the TCR are defined for a doubling with respect to the levels in the pre-industrial era. Because of potential changes in climate sensitivity, the climate system may warm by a different amount after a second doubling of from after a first doubling. The effect of any change in climate sensitivity is expected to be small or negligible in the first century after additional is released into the atmosphere. Estimates Historical estimates Svante Arrhenius in the 19th century was the first person to quantify global warming as a consequence of a doubling of the concentration of . In his first paper on the matter, he estimated that global temperature would rise by around if the quantity of was doubled. In later work, he revised that estimate to . Arrhenius used Samuel Pierpont Langley's observations of radiation emitted by the full moon to estimate the amount of radiation that was absorbed by water vapour and by . To account for water vapour feedback, he assumed that relative humidity would stay the same under global warming. The first calculation of climate sensitivity that used detailed measurements of absorption spectra, as well as the first calculation to use a computer for numerical integration of the radiative transfer through the atmosphere, was performed by Syukuro Manabe and Richard Wetherald in 1967. Assuming constant humidity, they computed an equilibrium climate sensitivity of 2.3 °C per doubling of , which they rounded to 2 °C, the value most often quoted from their work, in the abstract of the paper. The work has been called "arguably the greatest climate-science paper of all time" and "the most influential study of climate of all time." A committee on anthropogenic global warming, convened in 1979 by the United States National Academy of Sciences and chaired by Jule Charney, estimated equilibrium climate sensitivity to be , plus or minus . The Manabe and Wetherald estimate (), James