Start with the heat that must leave
Put a pot of water on a low flame. At first it warms; later, heat escaping into the room can balance the heat arriving from below. Earth also receives energy and loses it, although its main exchange with space is radiation. A change that makes escape harder starts a period of warming. The temperature stops rising only when outgoing energy again balances the input.
In 1967, Syukuro Manabe and Richard Wetherald asked how that balance changes when the atmosphere contains more carbon dioxide. They worked at the Geophysical Fluid Dynamics Laboratory in the United States. Their model followed one vertical column from the ground into the atmosphere. That deliberately simple geometry let them investigate radiation, rising air and water vapour together.
Air carries heat as well as absorbing radiation
The ground absorbs sunlight and emits infrared radiation. Atmospheric gases absorb and emit infrared at different wavelengths. Some radiation reaches space directly; some is absorbed and emitted again higher up. Across much of the infrared band affected by added carbon dioxide, the radiation escaping to space comes from higher, usually colder levels of the troposphere, the lower atmospheric layer where most weather occurs. A colder emitter sends out less energy, so the atmosphere and surface must adjust.
Radiation alone would allow the temperature to fall too sharply with height in parts of the model. Warm air below colder air can rise and carry heat upward. Manabe and Wetherald represented this overturning with a convective adjustment: when the vertical temperature gradient became too steep, the model redistributed heat. This coupled calculation gave a physically useful temperature profile instead of treating the atmosphere as one uniform layer.
Warming brings more water vapour
Water vapour added a second response. Warm air can contain more water vapour before it becomes saturated. The researchers prescribed a vertical distribution of relative humidity, the fraction of saturation reached at each level. As temperatures rose, keeping this fraction fixed allowed the amount of vapour to rise too. Water vapour absorbs infrared, so this response amplified the initial warming. A response that acts back on the original change is called a feedback.
Imagine starting with a warm, humid room and raising its temperature without changing the water content. Its relative humidity falls. Holding relative humidity fixed is a different experiment: water must be added as the room warms. Recognising that distinction is essential to reading the 1967 calculation. Relative humidity and the actual amount of water are different quantities.
A calculation of about two degrees
The paper reported that doubling atmospheric carbon dioxide warmed the model by about two degrees Celsius under its fixed-relative-humidity assumptions. That number belongs to this particular calculation, with its prescribed clouds, surface properties and vertical humidity structure. It made a concrete test of the mechanism possible: alter the gas, solve for the new balance, and compare the temperatures.
The model also produced cooling in the stratosphere above the warmer lower atmosphere. Different heights could respond differently because their radiation and heat exchanges differed. Looking at a vertical pattern offered more information than a single surface temperature.
One column opened a larger programme
Manabe and his collaborators went on to develop three-dimensional atmosphere models and coupled atmosphere–ocean models. Winds, rainfall, ice and ocean heat transport could then respond within a much richer system. The column remained valuable because its central question stayed clear: how does the temperature adjust when radiation and vertical heat transport must agree?
The 2021 prize recognised Manabe and Klaus Hasselmann for climate modelling, variability and the prediction of global warming. Wetherald’s coauthorship matters to the history of the 1967 result. Their column calculation made a warming prediction traceable. A change in radiation exchange altered the temperature; water vapour responded, and the surface and atmosphere found a warmer balance.