If the globe is warming how come relative humidity is decreasing?

Increasing temperature leads to a greater capacity for air to hold moisture (the specific humidity). The actual amount of moisture held in a sample of air as a proportion of the capacity of that air to hold moisture is termed the relative humidity and is usually given as a percentage.

Specific humidity (g/kg) versus temperature for air  earthobservatory.nasa.gov

All climate computer models usually assume a constant relative humidity.

NasaIn climate modeling, scientists have assumed that the relative humidity of the atmosphere will stay the same regardless of how the climate changes. In other words, they assume that even though air will be able to hold more moisture as the temperature goes up, proportionally more water vapor will be evaporated from the ocean surface and carried through the atmosphere so that the percentage of water in the air remains constant. Climate models that assume that future relative humidity will remain constant predict greater increases in the Earth’s temperature in response to increased carbon dioxide than models that allow relative humidity to change. The constant-relative-humidity assumption places extra water in the equation, which increases the heating.

Relative humidity has decreased steadily for over 60 years. All computer models which assume constant relative humidity will overestimate the feedback and the degree of warming.

Forbes carries an article about the mismatch between computer models and actual observations.

ForbesWater vapor is a much more potent greenhouse gas than carbon dioxide, so substantial increases in atmospheric water vapor can certainly cause significant warming. United Nations computer models are programmed to assume that absolute humidity (the total amount of water vapor in the atmosphere) will rise so much that even relative humidity (the percent of water vapor in the atmosphere) will at least keep pace and perhaps even increase. Warmer air is able to hold more water than cooler air, so absolute water vapor would have to increase quite substantially for relative humidity to remain constant or increase in a warming world.

Scientists, however, have been measuring relative humidity for many decades. Rather than keeping pace with modestly warming temperatures, relative humidity is declining. This decline has been ongoing, without interruption, for more than 60 years. After more than six decades of consistent data, we can say with strong confidence that absolute humidity is not rising rapidly enough for relative humidity to keep pace with warming temperatures.

Global Relative Humidity 300 -700mb

Global Relative Humidity 300 – 700mb  (300mb corresponds to about 9,000 m altitude)

The failure of relative humidity to hold constant or rise during recent decades is a lethal dagger in the heart of alarmist global warming claims. According to the UN computer models, rapidly rising absolute humidity will cause substantially more global warming than the modest warming directly caused by rising carbon dioxide levels. Given the potency of water vapor, even a small overstatement of atmospheric humidity levels in UN computer models will cause a very significant overstatement of future warming. And the data show UN computer models assume too much atmospheric humidity.

The effects of this overstatement are apparent in real-world temperature data this century. Precise atmospheric temperature measurements compiled by NASA and NOAA satellite instruments show there has been no global warming since late in the 20th century


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