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How do water vapour and carbon dioxide affect Earth’s temperature?

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Wispy cirrus clouds stretch across a bright blue sky above trees and a river at Pinawa Dam Provincial Park in Manitoba, Canada, with sunlight illuminating the scene. — Atmosphere The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.

Water vapour and carbon dioxide (CO₂) are both potent greenhouse gases that contribute to the greenhouse effect by absorbing infrared radiation emitted from Earth’s surface and re-emitting it downward, which keeps the lower atmosphere warmer than it would otherwise be.123

Carbon Dioxide

Carbon dioxide acts as a direct agent of radiative forcing. Because it is a biogeochemically active gas, changes in its atmospheric concentration are primary drivers used to explain and predict global temperature changes. Human activities, particularly the burning of fossil fuels, have significantly increased CO₂ levels, leading to concerns regarding global warming.23

Water Vapour

While water vapour is a more efficient greenhouse gas than CO₂, its role is primarily that of a climate feedback rather than a direct forcing agent. Its concentration is determined by air temperature; as the surface warms, evaporation increases, leading to higher concentrations of water vapour. This creates a positive feedback loop known as the "water vapour feedback," where the resulting increase in the greenhouse effect causes further warming.124