Why does water have a temperature that is lower than the atmosphere?

Climate FAQ 8.1 | Steam

As the largest contributor to the natural greenhouse effect, water vapor plays an essential role in the earth's climate. However, the amount of water vapor in the atmosphere is mainly regulated by the air temperature rather than by emissions. Therefore, scientists see it as a feedback factor rather than a driver of climate change. Anthropogenic water vapor emissions from irrigation or power plant cooling have a negligible impact on the global climate.

Water vapor is the most important greenhouse gas in the earth's atmosphere. The contribution of water vapor to the natural greenhouse effect compared to that of carbon dioxide (CO2) depends on the calculation method, but can be regarded as roughly two to three times greater. Additional water vapor is added to the atmosphere through anthropogenic activities, largely as a result of increased evaporation from irrigated crops, but also as a result of power plant cooling and, to a lesser extent, the burning of fossil fuels. One can therefore ask why there is such a strong focus on CO2 and not on water vapor as the driver of climate change.

Water vapor behaves differently from CO2 in one fundamental way: it can condense and precipitate. When air with high humidity cools down, some of the water vapor condenses into water droplets or ice particles and precipitates out as a precipitate. The usual retention time for water vapor in the atmosphere is ten days. The flow of water vapor into the atmosphere from anthropogenic sources is much less than from “natural” evaporation. Therefore it has a negligible influence on the total concentrations and does not contribute significantly to the long-term greenhouse effect. This is the main reason that tropospheric water vapor (usually below 10 km altitude) is not considered an anthropogenic gas that contributes to radiative forcing.

Anthropogenic emissions, on the other hand, have a significant influence on water vapor in the stratosphere, the layer in the atmosphere above about 10 km altitude. Increased methane concentrations (CH4) due to human activities lead to an additional water source via oxidation, which partly explains the observed changes in this layer of the atmosphere. This change in the stratospheric water has an impact on the radiation, is seen as a drive and can be estimated. Stratospheric water vapor concentrations have fluctuated significantly over the past few decades. The full extent of these fluctuations is not fully understood and is likely less of a driver than a feedback process that adds to natural variability. The contribution of stratospheric water vapor to warming, both as propulsion and through feedback, is much smaller than that of CH4 or CO2.