Overall, water on Earth is cheap on a per liter basis, but increasing population pressures have made fresh water a declining resource. Water is essential to life as we know it, and unfortunately, in those societies where water is short, much human suffering can result.
The Earth’s climate is part of a coupled system including the atmosphere, the oceans and the frozen icecaps. The lower atmosphere contains a large amount of water, which is close to its saturated vapor pressure in many places. The oceans and ice caps are in contact with the atmosphere and play a major role in regulating the climate.
Frozen Water and Climate
Water freezes in a variety of ways. On the surface, frozen water is often in the form of glaciers while precipitation in high latitudes produces low density snow which over decades becomes compacted into ice.
Fresh snow and glaciers are highly reflective. Heat from the sun is reflected back into space, cooling the surface. With overall climate warming, more ice and snow are melted. Less heat is then reflected back into space, warming the climate even more. Snow melt from land, such as Antarctica and the Greenland ice shelf release additional water into the sea, raising sea levels.
Liquid Water and Climate
The Earth is 70% covered with oceans. As the atmospheric temperature rises, heat is transferred to the deep ocean over decades. This leads to thermal expansion of the ocean and increasing sea levels.
In the long term, the heat exchange with the atmosphere can be accommodated by the deep ocean, but eventually, the ocean surface warms. Because of its contact with the atmosphere, warming oceans have climate impacts such as increased hurricane frequency.
Water Vapor and Climate
Water vapor is a potent greenhouse gas. As the climate changes, due, for example, to a CO2 increase, the temperature will rise and the saturated vapor pressure of water will also increase. the increased water vapor amounts will provide more warming of the Earth’s surface. Climate models suggest that the relative humidity (actual water vapor pressure divided by the saturated vapor pressure) will not change significantly in the future. If it does, it could have an important influence on cloud cover.
Clouds and Climate
Clouds are condensed water vapor. Both liquid and solid are frequently present, suspended in the atmosphere because of their small droplet or crystal sizes. Seen from space, they are highly reflective, so changes in cloud amounts will lead to less heat reaching the Earth’s surface.
Clouds generally form as soon as the saturated vapor pressure of water is reached. This is because of the presence of copious quantities of “condensation nuclei” such as dust and aerosols.
In principle, galactic cosmic rays could influence clouds by providing sites for water to condense. However, there is no evidence that increasing the amount of condensation nuclei this way has any effect on cloud formation. Hence, solar output change cannot contribute to climate change via this route.
Feedbacks in the Climate System Due to Water
The changes in water and the atmosphere lead to various “feedbacks”. Increasing temperatures melt more ice, which increases the temperatures further, a “positive feedback”. Increases in CO2 warm the sea with heat taken up by the oceans. Here, the oceans moderate the flux of heat from the atmosphere, lowering it from what it would otherwise be, a “negative feedback”.
Increases in water vapor in the first instance are a positive feedback since water vapor is a greenhouse gas. Increases in water vapor, could also lead to increased clouds, reflecting more sunlight back (negative feedback), or decreasing clouds, reflecting less sunlight back (positive feedback).
The conclusions from observations and climate models is that cloud feedbacks are neither positive nor negative. However, aerosols could play a major role in altering cloud feedbacks.
The predictions of climate modeling depend on a proper characterization of all the water vapor feedbacks in the atmosphere-ocean system. The main area where improved understanding is needed, is in the simulation of clouds. Nonetheless, consistent results are being obtained by a large number of laboratories world wide. These show unequivocal future climate warming due to greenhouse gas increases from human activities.