Climate and water on the planet Earth are closely linked. Water takes part in a large-scale exchange of mass and heat between the atmosphere, the ocean, and the land surface, thus influencing the climate, and also being influenced by the climate.
In the history of Earth's climate, there were time periods when much of the hydrosphere on the surface of the planet was in the solid form of glacial ice. Possibly, during the Cryogenian period, the range of sea ice extended nearly to the equator. There have been several ice ages in the history of the Earth, and the most recent retreat of glaciation is dated at some 10 000 years ago. Range and extent of ice sheets, glacier, and permanent snow areas remain a sensitive indicator of changes in the Earth's climate. After expansion during the Little Ice Age, they have been shrinking recently in response to the ongoing global warming.
Under normal pressure, water exists as a liquid over a large range of temperature from 0 to 100 ° C; hence, water remains as a liquid in most places on the Earth. Because water has a high specific heat (heat capacity) defined as the amount of energy required to increase the temperature of 1 g of a substance by 1 °C, a water body can absorb (or release) large amounts of heat when warming (or cooling). This large hidden energy is released in the atmosphere when water vapor condenses. Latent heat (water vapor) transport is a major component of the Earth's heat balance. Some 23% of the solar radiation that reaches the Earth is used for evaporating water. Solar engine lifts about 500 000 km3 of water a year, evaporating from the Earth's surface, therein 86% (430 000 km3) from the ocean and 14% (70 000 km3) from land.
Water plays a pivotal role in the redistribution of heat in the Earth's atmosphere, and in the Earth's thermal system. Due to high specific and latent heat, water moderates the Earth's climate, acting as air-conditioner in the Earth system. Most (1.338 billion km3, i.e., 96.5% of all the Earth's waters) is contained in the oceans and the very high heat capacity of this large volume of water buffers the Earth surface from strong temperature changes such as those occurring on the waterless Moon. Ocean acts as the principal heat storage component in the Earth system, a regulating flywheel in the Earth's heat engine. The principal characteristics that affect density and motion (currents) of ocean's water are its temperature and salinity. Since warm water is less dense (lighter) than cold water and salty water is heavier (more dense) than freshwater, the combination oftemperature and salinity of the oceanic water determines whether a water particle sinks to the bottom, rises to the surface, or stays at some intermediate depth. Thermohaline circulation can be interpreted as a conveyor belt of heat, responsible for the relatively mild climate of Europe. It is driven by the density of oceanic water, which, in turn, is impacted by freshwater influx to the ocean. Besides oceans and seas, surface water bodies, such as lakes, wetlands, and large rivers, also affect the local, or regional, climate and partake in temperature regulation processes. Enhanced evaporation in large water storage reservoirs is an important component of a water balance, especially in arid and semiarid areas, being a very essential part of the total water consumption in individual regions.
The hydrological cycle affects the energy budget of the Earth. Clouds alter Earth's radiation balance. Atmospheric water vapor (along with carbon dioxide and methane) is a powerful greenhouse gas, playing a significant role in the greenhouse effect. This effect, which can be described as absorbing the long-wavelength infrared radiation emitted by the Earth's surface, is responsible for maintaining the mean surface temperature about 33 °C higher than would be the case in the absence of the atmosphere. Condensation of water in clouds provides thermal energy, which drives the Earth's circulation. The atmospheric transport of water from equatorial to subtropical regions (where latent heat is released from water vapor) serves as an important mechanism for the transport of thermal energy. During 8-10 days that a water molecule resides, on average, in the atmosphere, it may travel about 1000 km.
Earth's climate has always been changing, reflecting regular shifts in its orbit and solar activity and radiation, and volcanic eruptions. However, a large part ofthe climate change being observed recently is due to human activity. The humankind has been carrying out a planetary-scale experiment, disturbing the natural composition of the atmosphere by increasing the contents of greenhouse gases. This takes place because of the increasing burning of fossil carbon (coal) and hydrocarbons (oil and natural gas), and large-scale deforestation (reduction of carbon sink). In consequence, carbon dioxide concentration in the Earth's atmosphere increases and the greenhouse effect becomes more intense, leading to global warming. The global mean temperature of the Earth has already visibly increased by over 0.74 °C since 1860 and further increase is projected, by up to 1.1-6.4 °C by 2100, depending on the socioeconomic (and - in consequence - carbon dioxide emission) scenarios. Apart from the warming, there are several further manifestations of climate change and its impacts, of direct importance to the hydrosphere.
Many climate-change impacts on freshwater resources have already been observed, and further (and more pronounced) impacts have been projected. There is a poleward shift of the belt of higher precipitation. Increased midsummer dryness in continental interiors has been observed. The effect ofclimate change on streamflow, lake levels, and groundwater recharge, which varies regionally, largely follows changes in the most important driver, precipitation. Effects of future climate change on average annual river runoff across the world in contemporary projections indicate increases in high latitudes and the wet tropics, and decreases in mid-latitudes and some parts of the dry tropics. The latter translates into lower water availability (lower river flows and stages, lake and groundwater levels, and soil moisture contents).
The weight of observational evidence indicates an ongoing intensification of the water cycle - very dry or very wet areas have increased, globally, from 20% to 38% in the last three decades. There is more water vapor in the atmosphere, and hence there is potential for more extreme precipitation. Based on the results of the climate models, it is projected that the water cycle will further intensify, with possible consequences to rendering extremes more extreme.
Warmer temperatures generate increased glacier melt; hence, widespread glacier retreat has been already observed, and many small glaciers disappear. High reductions in the mass of Northern Hemisphere glaciers are expected in the warming climate. As these glaciers retreat, rivers, which are sustained by glacier melt during the summer season, feature flow increase, but the contribution of glacier melt will gradually fall over the next few decades.
Water quality is likely generally to be degraded by higher water temperature, but this may be offset regionally by the dilution effect of increased flows. Warming-enhanced sea-level rise can lead to saltwater intrusion into fresh groundwater bodies. Thus, freshwater availability in coastal areas is likely to decrease in the warmer climate.
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