The hydrologic cycle is a variation on the concept of conservation of mass. It is the tracking of water in the Earth system as the water is stored in various reservoirs and transferred from one reservoir to another. In the process of transferring or storing water, the total mass of water in the Earth system is approximately constant.

Reservoirs: The major reservoirs of water are the oceans (~97% of all water on Earth), ice caps and glaciers (~2% of all water, ~77% of freshwater), groundwater (~0.6% of total, ~22% of freshwater), surface water, soil moisture, saline lakes, and the atmosphere (these last 4 reservoirs are extremely small compared with the others). Environmental issues typically involved changes in these reservoir sizes, particularly for the oceans (an increase in the reservoir size leads to sea level rise), ice caps and glaciers (a decrease in size diminishes freshwater supplies and increases the size of the ocean reservoir), and groundwater (a decrease in size in arid regions threatens water supplies for drinking, irrigation, and industry).

Ice caps and glaciers: The "cryosphere" has experienced a net loss of mass since industrialization. This is most directly related to increasing air temperatures. Recent satellite imagery has documented that Greenland and Antarctica gained mass from 1992-2003, likely as a result of increasing snowfall in these regions. The ice sheets' growth does not mean that temperatures have cooled and that global warming is false: warmer temperatures are likely driving the increase in snowfall, melt rates are likely increasing (but at a slower rate that the increase in snowfall), and the data are only for the last 11 years. Still, the growth of these ice sheets certainly helps to mitigate sea level rise.

Groundwater: The size of the groundwater system is a function of the rate that water seeps into the ground following rainstorms versus the rate that water is extracted naturally and artificially. On the whole, there is enough precipitation to balance what Americans extract annually. The problem is that precipitation and groundwater usage is not uniform. Some of the driest places in this country (and this is true worldwide, too) are also the places where groundwater is most heavily used. California's Central Valley is a great example. This region has seen extensive ground subsidence as the groundwater reservoir has been depleted. This is because the groundwater filling pore spaces between grains of soil and sediment offers more structural support to the overlying sediment than empty pore spaces do. Venice is likewise subsiding. Other environmental problems associated with groundwater mining include failure of wells that now bottom-out above the water table, and saltwater intrusion in coastal regions.

Oceans: Tide gages and satellites have documented that sea level has risen over the last century by ~20 cm. This rise has come from two sources: 1) an increase in the mass of water stored in the oceans, primarily as a result of melting ice caps and glaciers; and 2) a decrease in water density as the seawater has warmed. This "thermal expansion" of seawater occurs because water (>4°C) increases in volume as temperature increases:

ΔVolume=Volume*α*ΔTemperature

where α is a constant that captures the change in volume for a 1°C change in temperature. For water at 20°C, α=2x10^-4/°C. In practice, it is difficult to predict how thermal expansion will affect sea level without using a numerical model that captures the depth and magnitude of temperature change in the oceans. The current models predict that thermal expansion will be the primary cause of sea level rise in the next century, with a smaller amount of sea level rise caused by shrinking glaciers and Greenland. Antarctica is predicted to increase in size.

To get a sense of how much sea level could possibly rise in the future, you can look at the amount of water currently stored in the cryosphere. If all of the ice on Earth melted and was added to the oceans, and if the oceans kept their current surface area, then sea level would go up by ~80 m. Another approach is to look at reconstructions of sea level throughout geologic history. These reconstructions indicate that sea level in the past has been up to 400 m higher than modern sea level. Bryn Mawr and Haverford would be underwater, but homes in the Appalachians would have nice ocean views.