Geospatial Technologies

Geospatial technologies are systems that acquire and handle location-specific data about Earth. Remote sensing, the global positioning system (GPS), and geographic information systems (GIS) are important geospatial technologies.

Geothermal Energy

The word "geothermal" is derived from the Greek words geo (earth) and therme (heat), and "energy" is defined as usable power, such as heat or electricity. The temperature at the Earth's core (6,437 kilometers or 4,000 miles deep) may exceed 4,980°C (9,000°F).

Glaciers and Ice Sheets

Glaciers and ice sheets are perennial accumulations of ice and snow that flow downslope, slowly, due to their own weight. These terrestrial (land-based) ice masses often are classified by size as either glaciers, ice caps, or ice sheets.

Glaciers, Ice Sheets, and Climate Change

Glaciers and ice sheets both affect and are affected by changes in Earth's climate. They are frozen fresh-water reservoirs that change volume in response to changes in temperature and snowfall.

Global Warming and Glaciers

Evidence suggests that the Earth's average temperature is becoming warmer. Because atmospheric warming will be most readily apparent in the melting of perennial and permanent ice, and because most glaciers are small relative to vast ice sheets and expansive sea ice, glaciers are important indicators of climate change.

Global Warming and the Hydrologic Cycle

To maintain the global water balance, evaporation from oceans worldwide must be balanced by precipitation into the oceans plus runoff from the continents. Earth's atmosphere contains only 0.001 percent of the Earth's water, yet it is an essential component of the global hydrologic cycle: currents of air carry water vapor over land, and the resulting precipitation enables life on land.

Global Warming and the Ocean

The Earth's climate seems stable in respect to humankind's limited length of historical knowledge, but in reality, it is an ever-changing system. Climate change has been occurring since the Earth began, passing through long periods of fluctuating temperatures.

Global Warming: Policy-Making

Global warming is a typical case of Hardin's tragedy of unmanaged global commons.* As in most common property, policy-making and implementation for mitigating global warming remains a complex task that involves international agreements and protocols. In this situation, substantive actions on climate change will be taken by sets of nations if and only if each nation believes that it benefits on balance.

Globalization and Water

Globalization, which refers to the increasing integration and interdependence of countries, is a major trend shaping world affairs around the globe. Never before has it been so likely that the bananas we eat, the music CDs we listen to, the shirts we wear, the cars we drive, and the movies we watch were produced in another country.

Great Lakes

Located in north-central North America, the Great Lakes are five large fresh-water lakes interconnected by natural and artificial waterways: Lake Superior, Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario. Carved by ancient glaciers, these lakes contain approximately 20 percent of the world's surface fresh-water supply and 95 percent of the surface fresh water in the United States.


Water that occurs below the ground and is brought to the land surface by wells or springs is referred to as groundwater. Groundwater is a significant part of the hydrologic cycle, containing 21 percent of Earth's freshwater.

Groundwater, Age of

The age of groundwater is defined as the time that has elapsed since the water first entered the aquifer. For example, some of the rain that falls on an area percolates (trickles) down through soil and rock until it reaches the water table.

Groundwater Supplies, Exploration for

Although groundwater occurs beneath essentially every spot on the Earth, finding usable supplies can be a challenge. Groundwater commonly occurs within earth materials through which water moves very poorly, and thus its extraction may be impossible.

Hem, John D

Important information regarding the origin and evolution of water is contained within the chemical composition of the water. Perhaps no other individual contributed so much to the use of natural water chemistry as a tool in hydrological studies than John Hem.

Hoover Dam

In 1928, the U.S. Congress authorized the Boulder Canyon Project, later known as Hoover Dam.

Hot Springs and Geysers

Geysers are dynamic natural features on Earth' surface. The most dramatic examples of geysers are those that shoot tall columns of hot water and steam into the air.

Hot Springs on the Ocean Floor

In many areas of the ocean floor, wherever magma nears the seafloor, or where lava erupts directly at the seafloor surface, hot springs on the seafloor called hydrothermal vents commonly are found. Vent fields are generally associated with submarine (undersea) volcanoes where lava is erupting or preparing to erupt.

Hubbert, Marion King

The greatest contributions in science come from those that are able to see the big picture, to be able to see relationships between seemingly very different observations and bring them together into a coherent hypothesis. Marion King Hubbert is widely recognized for his ability to contribute in this manner.

Human Health and the Ocean

The interaction between people and the ocean—particularly the coastal ocean—allows for instances in which the ocean and its inhabitants can have negative effects on human health. The greatest biological risk posed to people is from eating tainted seafood, although other factors also can harm humans: for example, the discharge into waterways of organic and inorganic wastes, including toxic substances; the global transport of microorganisms by shipping; and marine animals that bite or sting.

Human Health and Water

All living creatures, including humankind, need water for survival. Humans directly and indirectly consume water for drinking, cooking, and food production.

Hutton, James

James Hutton was from Scotland and is considered to be the founder of modern geology. After a successful career as a farmer and chemist, he retired at the age of 42 in 1768 to devote the rest of his life to geology.

Hydroelectric Power

The damming of streams and rivers has been an integral part of human civilization from its early history. Controversy paralleled this use because impounding and diverting water for upstream users affects those who live downstream, and also modifies the local habitats of plants and animals.