Water covers approximately 71% of the Earth’s surface. Oceans hold about 96.5% of all Earth’s water. Only about 2.5% of Earth’s water is freshwater, and most of that is locked up in glaciers and ice caps. Less than 1% of freshwater is readily accessible for human use. Despite this, water is one of the most abundant resources on Earth and is essential for all life.
The total amount of water on Earth is about 1.39 billion cubic kilometers. To put that into perspective, if all the water on Earth was spread evenly across the entire globe, it would form a layer about 2.7 kilometers or 1.7 miles deep. Freshwater makes up only about 2.5% of that total, or about 35 million cubic kilometers. Most freshwater is locked up in glaciers and ice sheets, with only a fraction found in lakes, rivers, soil moisture, groundwater, and the atmosphere.
Oceans
The world’s oceans contain approximately 1.335 billion cubic kilometers of water. This makes up 96.5% of all the water on Earth. The five main oceans are:
- Pacific Ocean – 622 million cubic km
- Atlantic Ocean – 323 million cubic km
- Indian Ocean – 282 million cubic km
- Southern Ocean – 21 million cubic km
- Arctic Ocean – 18 million cubic km
The largest by far is the Pacific Ocean, which covers about 30% of the Earth’s surface. On average, oceans are about 3,688 meters or 12,100 feet deep. The deepest point is the Mariana Trench in the western North Pacific at 10,994 meters or 36,070 feet below sea level. Oceans play a critical role in regulating global climate, absorbing carbon dioxide from the atmosphere, and providing habitat for a huge diversity of marine organisms.
Freshwater
While the oceans contain the vast majority of water on Earth, humans rely primarily on freshwater sources for drinking, agriculture, and industry. Freshwater makes up only about 2.5% of the total water volume, equivalent to around 35 million cubic kilometers. The main freshwater reservoirs are:
- Glaciers and ice caps – 24 million cubic km (68.7% of freshwater)
- Groundwater – 23 million cubic km (30.1% of freshwater)
- Lakes – 0.125 million cubic km (0.26% of freshwater)
- Soil moisture – 0.065 million cubic km (0.05% of freshwater)
- Atmosphere – 0.013 million cubic km (0.04% of freshwater)
- Rivers – 0.0017 million cubic km (0.006% of freshwater)
The vast majority of freshwater is locked up in glaciers and ice sheets, such as those found in Greenland and Antarctica. Groundwater is the next largest reservoir, consisting of water that has seeped into the soil and permeable rock underground. This includes aquifers that store and transmit groundwater below the water table.
Although they contain a relatively small fraction of total freshwater reserves, rivers, lakes, and soil moisture are critically important for ecosystem health and human use. Rivers transport water from inland areas to the oceans. Large lakes store freshwater, regulate regional climate, and provide habitat for aquatic species. Soil moisture supports land vegetation and agriculture.
Distribution of Earth’s Water
To summarize, here is how Earth’s water is distributed among the major reservoirs:
| Reservoir | Volume (cubic km) | Percent of total water |
|---|---|---|
| Oceans | 1,335,000,000 | 96.54% |
| Glaciers and ice caps | 24,064,000 | 1.74% |
| Groundwater | 23,400,000 | 1.69% |
| Lakes | 125,000 | 0.009% |
| Soil moisture | 65,200 | 0.005% |
| Atmosphere | 12,900 | 0.001% |
| Rivers | 1,700 | 0.0001% |
| Total | 1,386,669,800 | 100% |
This table illustrates the massive size of the oceans compared to other water reservoirs on Earth. Glaciers and ice caps hold most of the planet’s freshwater, but less than 1% is found in lakes, rivers, soil, and the atmosphere – the sources humans rely on most for survival.
The Hydrologic Cycle
Although the total amount of water on Earth remains fairly constant over time, water is continuously cycling through the various reservoirs in what is known as the hydrologic cycle. This includes:
- Evaporation – Heat from the sun causes water from oceans, lakes, rivers, and soil to evaporate and enter the atmosphere as water vapor.
- Transpiration – Plants release water vapor through tiny pores in their leaves.
- Condensation – As moist air rises and cools, water vapor condenses to form clouds.
- Precipitation – Condensed water falls back to Earth as rain, snow, hail, and other forms of precipitation.
- Infiltration – Some precipitation soaks into the ground to become soil moisture or groundwater.
- Surface Runoff – Excess precipitation flows overland into lakes, rivers, and oceans.
This continuous cycling of water means that the water you drink could have passed through the body of a dinosaur millions of years ago! Over long time scales, the hydrologic cycle is in balance – water returns to the oceans, atmosphere, and land at about the same rate as it leaves. However, over shorter periods, some reservoirs may gain or lose large amounts of water.
Water Use and the Human Water Cycle
While the natural hydrologic cycle recycles Earth’s fixed water supply, human activities have altered water movement through what is called the human water cycle:
- Water withdrawals remove water from rivers, lakes, or aquifers for human use.
- Water consumption occurs when water is not returned immediately to the natural system, such as water used for irrigation.
- Water returns to the environment through wastewater disposal, agricultural runoff, or discharge from hydroelectric plants.
The biggest human use of water is for agriculture, accounting for about 70% of global water withdrawals. Other major human uses include industry, domestic water supply, and electricity generation. More than 80% of wastewater worldwide is discharged back into the environment without being treated. This can pollute water sources and disrupt aquatic ecosystems.
Water Scarcity and Conservation
Despite the vast scale of Earth’s water, less than 1% is readily available freshwater. And human water use continues to reduce and pollute surface and groundwater supplies. It’s estimated that by 2025, about 1.8 billion people will live in regions with absolute water scarcity.
Water scarcity occurs when demand for water exceeds the available supply. Causes include drought, high population density, pollution, and excessive withdrawals for human use. Climate change is likely to exacerbate water scarcity in many regions. To conserve water, we need to:
- Improve efficiency of irrigation and reduce agricultural water use
- Repair and upgrade water infrastructure to reduce leaks
- Treat and reuse wastewater
- Protect and restore wetlands and floodplains
- Reduce household water consumption
Careful stewardship of freshwater resources will only become more crucial as the global population grows. Although Earth possesses immense quantities of water, less than a fraction of 1% is readily accessible freshwater critical for all life.
Conclusion
While water appears abundant when looking at the blue expanse of the oceans, freshwater is actually a very limited resource. The world’s water is dominated by the oceans, which comprise 96.5% of the total global water volume. Freshwater essential for terrestrial life makes up only about 2.5%, with much of that trapped in glaciers and ice. Less than 1% of freshwater is found in lakes, rivers, aquifers, soil, and the atmosphere.
This relatively tiny fraction of readily usable freshwater is continuously recycled through the hydrologic cycle. However, increasing human water consumption could severely impact water availability in certain regions. Careful management of water resources will be crucial in the coming decades with population growth and climate change threatening water security. While the total quantity will remain stable, ensuring sustainable and equitable access to clean freshwater represents an enormous but vital challenge.
