The marine biome, encompassing the world’s oceans, seas, and connecting waters, represents the largest habitat on Earth, covering over 70% of the planet’s surface. Answering the question of its average rainfall is complex because the environment is a massive body of water, unlike a terrestrial biome where rainfall is an external input onto a dry surface. Since the ocean holds approximately 97% of all water on Earth, the total water budget focuses on maintaining a dynamic balance between water gained through precipitation and water lost through evaporation.
Defining Precipitation in the Marine Environment
In the marine environment, precipitation refers to any form of water (liquid or solid) that falls from the atmosphere directly onto the ocean surface. This includes rain, snow, sleet, or hail, collectively known as hydrometeors. This process introduces freshwater directly into the saline ocean, contrasting with land environments where rainfall leads to surface runoff and infiltration.
The ocean plays a dominant role in the global hydrologic cycle, acting as both a massive source and a sink for atmospheric moisture. Globally, about 78% of all precipitation falls over the oceans, while approximately 86% of global evaporation originates there. Other significant sources of freshwater input exist, such as river runoff from continents and meltwater from glaciers and ice sheets.
Global Estimates of Rainfall Over the Oceans
Determining a precise figure for the average annual rainfall over the entire ocean surface is challenging due to immense regional variability. Scientific estimates suggest that roughly 373,000 cubic kilometers of water falls over the oceans each year. Averaged over the entire surface area of the globe, including both land and sea, the mean annual precipitation is often cited around 1,000 millimeters (mm).
These global estimates rely heavily on advanced satellite technology, since direct measurement using traditional rain gauges is impossible across open water. Missions like NASA’s Global Precipitation Measurement (GPM) use a constellation of satellites equipped with dual-frequency radars and microwave imagers to estimate precipitation intensity and type. The Integrated Multi-satellitE Retrievals for GPM (IMERG) product combines data from these sensors to provide a high-resolution, quasi-global view of rainfall and snowfall.
The distribution of precipitation is far from uniform, with certain oceanic areas acting as “rain belts” and others as “ocean deserts.” The highest intensities are consistently found near the equator in the Intertropical Convergence Zone (ITCZ), where warm, moist air rises and condenses, leading to frequent and heavy tropical rainfall. Conversely, vast subtropical gyres in the mid-latitudes experience significantly lower rainfall, sometimes dropping below a few hundred millimeters annually, similar to desert conditions on land.
The Balance of Evaporation and Salinity
Understanding marine dynamics requires looking beyond precipitation alone and considering the net exchange of water, known as the precipitation-minus-evaporation (P-E) balance. Evaporation, the process where liquid water turns into vapor and is lost to the atmosphere, serves as the counter-force to precipitation. This balance dictates the amount of freshwater gained or lost by a specific region of the ocean.
In regions where evaporation exceeds precipitation (E > P), the removal of freshwater leaves the salt behind, resulting in higher surface salinity. These areas, like the subtropical oceans, become saltier. Conversely, in regions dominated by high rainfall, such as the ITCZ and high latitudes, the continuous input of freshwater dilutes the surface water, causing lower salinity.
This P-E balance is a fundamental driver of ocean properties, as both temperature and salinity determine the density of seawater. Surface waters made denser by high salinity or cold temperatures can sink, initiating global density-driven circulation patterns. Observations have shown an amplification of this pattern over recent decades, with “the fresh getting fresher and the salty getting saltier.” This indicates a strengthening of the water cycle and its effect on the ocean’s chemistry and circulation.