Evaporation is a fundamental process within Earth’s water cycle, transforming liquid water into water vapor. This occurs when water molecules gain enough energy, primarily from solar radiation, to ascend into the atmosphere. This movement of water plays a significant role in Earth’s energy balance and climate regulation.
Oceans and Seas
The vast majority of evaporation on Earth originates from its oceans and seas. Approximately 86% of global evaporation occurs over these expansive saltwater bodies. This disproportionate contribution is primarily due to their immense surface area, which covers over 70% of the planet’s surface. Oceans also possess a remarkable capacity to absorb and store large amounts of solar energy, leading to significant heating of their surface waters.
The rate at which water evaporates from oceans is influenced by several factors. Warmer ocean temperatures provide more kinetic energy to water molecules, enabling them to escape into the atmosphere more readily. Wind speed plays a significant role; stronger winds help remove the layer of saturated air directly above the water, allowing drier air to absorb more moisture. Humidity levels in the air above the ocean also affect evaporation, with lower humidity creating a greater driving force. The salinity of ocean water can slightly reduce evaporation compared to freshwater, but its impact is generally less pronounced than that of temperature and wind.
Land Surfaces and Freshwater Bodies
Following the oceans, land surfaces and freshwater bodies are the next major contributors to global evaporation. This encompasses both direct evaporation from the ground and water released by plants. Water evaporates directly from moist soil, puddles, and other wet surfaces on land, particularly after rainfall events, when exposed to solar energy and favorable atmospheric conditions.
Another significant process on land is transpiration, where plants release water vapor into the atmosphere through tiny pores on their leaves called stomata. This biological process is a vital part of a plant’s water movement and is often combined with direct land evaporation under the broader term “evapotranspiration.” While plants absorb substantial amounts of water from the soil, a large portion, sometimes over 97%, is released as water vapor through transpiration. Overall, about 14% of the global total atmospheric water vapor comes from land surfaces, with a considerable amount attributed to plant transpiration.
Freshwater bodies like lakes, rivers, and wetlands also contribute to atmospheric moisture. Although their combined surface area is considerably smaller than the oceans, they still represent an important source of evaporation. Recent research indicates that the amount of water evaporating from lakes globally is greater than previously estimated. Artificial reservoirs, in particular, are showing increasing rates of evaporative loss, sometimes playing an outsized role in regional water budgets.