Water vapor is the most abundant greenhouse gas in the atmosphere. Relative to all the absorbers of longwave radiation in the atmosphere, water vapor is responsible for an estimated 50% of the total current greenhouse effect, followed by clouds (25%), then CO2 (20%) (Schmidt et al., 2010). Without water vapor and the other greenhouse gases, the global mean average temperature would be a frigid -15 oC. Evaporation is the link between the energy and water balances, and on an annual basis, evaporation accounts for roughly 76% of the surface radiation budget. Critical to life in many aspects, water plays a vital role in regulating the Earth’s temperature through both positive and negative feedbacks in the climate system. For example, warming due to increases in atmospheric carbon dioxide concentrations can result in increased humidity thus additional warming (positive feedback), or increased cloud cover can result in surface cooling (negative feedback).

Driven by recent observations of increases in humidity, extreme events such as droughts and floods, and issues concerning food security and water quality, understanding how water enters the atmosphere through open water sources (e.g. lakes) and vegetation is critical to understand the changing environment. Despite this need, evaporation (and sublimation) conceptually appear as a simple process; just place a pan of water outside and measure change in water level. Compared to the other well-known greenhouse gas carbon dioxide, however, water vapor is inherently difficult to measure and shows complex patterns in both space and time. From a measurement perspective, water vapor exists in minute quantities in the atmosphere (roughly 4 ppm compared to 400 ppm for carbon dioxide), adheres to instruments such as sample tubes, and changes phase with changes in temperature and pressure. Moreover, evaporation shows large spatial and temporal patterns even over “simple” water bodies such as lakes and reservoirs. Control of transpiration from vegetation, responsible for roughly 60% of the terrestrial global evaporation (Schlesinger and Jasechko, 2014), also varies spatially from the scale of individual leaves to forests, and is influenced by a myriad of both biotic and abiotic factors.

Given the importance of water vapor, different measurement and modeling approaches to quantify the transfer of water from the Earth’s surface to the atmosphere have been attempted for decades. Starting with open water surfaces, a summary of evaporation studies from reservoirs to the Great Lakes, and from alpine wetlands and tundra to subalpine forests, will be presented. Challenges and lessons learned from a field-study perspective of this vital component of the water cycle will be discussed.