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The Global Meteoric Water Line (GMWL) describes the global annual average relationship between hydrogen and oxygen isotopes (Oxygen-18 and Deuterium) ratios in natural meteoric waters. This article describes the definition of GMWL, the cause of isotopic fractionation in GMWL, and its applications environmental geochemistry and hydrogeology.

Development and Definition of GMWL
When working on the global annual average isotopic composition of Oxygen-18 and Deuterium (2H) in meteoric water, the geochemist Harmon Craig observed a correlation between these two isotopes. The equation for Global Meteoric water line was then developed and defined by Harmon Craig :

$$\delta\mathrm D = 8.0\cdot\delta^{18}\mathrm O + 10{}^{0\!}\!/\!_{00}$$

Where δ18O and δ2H (also known as δD) are the ratio of heavy to light isotopes (e.g. 18O/16O, 2H/1H).

The relationship of 18O and 2H in meteoric water is caused by mass dependent fractionation between evaporation from ocean seawater and condensation from vapor. As oxygen isotopes (18O and 16O) and hydrogen isotopes (2H and 1H) have different masses, they behave differently in the evaporation and condensation processes, and thus resulting in the fractionation between 18O and 16O as well as 2H and 1H. Kinetic fractionation causes the isotope ratios of δ18O and δ2H to vary between localities within the area. The fractionation processes can be influenced by a number of factors including: temperature, latitude, continentality, and most importantly, humidity,.

Applications
Craig observed that δ18O and δ2H isotopic composition of cold meteoric water from sea ice in the Arctic and Antarctica are much more depleted than that in warm meteoric water from the tropic. A correlation between temperature (T) and δ18O was proposed later in the 1970s. Such correlation is then applied to study surface temperature change over time. The δ18O composition in ancient meteoric water, preserved in ice cores, can also be collected and applied to reconstruct paleoclimate.

A meteoric water line can be calculated for a given area, named as local meteoric water line (LMWL), and used as a baseline within that area. Local meteoric water lines can differ from the global meteoric water lines in slope and intercept. Such deviated slope and intercept is a result largely from humidity. In 1964, the concept of deuterium excess d (d=δ2H - 8δ18O) was proposed. Later, a relationship of deuterium excess as a function humidity has been established, as such the isotopic composition in local meteoric water can be applied to trace local relative humidity, study local climate and used as a tracer of climate change.

In hydrogeology, the δ18O and δ2H composition in groundwater or are often used to study the origin of groundwater and groundwater recharge.