Mojave Desert, near Joshua Tree N.P. This is one of the field sites where we have studied the potential for hydrogen isotopes in plant leaf waxes to record aridity.
Scientists understand past changes in the Earth's climate is through the use of 'proxies.' These are things that we can measure, such as tree-ring size or ice layer thickness or carbonate 18O content, that are well correlated with changes in some aspect of the Earth's climate. The deuterium (2H) content of organic leaf waxes in terrestrial plants is one such proxy, and we have multiple ongoing projects in my group that are both trying to understand how this proxy works, and to exploit it for understanding terrestrial climate change.
We usually think of plants as fixing carbon, but in a sense they also fix hydrogen. They take H (in the form of H2O) from their environment, and reduce it to yield carbon-bound organic H. It so happens that the relative abundance of deuterium in environmental water varies rather dramatically with climate: 2H increases when its hot and dry, and decreases when its cold and wet. This isotopic 'fingerprint' of environmental water is then recorded by the organic hydrogen in plants. It is not recorded perfectly, because there are large biologic effects that lead to systematic offsets in the deuterium content of water versus organic compounds. But if we can understand those systematic effects, and calibrate the relationship between organic compounds and water, then we would have a useful paleoclimate proxy. In particular, the long-chain hydrocarbons that make up the waxy coatings of leaves are key targets for study. They are not volatile, degrade very slowly, and are abundantly preserved in most lake sediments.
Ongoing projects in my group include efforts both to understand the biological processes that lead to fractionations between water and lipid H (ie, trying to figure out how the proxy works; see also biochemical studies described here), as well as using the proxy to gain preliminary insight into past changes in terrestrial climate. Study areas have included the Mojave Desert and Coastal Ranges of southern California, the Sierra Nevada, Santa Barbara Basin, Cariaco Basin, and Indonesia.