Scientific methods to study how forests affect climate are distinguished as environmental monitoring, experimental manipulation, or modeling. Meteorological measurements of air temperature and wind speed in forests and adjacent clearings characterize microclimates. More complex measurements of energy, water, and carbon dioxide fluxes obtained using principles of eddy covariance required sophisticated instruments on tall towers extending above the forest canopy. In situ measurements of leaves and individual trees reveal physiological functioning and can be extrapolated to an entire forest. Whole-ecosystem manipulations that warm the soil or enrich the air with carbon dioxide provide insight to ecosystem responses to environmental change. Ecosystem studies monitor carbon and elemental stocks and fluxes, and watershed studies monitor water flows. Remote sensing instruments that acquire radiative signatures of the land provide an indicator of vegetation type, health, and productivity. Numerical models of terrestrial ecosystems and climate provide a means to test theories and develop understanding of the biosphere-atmosphere system.