Around the world — from tundra to tropical forests, and a variety of ecosystems in between — environmental researchers have set up micrometeorological towers to monitor carbon, water, and energy fluxes, which are measurements of how carbon dioxide (CO2), water vapor and energy (heat) circulate between the soil, plants and atmosphere. Most of these sites have been continuously collecting data, some for nearly 25 years, monitoring ecosystem-level changes through periods of extreme droughts and rising global temperatures. Each of these sites contributes to a regional network — i.e. the European Network (Euroflux) or the Americas Network (AmeriFlux) — and the regional networks together comprise a global network called FLUXNET.