Understanding and addressing environmental challenges, including climate change, requires access to accurate data from many sources. In some cases, government agencies that operate Earth-observing satellites have been leaders in this regard - making their data freely available to all users. In fact, some of the earliest references to "open data" can be traced back to early government satellite projects. However, many governments continue to restrict access to their unclassified Earth-observing satellite data, and even those that now make their data freely available did not always do so. Open Data: The Global Effort for Open Access to Environmental Satellite Data examines how government agencies developed data sharing policies for their Earth observation satellites and how these data sharing policies changed over time.