climate by absorbing and reflecting solar radiation, and by altering fluxes of heat, water vapour, carbon dioxide and other trace gases. Detailed assessments — regional, global, daily and seasonal — of land use and land cover are needed to monitor biodiversity loss and ecosystem dynamics and to aid in reducing emissions from deforestation and forest degradation. Satellite imagery is the best source of such data, especially over large areas. Observations need to be extensive, regular and consistent to establish baselines and trends. But today, most satellite observations have limited coverage and compatibility, because they are controlled by the diverse objectives of national space programmes. In many cases, satellite data are restricted or charged for. A new era of open-access satellite data has arrived. In 2008, the US Geological Survey (USGS) released for free to the public its Landsat archive, which dates back to the 1970s and is the world’s largest collection of Earth imagery. Greater computing power is also enabling scientists to manipulate big data representing larger areas and with greater sophistication, to produce multibillion-pixel composite maps of land cover and change across regions, continents and the globe. Monitoring land-cover change in near-real time is now a reality. Obstacles remain. Data coverage in the Landsat archive is variable, in both space and time (see ‘Global coverage’). And few people have enough computing power and bandwidth to download and manipulate the data. Decision-makers remain largely unaware of the vastly improved opportunities for environ mental monitoring offered by the latest methods, and so are not yet using such data to their full potential. Scientists and policy-makers can support the shift to open-access satellite data, and coordinate efforts to deliver the detailed global monitoring required by international climate change and emissions-reduction programmes. Further, governments should open up their national satellite image archives and integrate compatible data to fill gaps. And satellite imagery from future missions should be freely accessible to all to promote innovation and use. Landsat is the longest-running civilian Earth-observing programme. The United States launched the first satellite in 1972, when public interest in space missions was high. In 1982, Landsat 4 began to deliver moredetailed imagery (with a spatial resolution of 30 metres, now considered a benchmark for historical analyses) and spectral channels at visible, nearand shortwave-infrared wavelengths, to track the unique signatures in reflected light of different types of vegetation. The latest in the unbroken series of Landsat satellites is Landsat 8, launched in 2013, which measures Earth’s surface with a resolution of 15–100 metres from visible to infrared wavelengths. Landsat data were archived from the outset, but early observing strategies and limitations to on-satellite storage and downlink capacity resulted in uneven global coverage. Access policies have changed over the years. In the 1980s, Landsat observations Make Earth observations open access
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