Indonesia's new planned capital in East Kalimantan is being touted as a "smart, green, beautiful and sustainable city" but has stoked fears of massive environmental damage to the island of Borneo, one of the world's most important biodiversity hotspots and carbon sinks. Precedents of other planned capitals can contribute to an understanding of the potential long-term impacts of Indonesia's new capital. We used historical nighttime lights to quantitatively assess the spatial growth footprint of

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... 2 previous planned capitals, and conducted land-use analyses to identify the potential environmental impacts on Borneo's natural environment. Our assessment suggests that it is likely that the direct footprint of the new capital could grow rapidly, expanding over 10 km from its core in less than two decades and over 30 km before mid-century. We identified sensitive ecosystems which may be affected by the new capital's direct and indirect footprint, such as forest reserves, mangrove and peat. Deforestation emissions from the new capital's direct (30 km) and indirect (200 km) footprint could be approximately 50 MtCO2e and 2326 MtCO2e respectively, equivalent to 2.7% and 126% of Indonesia's 2014 greenhouse gas emissions. We discuss how planned capitals can spatially restructure the socio-political geographies of cities and nation-states by interacting with meanings, symbolisms and power relations, which may aggravate environmental impacts but also be seized upon as a catalyst for improving environmental performance in Borneo and Indonesia. Finally, we recommend the use of best practices in impact assessment and sustainability as a necessary first step towards protecting Borneo.