This study explored the environmental impact of the large-scale projects, the 3rd Bridge across the Bosphorus and 3rd Airport, carried out in the last decade leading to the massive deterioration of the northern forest area in Istanbul. Destroyed forest area was assessed through relevant changes in land classification detected by multi-temporal Landsat data of Istanbul between 2009 and 2016. The magnitude of destroyed carbon stocks and related CO2 emission together with the reduction in the CO2 absorption potential inflicted by massive land-use change were also calculated. Observed results indicated that approximately 15,000 ha of forest area was destroyed in 7 years, corresponding to a 7% ultimate loss in the total forest area. The total land cover change for the same period was determined as 11.5% of the study area. The extent of land cover changes indicated that more than 4.4 million tons of CO2 were additionally emitted to the atmosphere, due to observed reduction of carbon stocks between 2009 and 2016. More than 70% of the total C/CO2 emission associated with land cover changes could be attributed to the loss of forest land. In addition, destroyed forestland corresponded to a CO2 absorption loss of 0.3 million tons CO2/year equivalent to the emission of 830,000 people in Istanbul.