This issue brief builds on past research (e.g., Prest et al. 2024; Prest 2022) to estimate the magnitude of emissions increases from a sustained expansion of oil and gas leasing on federal lands. Evaluating the emissions effects of expanded leasing is particularly challenging, as it involves understanding the anticipated effect of lease sales on oil and gas drilling on federal lands; the resulting increase in federal oil and gas production; the amount of production “leakage,” meaning partial substitution of production between federal lands and other sources of supply; and the greenhouse gas emissions resulting from the net increase in global oil and gas consumption. The central estimate suggests that perpetually expanded oil and gas leasing on federal lands at annual rates consistent with the fastest pace of leasing over the past decade would increase cumulative global greenhouse gas emissions by 1.2 gigatons (billion metric tons) of CO2 equivalent (GtCO2e) over 2024–2050, under a 100-year global warming potential (GWP). For reference, this 1.2 GtCO2e value corresponds to approximately 43 million tons CO2e per year over that 27-year window, or about 0.1 percent of current annual global greenhouse gas emissions. This increase is relative to a business-as-usual (BAU) baseline of about half that amount of leasing. An extensive set of sensitivity analyses suggests that emissions increases are highly unlikely to be outside the 0.6 to 2.1 GtCO2e range. On a territorial emissions accounting basis, US emissions account for approximately 0.2 GtCO2e of the global increase of 1.2 GtCO2e estimated in the central case. This analysis covers onshore development on federal lands and does not consider the potential impacts of expanded offshore development, which accounts for another 14 percent of national US oil production and 2 percent of national gas production.