The Glasgow Declaration called for scientifically based measurements of CO2 emissions in the tourism industry to achieve the goals of the Paris Agreement. Despite the economic and employment downturn caused by COVID-19, there are limited cases of environmental assessments related to tourism. This study estimates the carbon footprint of the tourism industry in major G20 countries before and after COVID-19. Understanding the characteristics of inbound tourism and domestic tourism is important as they impact different markets. GHG emissions from tourism mainly stem from transportation, but the type of travel and country differences also make souvenirs, accommodation, and food and beverages significant emission factors. The pandemic has greatly impacted the tourism industry. In 2020, GHG emissions from both domestic and inbound tourism significantly decreased, affecting the number of tourists. In some countries, measures against COVID-19 influenced these figures, and although signs of recovery were observed in 2021, the reduction rates varied by country. These emission reductions should be the goals pursued by the tourism industry in the post-COVID-19 era, requiring efforts to achieve sustainable tourism.