The International Civil Aviation Organization's (ICAO) global carbon offsetting system, the first of its kind for a single industry, is projected to hinder the increase of emissions from commercial flights. This initiative is estimated to cost the industry less than 2 percent of its revenues. While individual countries' governments still need to take action to enforce the agreement's restrictions, the system will transition from voluntary from 2021 to 2026 to mandatory from 2027 for states with larger aviation industries. Airlines will be required to purchase carbon credits from specific environmental projects worldwide to counterbalance the growth in emissions. According to Olumuyiwa Benard Aliu, president of ICAO's governing council, the system is a result of compromises and consensus. Aliu stated that objections from a few countries would not stop the plan from moving forward. He mentioned that with 65 countries involved, which account for over 80 percent of aviation activity in the initial phases, participation exceeded the agency's expectations and is expected to continue to increase. The main source of tension was between developed nations, who have been responsible for the majority of greenhouse gas emissions in the past, and emerging and developing countries concerned about the potential costs hindering growth. Both Russia and India have declined to participate in the initial phases, arguing that the deal unfairly burdens emerging countries. However, China has confirmed its intention to take part in the voluntary phase. Brazil has expressed support for the deal but has not confirmed if it will participate in the initial stages. Estimates suggest that the deal could cost airlines between $1.5 billion and $6.2 billion by 2025, with the cost not exceeding 1.8 percent of industry revenues by 2035. Despite slim profit margins for airlines, the deal is seen as less costly than alternative national and regional climate agreements. Paul Steele, a vice president of IATA, stated that the industry views the cost as manageable. The US Department of State, which advocated for a deal, stated that it would lead the industry towards sustainable, carbon-neutral growth. However, environmentalists argued that due to the voluntary phase and exceptions for smaller markets, the scheme would not achieve its own objectives. The International Council on Clean Transportation estimated that the agreement would only require airlines to offset around three-quarters of growth after 2021, or one-quarter of total international traffic. Some critics also raised concerns about the deal's reliance on offsets, with Transport and Environment director Bill Hemmings stating that it would not reduce demand for jet fuel and would not effectively combat climate change. Discussions will now focus on the technical aspects of the deal, particularly the types of offset credits that will be deemed acceptable. The agreement, approved by an assembly of ICAO's 191 member states in Montreal, will cover international passenger and cargo flights, as well as business jets that produce over 10,000 tonnes of emissions annually. Previous negotiations caused tension before the 2013 ICAO assembly, as the European Union, frustrated with slow progress, mandated foreign airlines to purchase credits under its own program, which led to concerns about violating sovereignty from countries like China. This agreement was reached the day after the Paris climate accord came into effect. Aviation was not included in the agreement, even though the industry is responsible for approximately 2% of carbon dioxide emissions, which is more than some industrialized nations. With the industry predicting that the number of passengers will double to 7 billion by 2034, it is essential to reduce aircraft pollution in order to meet the temperature goals set in Paris, according to Lou Leonard, a vice president at the World Wildlife Fund.