By January 2026, the BASIC countries (Brazil, South Africa, India, and China) still did not commit to join the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) initiated by the International Civil Aviation Organization (ICAO) and remain skeptical about its equity, creating uncertainty for the second phase of CORSIA. This study quantifies member states’ offsetting requirements in the second phase and assesses the impact of a potential withdrawal by BASIC countries. The results indicate that developing countries face substantial economic burdens. China, with 657.0 Mt of CO2 emissions, ranks second globally by offsetting requirements and is projected to incur USD 12.5 billion in offsetting costs. India, South Africa, and Brazil would pay USD 1.7, 0.51, and 0.20 billion, respectively. If the BASIC countries collectively withdraw from CORSIA, the overall effectiveness of CORSIA would be significantly undermined, reducing total offsetting requirements by 347.4 Mt of CO2 emissions, equivalent to 23% of total amount. This study recommends refining the mechanism design of CORSIA and exploring feasible avenues for its improvement.