For example, the US National Research Council has commented: The question of whether there exists a "safe" level of concentration of greenhouse gases cannot be answered directly because it would require a value judgment of what constitutes an acceptable risk to human welfare and ecosystems in various parts of the world, as well as a more quantitative assessment of the risks and costs associated with the various impacts of global warming.
例如，美國國家科學研究委員會已经建议： 是否存在一个温室气体浓度的“安全”级别这一问题是无法直接回答的，因为这需要对怎样的风险才是人类福利和世界各地生态环境能够接受的进行价值判断，并对全球变暖的各种影响相关的风险和成本进行更加量化的评估。