Sentence:  Moving to the climate change context, as long as there is not a comprehensive climate policy in place, respect for the principles of polluter pays and proportionality will require a mechanism to determine the level of pollution created by each polluter, so as to avoid that some polluters are overburdened while others are <WSD>underburdened</WSD>. 
Meaning of the word marked with <WSD> tags:  To give too light a burden; to assign less than a fair share.