The world's top climate scientists, and a handful of diplomats, have just approved a summary of "policy relevant" conclusions on the physical science of climate change. The most controversial finding of the report is also the most significant one for us.

The "carbon budget" was the last part of the summary to be decided, and the subject of hours of heated discussions in the early hours of Sept 27.

The political implication is clear. Annual emission targets pit old against new emitters and point the finger at economic growth in developing countries, ignoring the historical emissions of the developed countries. The problem is that it also deals with the symptoms and ignores the causes of these emissions.

A carbon budget will consider the activities that generate these emissions in the course of economic development, leading to very different conclusions. It recognizes that the bio-physical and social systems are intertwined in such a way that the conditions and responses to external forcing, like increases in carbon dioxide in the atmosphere, are based on the synergy between the two sub-systems. Consequently, the full global system has to be studied rather than its individual components, as none of the challenges can be fully studied without addressing the other challenges. So far the projections about the future have focused on environmental damage rather than the relationship between people and planet.

The policy issue a carbon budget raises is to identify the longer-term trends that need to be modified, rather than agree to short-term targets. With the global middle class expected to triple by 2030, the central issue is how an acceptable level of well-being for each person can be achieved without having a serious impact on the planet. How the limits are approached depends on what is regarded as essential for human well-being.