Several years ago Bill McKibben contacted Dr. Hansen to ask for help in providing the scientific basis for a CO2 target of 450 ppm, as he was planning to form an organization 450.org. Dr. Hansen asked Bill to hold off while he worked on a relevant paper with several of the leading experts on the global carbon cycle and global climate. After a few months the team provided results to McKibben and within a year published a monumental paper (Target atmospheric CO2:Where should humanity aim?). Dr. Hansen and team concluded that maintaining a long-term CO2 level of 450 ppm would have highly deleterious impacts, and that a more appropriate long-term goal was 350 ppm of CO2. Subsequent fundamental work on Earth’s energy imbalance (Earth’s energy imbalance and implications) confirmed the accuracy of that estimate, inspiring McKibben to form 350.org instead of 450.org.
Thus began an effective collaboration, which continues today. Current cooperation includes their efforts to stop construction of the Keystone XL Pipeline, because such infrastructure would lead to eventual exploitation of much of the huge tar sands deposits.