Unwanted outcomes from the combination of many small-scale decisions.

How do you inclue consequences and incentivize against unwanted outcomes that may arise from the aggregate of many small-scale decisions that individually work towards a positive goal?

For example:

It is assumed that afforestation sequesters carbon and therefore has a global benefit by reducing greenhouse gas concentrations.  Each individual afforestation project may then appear to have positive effects, but large spatial scale changes in plant cover (from the shift to forest) from the combination of many small-scale decisions can have unwanted climate effects such as changes in precipitation patterns.

How should this potential for climate effects from the combination of many small projects be included in the decision making process?