How can we balance conflicting moral obligations to the future and to the present? Things we do today--say, burn fossil fuels--improve the lives of people living now, while, science suggests, creating very serious problems for the future that may be impossible to aleviate once the negative effects are felt. Activities which may cause very specific good effects now have negative effects which are vanishingly small for a single instance but cumulatively disasterous in the future (this is, perhaps, an entirely different but related issue).
This seems analogous in some ways to the trolley problem; if you would pull the lever to save five people while killing one, does it matter when the five people would have been killed? What if it were five people with ninety percent certainty one hundred years in the future?