What is an example of the difference between long-term and short-term interests as it pertains to ethics?

Ethics is concerned with the valuation of behavior as right or wrong, good or bad. John Stuart Mill's theory of Utilitarianism views ethical behavior as doing the greatest amount of good for the greatest number of people. Other thinkers have developed their own ethical systems, but let us use Mill's for the sake of clarity. Now, why would a behavior be good in the short term yet bad in the long term, or vise-versa?
An example of the short-term vs. long-term dilemma is what to do about climate change. The vast majority of climate scientists believe that climate change is real, caused by human behavior and is a threat to our survival in the long-term. If we take a short-term ethical view of this issue, we may say to continue relying on fossil fuels the way we have been because this reliance has proven to benefit the greatest number of people since the start of the industrial revolution. Of course by doing so the trade off is that we risk the long-term disasters predicted by the majority of scientists.
If, on the other hand, we take a long-term ethical view, we would diminish our reliance on fossil fuels as quickly and completely as possible. This would give us the best chance of insuring the survival of the greatest number of people in the long-term. The trade off here is that in the short-term the way of life we have come to know thanks to the power of fossil fuels would be diminished.
There is no easy answer as we can see from the political disagreements over what to do about this threat to the survival of the species. Short-term vs. long-term dilemmas arise in all walks of life. As mere humans, we often want the short-term benefit and end up rolling the dice on what happens long-term.

Let's start with ethics. The Oxford dictionary defines ethics as the "moral principles that govern a person's behavior or the conducting of an activity." That sounds like a great definition; however, "moral principles" will differ from person to person. This means that long term and short term ethical interests for one person might not exist as possibilities for all other people.
I think parenting could offer some good examples. In the short term, a parent wants to be able to provide for his or her family. That parent also wants that child to grow up to be a "good" person. Does a parent compromise his or her modeling of right and wrong to steal food in order to feed the family? In the short term, the family stays fed; however, the consequences might be that the children learn that stealing at certain times is acceptable.
The business world and city governance could offer a good example too. Let's use something like traffic congestion. A city planner could decide that it is beneficial to the largest number of people to make a road change that will increase movement through the city and increase the overall business gains of the city. That would appear to be the right decision and a "good" decision; however, that decision might come at the cost of destroying a beautiful city park or the city's focus on historical preservation. The "right" decision in this case is cloudy because the long term city interests are in contrast with the immediate, short term interests.