Attempts to Guess the Future

Reducing Carbon Emissions
The fundamental problem handed to the 21st century by the 20th is the fact that the number of humans have been growing exponentially and are still expanding rapidly, and that the increasing needs for sustenance and the strong desire for a better standard of living can only be satisfied (if at all) at the cost of increased energy use.

There are a number of reasons why reducing the use of carbon-based energy and the greenhouse gas emissions that go with it is difficult. Perhaps the two more important ones are as follows:
The Role of Scientists
As seen with the ozone hole and the Montreal Protocol, governments will act when the danger ahead is quite well defined and not too far in the future and when the remedy is obvious and not too expensive. Therefore, if a scientist's advice is to be useful to government it needs to define the danger that the future might bring, how soon it may be expected, and what types of action might avert the danger anticipated. Guessing the future climate, then, is an attempt to make science useful to human affairs. There are several circumstances that make such guessing very difficult, as follows:
Coming Up with Realistic Scenarios
In a sense, the best scientists can do is to assume the future rate of emissions, and combine this with the best knowledge of physics of the climate system, and then let their computers paint pictures of the future. Such pictures, called “scenarios,” are not predictions in the usual sense of a weather forecast for the next few days. But they can give a range of possible outcomes for what Roger Revelle called the “great one-time geophysical experiment” that humankind is now doing on the planet by burning up the supplies of fossil fuels.