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:

Carbon-based energy is relatively cheap, and it is readily transported from one place to another as coal and oil. The methods for converting coal and oil to electric energy are well known, and the energy derived lacks the stigma accorded nuclear power in many countries. Also, it is difficult to find ways to power cars efficiently other than with fuel derived from petroleum. (Electric cars, of course, also use fuel, except that it is burned not in the car but elsewhere.)
The downside of greenhouse gas emissions from generating power and from moving vehicles is not immediately obvious and is not localized. The benefit of using fossil fuels is readily obvious to the user, while the cost is distributed generally to all the Earth's atmosphere, and hence all of the Earth's people. Thus, it is difficult to motivate people to do something about it.

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:

The future of emissions involves the behavior of people and the economic development of nations. Both are notoriously difficult to predict.
The plain physical greenhouse warming from given rates of emissions is difficult to calculate, even when all other factors are kept constant.
A lot of factors play a role that are by no means constant. Warming, and especially unequal warming, generates all sorts of changes which in turn slow the warming (like heat uptake by the ocean) or accelerate it (like darkening of the ground by melting snow). The ultimate outcome of such calculations have a large margin of error.
Non-linear effects of global warming (threshold effects such as stopping the ocean's deep mixing, or runaway effects such as producing methane from melting permafrost or from a warming sea floor) introduce potentially important dynamics which are very poorly understood and cannot be predicted with much confidence at all.

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.