Answer: Here are some suggested answers.
1. The obvious starting point is to say that it depends on how
much fossil fuels are used. The less we use, the longer they
will last. More and more industrialisation all over the world
increases their consumption.
However, we assume that you mean how long they will last, if
they are used at their current rates.
As far as coal is concerned, the world still has huge reserves
which should last for hundreds of years. The demand for coal
has not increased as much as predicted, partly due to coal being
seen as more of a pollutant than oil or gas and also due to the
adverse conditions experienced in mining it. Concerns were also
expressed about the effect of acid rain due to the emissions
from coal burning power stations. It is possible that given improving
technologies, gas will be more cleanly and readily extracted
from coal, which would of course, increase its usage and therefore
the stocks will not last so long.
Crude oil is being used at an extremely high rate and present
reserves could run out in 30 - 50 years time, unless we use less
or something is found as an adequate replacement or new methods
are discovered to extract the pools of oil which presently cannot
Natural gas is as exhaustible as the other fossil fuels, however,
as the large natural gas fields dry up, new ones are being discovered.
But locating these and drilling for them is very costly.
2. There are a number of emerging technologies which could make
the petroleum reserves last longer.
Solar energy which can be divided into three main groups
- heating and cooling applications; electricity generation; and
fuels from biomass.
Heating and cooling: drawing on the heat of the sun to
provide hot water and heating for households and industry. In
the cooling, which is not widely used, a liquid is cooled by
first being heated to a temperature sufficient to drive a refrigeration
Electricity generation: this can use a variety of technologies
which arguably depend on the effects of solar radiation. Windmills,
turbines, hydro-electric installations and wave power fall into
this category. Photovoltaic cells convert sunlight directly into
electricity and used in remote areas, such as orbiting space
satellites, but this is costly. Ocean thermal conversion (OTC)
generates electricity on offshore platforms.
Biomass: this happens where any solid liquid or gaseous
fuel is produced from organic matter. Wood and dung are frequently
used as well as other industrial, agricultural and domestic waste.
Geothermal energy: where steam trapped deep in the
earth is brought to the surface to drive a turbine to produce
Synthetic fuels are made from substances found in nature;
such as gasohol, a mixture of gasoline and alcohol made from
living plants. This is done on a relatively large scale in Brazil.
Nuclear energy: where fission of uranium is used to
produce heat which eventually drives a turbine to produce electricity.
There are drawbacks for the environment with most of these,
e.g. nuclear energy - the problems of nuclear waste.
With synthetic fuels countries are encouraged to grow cash crops
rather than agricultural for food.
Hydro-electricity also can have adverse effects on the environment
when dams are built.
Within all this you cannot discount the fact that the industrialised
nations have to become more energy aware and make serious attempts
at reducing dependency on fossil fuels.