People talk as if we can only use energy and materials one way: we take fossil fuels from the ground, use them up, and when they are gone our world will end. We mine metals and other essential substances, and when they are finished, so are we.

Science says something else. Energy is simply the ability to do work – move an object, heat it, or create light – and the universe is awash with it. Every year our sun releases 20 trillion times the energy our economy uses. Raw materials, meanwhile, are just the hundred or so chemical elements that everything is made of. Anything we need can be assembled from these.

Fossil fuels have become too problematic, but there are other sources of energy we can tap into. Most obvious is the heat and light from the sun, but there is also the tidal energy produced as the gravity of the sun and moon moves the oceans, and the geothermal heat leaking slowly from the Earth’s core.

Finally, there is nuclear energy, which can be obtained by fission (splitting) of elements such as uranium, or fusion of elements such as hydrogen.

The question is: would it be practical, with current or reasonably expected knowledge, to get all the energy we need from such sources? To analyse this, we must introduce a very large unit of energy – the exajoule. One exajoule is equal to five times the energy of the biggest nuclear weapon ever detonated, and the world economy uses roughly 500 exajoules every year.

Alternative resources
Tidal power resources are quite limited, being practical at only a few locations where tides are forced between islands, through straits or up into inlets. They are estimated at five exajoules per year, around one percent of current needs. Tidal power is, however, completely predictable – being linked to the cycles of the sun and moon.

Geothermal power is most abundant where there are volcanoes, but by drilling kilometres down can be harnessed anywhere. Rocks can be fractured at depth and water pumped in and out to extract natural heat, and from this electricity. Roughly 1,500 exajoules of geothermal energy flow out of the Earth each year, but it is only be practical to extract a small portion of this – a few percent of current energy use.

Bioenergy – solar energy trapped in plants – is also limited. Turning crops into fuels competes with food production and drives up prices, threatening the world’s poor. But we can grow algae in lakes, seawater or desert greenhouses, producing from them oils very like diesel. Fuel crops can also be grown on marginal land. And we can use biological wastes like wheat chaff and sawdust as fuel.

The carbon dioxide produced merely returns to the atmosphere what the plants removed, and solid waste can be returned to the soil. Using biological waste that is already produced could sustainably yield 50 exajoules a year – 10 percent of current energy needs. Energy crops that don’t compete with food could double this, but might disrupt certain habitats.

Accessible wave energy resources are estimated at 70 exajoules per year (14 percent of current energy use), concentrated mainly on west-facing coasts in the temperate zones. However, wave power estimates cannot be added to estimates for offshore wind because locations suitable for wind turbines are often also the ones we would use for wave harvesters.

Wind power is very promising: offshore wind alone represents a vast resource. There are 360 million sq. km of ocean, but perhaps just one percent of this is shallow and breezy enough for turbines. Assuming we could put 15 turbines of five megawatt capacity on each square kilometre, operating at an average 25 percent of that capacity, we get an average output of nearly 20 megawatts per square kilometre. This scales up to 2,000 exajoules a year – four times current energy needs.

The nuclear options
Nuclear fission is a mixed picture. Known resources of uranium-235, which can be used directly in reactors once refined, contain a total of 2,000 exajoules of recoverable energy, only four years of current total energy use. However, resources of uranium-238, which must be converted in breeder reactors before use, are equivalent to over 200,000 exajoules of energy, or half a millennium of current needs. Resources of other suitable elements such as thorium-232 would extend this further. The downsides are that breeder technology can be used to produce highly enriched plutonium for nuclear weapons – potentially making weapons proliferation much worse; and that although the statistical death rate from nuclear power is far lower than other energy sources, one big accident can make a large part of a country uninhabitable.

Nuclear fusion would be better. The relevant forms of hydrogen – deuterium from seawater and tritium made from lithium using neutrons from the reactor itself – are sufficiently abundant to meet current energy needs for millenia. Only mildly radioactive waste is produced and the technology cannot be used to make nuclear weapons. However, progress with fusion has only inched forward over recent decades and will take decades more to reach fruition – if at all.

The most promising energy resource is solar. Covering one percent of all land area (itself one third of the Earth’s surface) in solar collectors that operate at 20 percent efficiency would yield more than 3,000 exajoules per year – seven times current energy usage. This could be done using solar photovoltaic panels, solar thermal facilities (where sunlight heats fluids that then generate electricity) or other methods. Solar panels could be placed in desert arrays or on the roofs of buildings, and hot fluids from solar thermal plants could be stored to smooth intermittency. By comparison, over one percent of land use today is urban – making the scale of construction needed for solar power seem feasible, especially when we remember that cities are more complex than solar parks.

Closed cycles
In short, we have enough energy to support an economy at least 10 times bigger than today’s.  But energy is just one aspect of sustainability. When it comes to raw materials and the environment, a key phrase is ‘closed cycles.’ Living things have used the same finite pool of essential elements (carbon, oxygen, hydrogen, nitrogen and others) for billions of years in an endless cycle of birth, death, decay and re-birth.

There is no reason the industrial economy cannot do the same: endlessly re-using a finite pool of iron, copper, nitrates, water, topsoil and other essential materials. Metals can be recycled. Water supplies can be maintained by using efficient industrial and agricultural processes that use it more slowly than aquifers are naturally replenished. Desalination of seawater may also be feasible. Topsoil depleted through successive harvests can be replenished through composting or by adding biochar. Fertiliser minerals lost through water runoff could be recaptured, or fertilisers could be bound to materials that prevent run-off.

Closed cycles intrinsically entail far less release of wastes, reducing pollution to levels lower than the rates at which natural processes eliminate pollutants.

The key question is what ‘rate of flux’ can be supported in a closed cycle. If the world has 50bn tons of some material, and the world economy needs to process five billion tons of it every year, is this feasible? At some point, there will be a limit where the percentage of a given resource that is processed in a year cannot be increased.

Closed cycles in water, topsoil and fertiliser minerals could all face such limits not far above current levels of use. But agricultural and water scientists have found efficient ways we could get much more sustenance from current fluxes of these, so we should be able to comfortably feed and water the 10bn or so people that world population is forecast to plateau at. Other materials, such as metals, could be recycled at a flux rate far higher than current use and be used much more efficiently – so sectors of the economy other than agriculture still have vast capacity for growth.

There are many technical hurdles to be overcome, and economics will decide exactly which technologies are adopted. But there are no physical limits to maintaining our way of life.

Sean Harkin is author of The 21st-Century Case for a Managed Economy