Non-renewable energy supplies, exist on earth only in limited amounts. The following table indicates the limits of the earth's reserves of each energy source. Based on typical estimates, it can be assumed that the world will have to find alternatives for the 70% of global energy currently supplied by petroleum and natural gas within 50 years. The transportation sector in particular will have to shift to a new dominant energy source.

Renewable energy, by definition is not limited in terms of supply, but is limited in terms of the total amount that what can be produced at a certain place or time for a reasoable price. Since the feasibility of renewable energy ultimately is a question of costs, here are some estimates.

Why Carbon Fuels Will Dominate The 21st Century’s Global Energy Economy

• Global energy demand has grown only slowly since the mid-1970s. This is predicated to continue, even without actions to limit/reduce CO2 emissions. Nevertheless, increasing energy use is required to eliminate energy poverty, not only for the 2 to 3 billion of the world's present population which suffer from the condition, but also for the net additional 3 billion inhabitants of planet earth by 2050. Sustainability for a more populous 21st century world depends on this development.
• Carbon fuel use since 1970 has been much below expectations at that time. Consequently, proven reserves have expanded to an all-time high. These provide a secure base for supply growth of coal, oil and gas. Collectively their present 90% share of global energy use will decline only very slowly, viz. to 80% by mid-century and still over 55% by 2100.
• Conventional expectations for a high growth rate in coal's use are unrealistic. At most, its contribution to fossil fuel supplies will be 30%. Thus hydrocarbons will remain dominant supplying over 50% of global energy use until post-2070. But their relative contributions will change dramatically. As gas supply grows more than five-fold over the century, it will contribute 57% of hydrocarbons supply (oil; 43%). Gas will overtake coal by 2010 and oil by 2040. Conventional and non-conventional gas reserves and resources will be adequate to maintain gas as a growth industry until post-2090 - even without a contribution from gas hydrates and geo-pressured gas.
• Oil supply will likely be more demand than resources constrained - given the demand-side preference for gas for technical and commercial, as well as environmental, reasons. The remaining 2000x109 barrels of recoverable conventional oil resources will sustain over 50% of oil demand until almost 2060. Thereafter non-conventional oil will take the leading role. The latter's output will peak only in the penultimate decade of the century and by 2100 its resource-base will be little more than half-depleted. In spite of oil's relative decline as an energy source the industry will, nevertheless, be larger in 2100 than it was in 2000.
• The historic and contemporary geo-political significance of the oil industry will, however, be undermined over the coming two decades. Thereafter, with its falling contribution to global energy supply and an increasingly dispersed geographical pattern of production, it will become "just another source of energy" in a competitive market.
• Serious medium- to long-term upward pressures on oil and gas prices are unlikely. Higher finding/exploitation costs will be offset by technologically-derived savings. Modest real price increases from post-1986 levels (of 10 to 20% by 2020 and 20 to 30% by 2040) will subsequently be constrained by lower-cost renewable energy supplies."

Assumptions made by EREC- the European Renewable Energy Council- together with its member associations (EPIA, ESHA, ESTIF, EUBIA, EUREC Agency, EWEA, AEBIOM
and EGEC) based on experiences and cumulative knowledge lead to assumptions on expected annual installations growth rates for different technologies and show that by 2040, a share of renewable energy up to 50% worldwide is possible. To reach such a share, advanced, intelligent and reliable policy measures have to be implemented at least in the majority of countries worldwide.

Policy measures such as the implementation of the Kyoto protocol, internalisation of external costs for conventional energy supply, ending subsidies to conventional, polluting energy sources and other initiatives have to be adapted to make the assumptions a reality. If these measures are not adopted in significant parts of the world, the deployment of renewable energy sources will be much slower. But even then the natural benefits of renewables will be used to supply 27 % of the world’s energy needs.

WEC Statment 2005 - Delivering Sustainability: Challenges and Opportunities for the Energy Industry
The World Energy Council (WEC) endorses this conclusion but would add that we do not have a century to act! We can and must act now to put energy development on a fully sustainable path. The keys to delivering energy sustainability, as reaffirmed at Sydney, are to:

• Keep all energy options open No technology should be idolised or demonised and energy efficiency must be increased.
• Ensure the necessary investment in energy infrastructure For this, cost-reflective energy prices are essential - systems which do not pay for themselves are ultimately unsustainable.
• Adopt a pragmatic approach to market reform This would accommodate specific policy measures to achieve certain objectives, while allowing for the operation of the market to the maximum possible extent.
• Place priority on the measures needed to ensure reliability of supply Above all this depends on energy diversity, supported by sound market design and improved generating plant performance.
• Promote regional integration of energy supply systems This further supports reliability and calls for stronger regional collaboration.
• Exploit the "win-win" opportunities of emerging climate change responses The mechanisms, whether voluntary or regulated, should embrace least cost emissions reduction, encouraging transfer of clean technology from industrialised to developing countries.
• Ensure technical innovation It is vital to reconciling development with environmental protection and calls for strong, sustained support for R&D
• Foster and sustain public understanding and trust This in turn depends on energy sector transparency and better public information, starting in particular with young people.

No single player in the energy system can act alone: government policy, stable and clear regulation, industry investment, civil society and the final consumers of energy services all have their individual roles to play. As the sole global, non-governmental body looking at energy in its totality, it is WEC's mission to pull the stakeholders together, to get the facts out, and to advocate action where this is necessary.