IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change

4.3 Primary energy resource potentials, supply chain and conversion technologies

This section discusses primary-supply and secondary-energy (carrier) technologies. Technologies that have developed little since the TAR are covered in detail elsewhere (e.g., IEA, 2006a). Energy flows proceed from primary sources through carriers to provide services for end-users (Figure 4.3). The status of energy sources and carriers is reviewed here along with their available resource potential and usage, conversion technologies, costs and environmental impacts. An analysis is made of the potential contributions due to further technological development for each resource to meet the world’s growing energy needs, but also to reduce atmospheric GHG emissions. Assessments of global energy reserves, resources and fluxes, together with cost ranges and sustainability issues, are summarized in Table 4.2.

Table 4.2: Generalized data for global energy resources (including potential reserves), annual rate of use (490 EJ in 2005), share of primary energy supply and comments on associated environmental impacts.

Energy class Specific energy sourcea Estimated available energy resourceb (EJ) Rate of use in 2005 (EJ/yr)c 2005 share of total supply (%) Comments on environmental impacts 
Fossil energy Coal (conventional) >100,000 120 25 Average 92.0 gCO2/MJ 
Coal (unconventional) 32,000     
Peatd large 0.2 <0.1   
Gas (conventional) 13,500 100 21 Average 52.4 gCO2/MJ 
Gas (unconventional) 18,000 Small   Unknown, likely higher 
Coalbed methane  >8,000?  1.5 0.3  
Tight sands  8,000  3.3 0.7  
Hydrates  >60,000   
Oil (conventional) 10,000 160 33 Average 76.3 gCO2/MJ 
Oil (unconventional) 35,000 0.6 Unknown, likely higher 
Nuclear Uraniume 7,400 26 5.3 Spent fuel disposition 
Uranium recyclef 220,000 Very small   Waste disposal 
Fusion 5 x 109 estimated   Tritium handling 
Renewableg Hydro (>10 MW) 60 /yr 25 5.1 Land-use impacts 
Hydro (< 10 MW) 2 /yr 0.8 0.2  
Wind 600 /yr 0.95 0.2  
Biomass (modern) 250 /yr 1.8 Likely land-use for crops 
Biomass (traditional)  37 7.6 Air pollution 
Geothermal 5,000 /yr 0.4 Waterway contamination 
Solar PV 1,600 /yr  0.2 <0.1 Toxics in manufacturing 
Concentrating solar 50 /yrh 0.03 0.1 Small 
Ocean (all sources) 7/yr (exploitable) <1 Land and coastal issues. 


a See Glossary for definitions of conventional and unconventional.

b Various sources contain ranges, some wider than others (e.g., those for conventional oil cluster much more closely than those for biomass). For the purposes of this assessment of mitigation potentials these values, generalized to a first approximation with some very uncertain, are more than adequate.

c Hydro and wind are treated as equivalent energy to fossil and biomass since the conversion losses are much less (www.iea.org/textbase/stats/questionaire/faq.asp)

d Peat land area under active production is approximately 230,000 ha. This is about 0.05% of the global peat land area of 400 million hectares (WEC, 2004c).

e Once-through thermal reactors.

f Light-water and fast-spectrum reactors with plutonium recycle

g Data from 2005 is at www.ren21.net/globalstatusreport/issuesGroup.asp

h Very uncertain. The potential of the Mediterranean area alone has been estimated by one source to be 8000 EJ/yr (http:/www.dlr.de/tt/med-csp)

Sources: Data from BP, 2006; WEC, 2004c; IEA, 2006b; IAEA, 2005c; USGS, 2000; Martinot, 2005; Johansson, 2004; Hall, 2003; Encyclopaedia of Energy, 2004.

4.3.1 Fossil fuels

Fossil energy resources remain abundant but contain significant amounts of carbon that are normally released during combustion. The proven and probable reserves of oil and gas are enough to last for decades and in the case of coal, centuries (Table. 4.2). Possible undiscovered resources extend these projections even further.

Fossil fuels supplied 80% of world primary energy demand in 2004 (IEA, 2006b) and their use is expected to grow in absolute terms over the next 20–30 years in the absence of policies to promote low-carbon emission sources. Excluding traditional biomass, the largest constituent was oil (35%), then coal (25%) and gas (21%) (BP, 2005). In 2003 alone, world oil consumption increased by 3.4%, gas by 3.3% and coal by 6.3% (WEC, 2004a). Oil accounted for 95% of the land-, water- and air-transport sector demand (IEA, 2005d) and, since there is no evidence of saturation in the market for transportation services (WEC, 2004a), this percentage is projected to rise (IEA, 2003c). IEA (2005b) projected that oil demand will grow between 2002 and 2030 (by 44% in absolute terms), gas demand will almost double, and CO2 emissions will increase by 62% (which lies between the SRES A1 and B2 scenario estimates of +101% and +55%, respectively; Table 4.1).

Fossil energy use is responsible for about 85% of the anthropogenic CO2 emissions produced annually (IEA, 2003d). Natural gas is the fossil fuel that produces the lowest amount of GHG per unit of energy consumed and is therefore favoured in mitigation strategies. Fossil fuels have enjoyed economic advantages that other technologies may not be able to overcome, although there has been a recent trend for fossil fuel prices to increase and renewable energy prices to decrease because of continued productivity improvements and economies of scale. All fossil fuel options will continue to be used if matters are left solely to the market place to determine choice of energy conversion technologies. If GHGs are to be reduced significantly, either current uses of fossil energy will have to shift toward low- and zero-carbon sources, and/or technologies will have to be adopted that capture and store the CO2 emissions. The development and implementation of low-carbon technologies and deployment on a larger scale requires considerable investment, which, however, should be compared with overall high investments in future energy infrastructure (see Section 4.1).