Energy efficiency will offer the biggest scope for cutting emissions

International Energy Agency : Economist, engineer
End-use efficiency is the largest contributor to CO2 emissions abatement in 2030, accounting for more than half of total savings in the 450 Scenario, compared with the Reference Scenario.
Energy-efficiency investments in buildings, industry and transport usually have short pay-back periods and negative net abatement costs, as the fuel-cost savings over the lifetime of the capital stock often outweigh the additional capital cost of the efficiency measure, even when future savings are discounted. Decarbonisation of the power sector also plays a central role in reducing emissions. Power generation accounts for more than two-thirds of the savings in the 450 Scenario (of which 40% results from lower electricity demand). There is a big shift in the mix of fuels and technologies in power generation: coal-based generation is reduced by half, compared with the Reference Scenario in 2030, while nuclear power and renewables make much bigger contributions. The United States and China together contribute about half of the reduction in global power-sector emissions. Carbon capture and storage (CCS) in the power sector and in industry represents 10% of total emissions savings in 2030, relative to the Reference Scenario. Measures in the transport sector to improve fuel economy, expand biofuels and promote the uptake of new vehicle technologies — notably hybrid and electric vehicles — lead to a big reduction in oil demand. By 2030, transport demand for oil is cut by 12 mb/d, equal to more than 70% of all the oil savings in the 450 Scenario. Road transport accounts for the vast majority of these transport-related oil savings. A dramatic shift in car sales occurs; by 2030, conventional internal combustion engines represent only about 40% of sales, down from more than 90% in the Reference Scenario, as hybrids take up 30% of sales and plug-in hybrids and electric vehicles account for the remainder. Efficiency improvements in new aircraft and the use of biofuels in aviation save 1.6 mb/d of oil demand by 2030.

WEO 2009