We begin by examining reference scenarios that represent the range (but not necessarily the distribution) of scenarios found in the open literature and explore the role technology plays in shaping fossil-fuel carbon emissions. The main conclusion is that scenarios are generally optimistic about future development and deployment of technologies that provide energy services without carbon emissions. Yet, as with the IS92a scenario, assumed technology improvements do not guarantee that the concentration of atmospheric CO2 will be stabilized in any given scenario. In most instances there remains a gap between emissions in the scenario and an emissions trajectory that would stabilize CO2 concentrations. The size of this gap depends on the CO2 stabilization concentration and the degree to which advanced energy technologies—both supply and energy efficiency—are assumed to have already displaced carbon-emitting energy technologies.

A wide array of scenarios have been developed that explore future energy, industrial, and land use carbon emissions. One of the most recent examinations is the IPCC Special Report on Emission Scenarios (SRES) (Nakicenovic et al. 2000). This document examines scenarios published in the open literature and develops an array of new global scenarios looking forward through the 21st century. (The many scenarios contained in the SRES are organized into four major groups labeled A1, A2, B1, and B2. The characteristics of each group are discussed in Colorplate 4, and the associated primary energy requirements are shown in Colorplate 6.) The literature is rich, containing scenarios that range from those with rapidly rising emissions to those in which emissions follow a pattern consistent with the stabilization of greenhouse gas concentrations. Some scenarios exhibit more rapid growth in emissions than IS92a, and some are slower. This range of anthropogenic CO2 emissions for both fossil fuels and land use change is shown in Colorplate 6, from Nakicenovic et al. (2000).

The scenarios with lower energy requirements (Colorplate 6) represent sustainable futures with a transition to very efficient energy use and high degrees of conservation. Generally, these are also the scenarios in which energy sources with low carbon intensity play an important role. The scenarios with higher energy requirements generally rep-

10,000 9,000 8,000 7,000


3,000 2,000 1,000

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100


Figure 4.2. Effects of energy intensity improvements on energy demands in SRES illustrative scenarios.

resent futures with more rapid rates of economic growth or low rates of economic growth combined with high population.

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