This section summarizes recent developments that are likely to affect the construction of scenarios over the coming few years. One of these developments is construction of the new SRES emission scenarios. Some features of these scenarios and their implications for atmospheric composition, global climate, and sea level are described below. In addition, a brief review of possible regional climate changes during the 21st century is presented, followed by discussions of stabilization scenarios and changes in climate variability and extreme events—key issues in constructing scenarios for policy-relevant impact and adaptation assessments.

Development of the SRES scenarios (Nakicenovic et al., 2000) is outlined in Section 3.2.4.1. The 40 scenarios, 35 of which are fully quantified, are based on four different narrative storylines and associated scenario families. Each storyline describes a different world evolving through the 21st century, and each may lead to quite different GHG emissions trajectories. Four of the scenarios are designated as "markers," each characterizing one of four "scenario families"; two additional scenarios illustrate alternative energy developments in one of the families. The storylines and scenario families are as follows:

• A1: A future world of very rapid economic growth, global population that peaks mid-century and declines thereafter, and rapid introduction of new and more efficient technologies. Major underlying themes are economic and cultural convergence and capacity-building, with a substantial reduction in regional differences in per capita income. The A1 scenario family develops into three groups that describe alternative directions of technological change in the energy system: fossil-intensive (A1FI), nonfossil energy sources (A1T), and a balance across all sources (A1B).
• A2: A differentiated world. The underlying theme is self-reliance and preservation of local identities. Fertility patterns across regions converge very slowly, resulting in continuously increasing population. Economic development is primarily regionally orientated, and per capita economic growth and technological change are more fragmented and slower than other storylines.
• B1: A convergent world with rapid change in economic structures toward a service and information economy, reductions in material intensity, and introduction of clean technologies. The emphasis is on global solutions to economic, social, and environmental sustainability, including improving equity, but without additional climate change policies.
• B2: A world in which the emphasis is on local solutions to economic, social, and environmental sustainability. This is a world with continuously increasing global population at a lower rate than in scenario A2, intermediate levels of economic development, and less rapid and more diverse technological change than in the A1 and B1 storylines. Although this scenario also is orientated toward environmental protection and social equity, it focuses on the local and regional levels.

Measures of global population, economic development (expressed in annual GDP), and equity (per capita income ratio) for 2050 and 2100 that are implied under the SRES scenarios are shown in Table 3-9, alongside the IS92a scenario and estimates for the present day. Attempts are underway to "downscale" aspects of these global scenarios for use in regional impact assessment (e.g., Lorenzoni et al., 2000).

Table 3-10: Examples of impacts resulting from projected changes in extreme climate events.
Projected Changes during the 21 st Century in Extreme Climate Phenomena and their Likelihooda	Representative Examples of Projected Impactsb (all high confidence of occurrence in some areasc)
Simple Extremes
Higher maximum temperatures; more hot days and heat wavesd over nearly all land areas (Very Likelya)	Increased incidence of death and serious illness in older age groups and urban poor Increased heat stress in livestock and wildlife Shift in tourist destinations Increased risk of damage to a number of crops Increased electric cooling demand and reduced energy supply reliability
Higher (increasing) minimum temperatures; fewer cold days, frost days, and cold wavesdover nearly all land areas (Very Likelya)	Decreased cold-related human morbidity and mortality Decreased risk of damage to a number of crops, and increased risk to others Extended range and activity of some pest and disease vectors Reduced heating energy demand
More intense precipitation events (Very Likelya over many areas)	Increased flood, landslide, avalanche, and mudslide damage Increased soil erosion Increased flood runoff could increase recharge of some floodplain aquifers Increased pressure on government and private flood insurance systems and disaster relief
Complex Extremes
Increased summer drying over most mid-latitude continental interiors and associated risk of drought (Likelya)	Decreased crop yields Increased damage to building foundations caused by ground shrinkage Decreased water resource quantity and quality Increased risk of forest fire
Increase in tropical cyclone peak wind intensities, mean and peak precipitation intensities (Likelya over some areas)e	Increased risks to human life, risk of infectious disease epidemics, and many other risks Increased coastal erosion and damage to coastal buildings and infrastructure Increased damage to coastal ecosystems such as coral reefs and mangroves
Intensified droughts and floods associated with El Niño events in many different regions (Likelya) (see also under droughts and intense precipitation events)	Decreased agricultural and rangeland productivity in drought- and flood-prone regions Decreased hydro-power potential in drought-prone regions
Increased Asian summer monsoon precipitation variability (Likelya)	Increase in flood and drought magnitude and damages in temperate and tropical Asia
Increased intensity of mid-latitude storms (little agreement between current models)d	Increased risks to human life and health Increased property and infrastructure losses Increased damage to coastal ecosystems
a Likelihood refers to judgmental estimates of confidence used by TAR WGI: very likely (90-99% chance); likely (66-90% chance). Unless otherwise stated, information on climate phenomena is taken from the Summary for Policymakers, TAR WGI. b These impacts can be lessened by appropriate response measures. c Based on information from chapters in this report; high confidence refers to probabilities between 67 and 95% as described in Footnote 6 of TAR WGII, Summary for Policymakers. d Information from TAR WGI, Technical Summary, Section F.5. e Changes in regional distribution of tropical cyclones are possible but have not been established.

IPCC Homepage