The Special Report on Emissions Scenarios (SRES) is a report by the Intergovernmental Panel on Climate Change (IPCC) that was published in 2000. The greenhouse gas emissions scenarios described in the Report have been used to make projections of possible future climate change. The SRES scenarios, as they are often called, were used in the IPCC Third Assessment Report (TAR), published in 2001, and in the IPCC Fourth Assessment Report (AR4), published in 2007. The SRES scenarios were designed to improve upon some aspects of the IS92 scenarios, which had been used in the earlier IPCC Second Assessment Report of 1995. The SRES scenarios are "baseline" (or "reference") scenarios, which means that they do not take into account any current or future measures to limit greenhouse gas (GHG) emissions (e.g., the Kyoto Protocol to the United Nations Framework Convention on Climate Change).
Emissions projections of the SRES scenarios are broadly comparable in range to the baseline emissions scenarios that have been developed by the scientific community. The SRES scenarios, however, do not encompass the full range of possible futures: emissions may change less than the scenarios imply, or they could change more.
SRES was superseded by Representative Concentration Pathways (RCPs) in the IPCC fifth assessment report in 2014.
There have been a number of comments on the SRES. It has been called "a substantial advance from prior scenarios". At the same time, there have been criticisms of the SRES. The most prominently publicized criticism of SRES focused on the fact that all but one of the participating models compared gross domestic product (GDP) across regions using market exchange rates (MER), instead of the more correct purchasing-power parity (PPP) approach.
Purpose
Because projections of climate change depend heavily upon future human activity, climate models are run against scenarios. There are 40 different scenarios, each making different assumptions for future greenhouse gas pollution, land-use and other driving forces. Assumptions about future technological development as well as the future economic development are thus made for each scenario. Most include an increase in the consumption of fossil fuels; some versions of B1 have lower levels of consumption by 2100 than in 1990. Overall global GDP will grow by a factor of between 5–25 in the emissions scenarios.
These emissions scenarios are organized into families, which contain scenarios that are similar to each other in some respects. IPCC assessment report projections for the future are often made in the context of a specific scenario family.
According to the IPCC, all SRES scenarios are considered "neutral". None of the SRES scenarios project future disasters or catastrophes, e.g., wars and conflicts, and/or environmental collapse.
The scenarios are not described by the IPCC as representing good or bad pathways of future social and economic development.
Scenario families
Scenario families contain individual scenarios with common themes. The six families of scenarios discussed in the IPCC's Third Assessment Report (TAR) and Fourth Assessment Report (AR4) are A1FI, A1B, A1T, A2, B1, and B2.
The IPCC did not state that any of the SRES scenarios were more likely to occur than others, therefore none of the SRES scenarios represent a "best guess" of future emissions.
Scenario descriptions are based on those in AR4, which are identical to those in TAR.
A1
The A1 scenarios are of a more integrated world. The A1 family of scenarios is characterized by: Rapid economic growth. A global population that reaches 9 billion in 2050 and then gradually declines. The quick spread of new and efficient technologies. A convergent world - income and way of life converge between regions. Extensive social and cultural interactions worldwide.
There are subsets to the A1 family based on their technological emphasis: A1FI - An emphasis on fossil-fuels (Fossil Intensive). A1B - A balanced emphasis on all energy sources. A1T - Emphasis on non-fossil energy sources.....
A2
The A2 scenarios are of a more divided world. The A2 family of scenarios is characterized by: A world of independently operating, self-reliant nations. Continuously increasing population. Regionally oriented economic development. High emissions