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.Section 1.5: Why New IPCC Emissions Scenarios?, in . 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)., in
Emissions projections of the SRES scenarios are broadly comparable in range to the baseline emissions scenarios that have been developed by the scientific community., in 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SRES was superseded by Representative Concentration Pathways (RCPs) in the IPCC fifth assessment report in 2014.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"., in At the same time, there have been criticisms of the SRES., in 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., in
Purpose The four SRES scenario familiesaccording to: Canadian Institute for Climate Studies, CCIS project: Frequently Asked Questions Table 4-2: Overview of SRES scenario quantifications, in .Figure 2.11: Schematic illustration of SRES scenarios, in . of the Fourth Assessment Report vs. projected global average surface warming until 2100AR4 (Summary; PDF)More economic focusMore environmental focusGlobalisation(homogeneous world)A1 rapid economic growth(groups: A1T; A1B; A1Fl)1.4 − 6.4 °C B1global environmental sustainability1.1 − 2.9 °CRegionalisation(heterogeneous world) A2regionally orientedeconomic development2.0 − 5.4 °C B2local environmental sustainability1.4 − 3.8 °C
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"., in 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., in
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., in
Scenario descriptions are based on those in AR4, which are identical to those in TAR.Summary for Policymakers, in .
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
B1 The B1 scenarios are of a world more integrated, and more ecologically friendly. The B1 scenarios are characterized by: Rapid economic growth as in A1, but with rapid changes towards a service and information economy. Population rising to 9 billion in 2050 and then declining as in A1. Reductions in material intensity and the introduction of clean and resource efficient technologies. An emphasis on global solutions to economic, social and environmental stability.
B2 The B2 scenarios are of a world more divided, but more ecologically friendly. The B2 scenarios are characterized by: Continuously increasing population, but at a slower rate than in A2. Emphasis on local rather than global solutions to economic, social and environmental stability. Intermediate levels of economic development. Less rapid and more fragmented technological change than in A1 and B1.
SRES scenarios and climate change initiatives While some scenarios assume a more environmentally friendly world than others, none include any climate-specific initiatives, such as the Kyoto Protocol.
Atmospheric GHG concentrations
The SRES scenarios have been used to project future atmospheric GHG concentrations. Under the six illustrative SRES scenarios, the IPCC Third Assessment Report (2001), in projects the atmospheric concentration of carbon dioxide () in the year 2100 as between 540 and 970 parts per million (ppm). In this estimate, there are uncertainties over the future removal of carbon from the atmosphere by carbon sinks. There are also uncertainties regarding future changes in the Earth's biosphere and feedbacks in the climate system. The estimated effect of these uncertainties mean that the total projected concentration ranges from 490 to 1,260 ppm. This compares to a pre-industrial (taken as the year 1750) concentration of about 280 ppm, and a concentration of about 368 ppm in the year 2000.
The United States Environmental Protection Agency has also produced projections of future atmospheric GHG concentrations using the SRES scenarios. These projections are shown opposite, and are subject to the uncertainty described earlier regarding the future role of carbon sinks and changes to the Earth's biosphere.
Observed emissions rates
Between the 1990s and 2000s, the growth rate in CO2 emissions from fossil fuel burning and industrial processes increased (McMullen and Jabbour, 2009, p. 8). The growth rate from 1990 to 1999 averaged 1.1% per year.
Between the years 2000–2009, growth in emissions from fossil fuel burning was, on average, 3% per year, which exceeds the growth estimated by 35 of the 40 SRES scenarios (34 if the trend is computed with end points instead of a linear fit). Human-caused greenhouse gas emissions set a record in 2010, a 6% jump on 2009 emissions, exceeding even the "worst case" scenario cited in the IPCC Fourth Assessment Report.
Views and analysis
MER and PPP
The SRES scenarios were criticised by Ian Castles and David Henderson.Castles and Henderson (2003), Energy and Environment, 14:159-185Castles and Henderson (2003), Energy and Environment, 14:415-435 The core of their critique was the use of market exchange rates (MER) for international comparison, in lieu of the theoretically favoured PPP exchange rate which corrects for differences in purchasing power.Nordhaus (2007), Energy Economics, 29:349–372 The IPCC rebutted this criticism.Economist (Feb 13, 2003) Hot Potato: The IPCC had better check its calculations,Economist (Nov 6, 2003) Hot Potato Revisited: A lack-of-progress report on the IPCCEconomist (May 27, 2004) Measuring Economies: Garbage In, Garbage Out
The positions in the debate can be summarised as follows. Using MER, the SRES scenarios overstate income differences in past and present, and overestimate future economic growth in developing countries. This, Castles and Henderson originally argued, leads to an overestimate of future greenhouse gas emissions. The IPCC future climate change projections would have been overestimated.
However, the difference in economic growth is offset by a difference in energy intensity. Some say these two opposite effects fully cancel,Gruebler et al. (2004), Energy and Environment, 15:11-24 some say this is only partial.Holtsmark and Alfsen (2005), Climatic Change, 68:11-19 Overall, the effect of a switch from MER to PPP is likely to have a minimal effect on carbon dioxide concentrations in the atmosphere.Manne et al. (2005), Climatic Change, 71:1-8 Castles and Henderson later accepted this and acknowledged that they were mistaken that future greenhouse gas emissions had been significantly overestimated.
But even though global climate change is not affected, it has been argued that the regional distribution of emissions and incomes is very different between an MER and a PPP scenario. This would influence the political debate: in a PPP scenario, China and India have a much smaller share of global emissions. It would also affect vulnerability to climate change: in a PPP scenario, poor countries grow more slowly and would face greater impacts.
Availability of fossil fuels
As part of the SRES, IPCC authors assessed the potential future availability of fossil fuels for energy use.Section 4.4.6. Resource Availability, in Chapter 4: An Overview of Scenarios, in . SRES assumptions about availability of fossil fuels is largely based on a 1997 study done by Rogner, who goes to great lengths to claim that there are enough fossil resources, i.e. hydrocarbon molecules in the crust, to theoretically sustain production for an extended period of time.
The issue of whether or not the future availability of fossil fuels would limit future carbon emissions was considered in the Third Assessment Report;,Synthesis Report 7.27, Question 7 pp. 119–120 it concluded that limits on fossil fuel resources would not limit carbon emissions in the 21st century. Their estimate for conventional coal reserves was around 1,000 gigatonnes of carbon (GtC), with an upper estimate of between 3,500 and 4,000 GtC.Figure 7-5, p. 121 (PDF), in . This compares with cumulative carbon emissions up to the year 2100 of about 1,000 GtC for the SRES B1 scenario, and about 2,000 GtC for the SRES A1FI scenario.