7.12 IPCC SECOND ASSESSMENT SYNTHESIS OF SCIENTIFIC-TECHNICAL INFORMATION RELEVANT TO INTERPRETING ARTICLE 2 OF THE UN FRAMEWORK CONVENTION ON CLIMATE CHANGE Cost estimates for a number of specific approaches to mitigating emissions or enhancing sinks of greenhouse gases vary widely and depend on site-specific characteristics. This is true for renewable energy technologies, for example, as well as carbon sequestration options. The latter could offset as much as 15-30% of 1990 global energy-related emissions each year in forests for the next 50 years. The costs of carbon sequestration, which are competitive with source control options, differ among regions of the world. 7.13 Control of emissions of other greenhouse gases, especially methane and nitrous oxide, can provide significant cost-effective opportunities in some countries. About 10% of anthropogenic methane emissions could be reduced at negative or low cost using available mitigation options for such methane sources as natural gas systems, waste management and agriculture. Costs differ between countries and regions for some of these options. Subsidies, market imperfections and barriers 7.14 The world economy and indeed some individual national economies suffer from a number of price distortions which increase greenhouse gas emissions, such as some agricultural and fuel subsidies and distortions in transport pricing. A number of studies of this issue indicate that global emissions reductions of 4-18 % together with increases in real incomes are possible from phasing out fuel subsidies. 7.15 Progress has been made in a number of countries in costeffectively reducing imperfections and institutional barriers in markets through policy instruments based on voluntary agreements, energy efficiency incentives, product efficiency standards and energy efficiency procurement programmes involving manufacturers and utility regulatory reforms. Where empirical evaluations have been made, many have found that the benefit-cost ratio of increasing energy efficiency was favourable, suggesting the practical feasibility of realizing "no regrets" potentials at negative net cost. Value of better information and research 7.16 The value of better information about the processes, impacts of and responses to climate change is likely to be great. Analysis of economic and social issues related to climate change, especially in developing countries, is a high priority for research. Further analysis is required concerning effects of response options on employment, inflation, trade, competitiveness and other public issues. 8.1 The scientific, technical, economic and social science literature does suggest ways to move forward towards the ultimate objective of the Convention. Possible actions include mitigation of climate change through reductions of emissions of greenhouse gases and enhancement of their removal by sinks, adaptation to observed and/or anticipated climate change, and research, development and demonstration to improve our knowledge of the risks of climate change and possible responses. 8.2 Uncertainties remain which are relevant to judgement of what constitutes dangerous anthropogenic interference with the climate system and what needs to be done to prevent such interference. The literature indicates, however, that significant "no regrets" opportunities are available in most countries and that the risk of aggregate net damage due to climate change, consideration of risk aversion and the precautionary approach, provide rationales for actions beyond "no regrets". The challenge is not to find the best policy today for the next 100 years, but to select a prudent strategy and to adjust it over time in the light of new information. adaptation costs, or can increase the cost-effectiveness of emission reduction measures. Appropriate long-run signals are required to allow producers and consumers to adapt cost-effectively to constraints on greenhouse gas emissions and to encourage investment, research, development and demonstration. 8.4 Many of the policies and decisions to reduce emissions of greenhouse gases and enhance their sinks, and eventually stabilize their atmospheric concentration, would provide opportunities and challenges for the private and public sectors. A carefully selected portfolio of national and international responses of actions aimed at mitigation, adaptation and improvement of knowledge can reduce the risks posed by climate change to ecosystems, food security, water resources, human health and other natural and socio-economic systems. There are large differences in the cost of reducing greenhouse gas emissions, and enhancing sinks, among countries due to their state of economic development, infrastructure choices and natural resource base. International cooperation in a framework of bilateral, regional or international agreements could significantly reduce the global costs of reducing emissions and lessening emission leakages. If carried out with care, these responses would help to meet the challenge of climate change and enhance the prospects for sustainable economic development for all peoples and nations. CLIMATE CHANGE 1995: IPCC SECOND ASSESSMENT REPORT Drafting team for the synthesis Bert Bolin (Chairman of the IPCC and Chairman of the Drafting References 1. IPCC, 1990: Narasimhan (1) Climate Change, The IPCC Scientific Assessment (ii) Climate Change, The IPCC Impacts Assessment (iii) Climate Change, The IPCC Response Strategies (iv) Overview and Policymakers Summary 2. IPCC, 1992: (i) Climate Change 1992, The Supplementary Report to the IPCC Scientific Assessment (ii) Climate Change 1992, The Supplementary Report to the IPCC Impacts Assessment 3. IPCC, 1994: Climate Change 1994, Radiative Forcing of Climate Change and an Evaluation of the IPCC IS92 Emission Scenarios 4. IPCC, 1995: (1) Climate Change 1995, The IPCC Second Assessment Synthesis of Scientific-Technical Information Relevant to Interpreting Article 2 of the UN Framework Convention on Climate Change (ii) Climate Change 1995, The Science of Climate Change (iii) Climate Change 1995, Scientific-Technical Analyses of Impacts, Adaptations and Mitigation of Climate Change (iv) Climate Change 1995, The Economic and Social Dimensions of Climate Change SUMMARY FOR POLICYMAKERS: THE SCIENCE OF CLIMATE CHANGE Increases in greenhouse gas concentrations since pre-industrial times (i.e., since about 1750) have led to a positive radiative forcing2 of climate, tending to warm the surface and to produce other changes of climate. • The atmospheric concentrations of greenhouse gases, inter alia, carbon dioxide (CO2), methane (CH) and nitrous oxide (N2O) have grown significantly: by about 30%, 145%, and 15%, respectively (values for 1992). These trends can be attributed largely to human activities, mostly fossil-fuel use, land-use change and agriculture. • The growth rates of CO2, CH, and N2O concentrations were low during the early 1990s. While this apparently natural variation is not yet fully explained, recent data indicate that the growth rates are currently comparable to those averaged over the 1980s. • The direct radiative forcing of the long-lived greenhouse gases (2.45 Wm-2) is due primarily to increases in the concentrations of CO2 (1.56 Wm-2), CH4 (0.47 Wm-2) and N2O (0.14 Wm2) (values for 1992). • Many greenhouse gases remain in the atmosphere for a long time (for CO and N2O, many decades to centuries), hence they affect radiative forcing on long time-scales. • The direct radiative forcing due to the CFCs and HCFCs combined is 0.25 Wm2. However, their net radiative forcing is reduced by about 0.1 Wm2 because they have caused stratospheric ozone depletion which gives rise to a negative radiative forcing. • Growth in the concentration of CFCs, but not HCFCs, has slowed to about zero. The concentrations of both CFCs and HCFCs, and their consequent ozone depletion, are expected to decrease substantially by 2050 through implementation of the Montreal Protocol and its Adjustments and Amendments. • At present, some long-lived greenhouse gases (particularly HFCs (a CFC substitute), PFCs and SF) contribute little to radiative forcing but their projected growth could contribute several per cent to radiative forcing during the 21st century. • If carbon dioxide emissions were maintained at near current (1994) levels, they would lead to a nearly constant rate of increase in atmospheric concentrations for at least two centuries, reaching about 500 ppmv (approaching twice the pre-industrial concentration of 280 ppmv) by the end of the 21st century. levels by, respectively, approximately 40, 140 or 240 years from now, and drop substantially below 1990 levels subsequently. • Any eventual stabilized concentration is governed more by the accumulated anthropogenic CO2 emissions from now until the time of stabilization than by the way those emissions change over the period. This means that, for a given stabilized concentration value, higher emissions in early decades require lower emissions later on. Among the range of stabilization cases studied, for stabilization at 450, 650 or 1000 ppmv, accumulated anthropogenic emissions over the period 1991 to 2100 are 630 GtC3, 1030 GtC and 1410 GtC, respectively (± approximately 15% in each case). For comparison the corresponding accumulated emissions for IPCC IS92 emission scenarios range from 770 to 2190 GtC. • Stabilization of CH, and N2O concentrations at today's levels would involve reductions in anthropogenic emissions of 8% and more than 50% respectively. • There is evidence that tropospheric ozone concentrations in the Northern Hemisphere have increased since pre-industrial times because of human activity and that this has resulted in a positive radiative forcing. This forcing is not yet well characterized, but it is estimated to be about 0.4 Wm-2 (15% of that from the long-lived greenhouse gases). However, the observations of the most recent decade show that the upward trend has slowed significantly or stopped. 1 Climate change in IPCC Working Group I usage refers to any change in climate over time whether due to natural variability or as a result of human activity. This differs from the usage in the UN Framework Convention on Climate Change where "climate change" refers to a change of climate which Is attributed directly or indirectly to human activity that alters the composi tion of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods. 2 A simple measure of the importance of a potential climate change mechanism. Radiative forcing is the perturbation to the energy balance of the Earth-atmosphere system (in Watts per square metre (Wm2]). 31 GtC = 1 billion tonnes of carbon. CLIMATE CHANGE 1995: IPCC SECOND ASSESSMENT REPORT • Global mean surface air temperature has increased by between about 0.3 and 0.6°C since the late 19th century; the additional data available since 1990 and the re-analyses since then have not significantly changed this range of estimated increase. • Recent years have been among the warmest since 1860, i.e., in the period of instrumental record, despite the cooling effect of the 1991 Mt Pinatubo volcanic eruption. ⚫ Night-time temperatures over land have generally increased more than daytime temperatures. • Regional changes are also evident. For example, the recent warming has been greatest over the mid-latitude continents in winter and spring, with a few areas of cooling, such as the North Atlantic ocean. Precipitation has increased over land in high latitudes of the Northern Hemisphere, especially during the cold season. • Global sea level has risen by between 10 and 25 cm over the past 100 years and much of the rise may be related to the increase in global mean temperature. • There are inadequate data to determine whether consistent global changes in climate variability or weather extremes have occurred over the 20th century. On regional scales there is clear evidence of changes in some extremes and climate variability indicators (e.g., fewer frosts in several widespread areas; an increase in the proportion of rainfall from extreme events over the contiguous states of the USA). Some of these changes have been toward greater variability; some have been toward lower variability. • The 1990 to mid-1995 persistent warm-phase of the El NiñoSouthern Oscillation (which causes droughts and floods in many areas) was unusual in the context of the last 120 years. Since the 1990 IPCC Report, considerable progress has been made in attempts to distinguish between natural and anthropogenic influences on climate. This progress has been achieved by including effects of sulphate aerosols in addition to greenhouse gases, thus leading to more realistic estimates of human-induced radiative forcing. These have then been used in climate models to provide more complete simulations of the human-induced climatechange 'signal'. In addition, new simulations with coupled atmosphere-ocean models have provided important information about decade to century time-scale natural internal climate variability. A further major area of progress is the shift of focus from studies of global-mean changes to comparisons of modelled and observed spatial and temporal patterns of climate change. The most important results related to the issues of detection and attribution are: • The limited available evidence from proxy climate indicators suggests that the 20th century global mean temperature is at least as warm as any other century since at least 1400 A.D. Data prior to 1400 are too sparse to allow the reliable estimation of global mean temperature. • Assessments of the statistical significance of the observed global mean surface air temperature trend over the last century have used a variety of new estimates of natural internal and externally-forced variability. These are derived from instrumental data, palaeodata, simple and complex climate models, and statistical models fitted to observations. Most of these studies have detected a significant change and show that the observed warming trend is unlikely to be entirely natural in origin. • More convincing recent evidence for the attribution of a human effect on climate is emerging from pattern-based studies, in which the modelled climate response to combined forcing by greenhouse gases and anthropogenic sulphate aerosols is compared with observed geographical, seasonal and vertical patterns of atmospheric temperature change. These studies show that such pattern correspondences increase with time, as one would expect, as an anthropogenic signal increases in strength. Furthermore, the probability is very low that these correspondences could occur by chance as a result of natural internal variability only. The vertical patterns of change are also inconsistent with those expected for solar and volcanic forcing. ⚫ Our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural variability, and because there are uncertainties in key factors. These include the magnitude and patterns of longterm natural variability and the time-evolving pattern of forcing by, and response to, changes in concentrations of greenhouse gases and aerosols, and land surface changes. Nevertheless, the balance of evidence suggests that there is a discernible human influence on global climate. Any human-induced effect on climate will be superimposed on the CLIMATE IS EXPECTED TO CONTINUE TO CHANGE IN THE FUTURE The IPCC has developed a range of scenarios, IS92a-f, of future greenhouse gas and aerosol precursor emissions based on |