IPCC Working Group I 1995 Summary for Policymakers Considerable progress has been made in the understanding of climate change1 science since 1990 and new data and analyses have become available. Greenhouse gas concentrations have continued to increase 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 (CH4) 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, CH4 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 Wm2) and N20 (0.14 Wm-2) (values for 1992). · Many greenhouse gases remain in the atmosphere for a long time (for CO2 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 Wm-2. However, their net radiative forcing is reduced by about 0.1 Wm-2 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 SF6) 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. 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 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 composition 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 [Wm 21). IPCC Working Group I 1995 Summary for Policymakers A range of carbon cycle models indicates that stabilisation of atmospheric CO2 concentrations at 450, 650 or 1000 ppmv could be achieved only if global anthropogenic CO2 emissions drop to 1990 levels by, respectively, approximately 40, 110 or 240 years from now, and drop substantially below 1990 levels subsequently. Any eventual stabilised concentration is governed more by the accumulated anthropogenic CO2 emissions from now until the time of stabilisation, than by the way those emissions change over the period. This means that, for a given stabilised concentration value, higher emissions in early decades require lower emissions later on. Among the range of stabilisation cases studied, for stabilisation at 450, 650 or 1000 ppmv accumulated anthropogenic emissions over the period 1991 to 2100 are 630 GtC1, 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. Stabilisation of CH4 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 characterised, 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. Anthropogenic aerosols tend to produce negative radiative forcings • Tropospheric aerosols (microscopic airborne particles) resulting from combustion of fossil fuels, biomass burning and other sources have led to a negative direct forcing of about 0.5 Wm-2, as a global average, and possibly also to a negative indirect forcing of a similar magnitude. While the negative forcing is focused in particular regions and subcontinental areas, it can have continental to hemispheric scale effects on climate patterns. Locally, the aerosol forcing can be large enough to more than offset the positive forcing due to greenhouse gases. In contrast to the long-lived greenhouse gases, anthropogenic aerosols are very shortlived in the atmosphere, hence their radiative forcing adjusts rapidly to increases or decreases in emissions. Climate has changed over the past century At any one location year-to-year variations in weather can be large, but analyses of meteorological and other data over large areas and over periods of decades or more have provided evidence for some important systematic changes. 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. IPCC Working Group I 1995 Summary for Policymakers 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 Niño-Southern Oscillation (which causes droughts and floods in many areas) was unusual in the context of the last 120 years. The balance of evidence suggests a discernible human influence on global climate Any human-induced effect on climate will be superimposed on the background "noise" of natural climate variability, which results both from internal fluctuations and from external causes such as solar variability or volcanic eruptions. Detection and attribution studies attempt to distinguish between anthropogenic and natural influences. "Detection of change" is the process of demonstrating that an observed change in climate is highly unusual in a statistical sense, but does not provide a reason for the change. "Attribution" is the process of establishing cause and effect relations, including the testing of competing hypotheses. 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 climate-change '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 AD. Data prior to 1400 are too sparse to allow the reliable estimation of global |