A1b A2 A1fi

Figure TS.4. Global temperature changes for selected time periods, relative to 1980-1999, projected for SRES and stabilisation scenarios. To express the temperature change relative to 1850-1899, add 0.5°C. More detail is provided in Chapter 2 [Box 2.8]. Estimates are for the 2020s, 2050s and 2080s, (the time periods used by the IPCC Data Distribution Centre and therefore in many impact studies) and for the 2090s. SRES-based projections are shown using two different approaches. Middle panel: projections from the WGIAR4 SPM based on multiple sources. Best estimates are based on AOGCMs (coloured dots). Uncertainty ranges, available only for the 2090s, are based on models, observational constraints and expert judgement. Lower panel: best estimates and uncertainty ranges based on a simple climate model (SCM), also from WGI AR4 (Chapter 10). Upper panel: best estimates and uncertainty ranges for four CO2-stabilisation scenarios using an SCM. Results are from the TAR because comparable projections for the 21st century are not available in the AR4. However, estimates of equilibrium warming are reported in the WGI AR4 for CO2-equivalent stabilisation11. Note that equilibrium temperatures would not be reached until decades or centuries after greenhouse gas stabilisation. Uncertainty ranges: middle panel, likely range (> 66% probability); lower panel, range between 19 estimates calculated assuming low carbon-cycle feedbacks (mean -1 standard deviation) and those assuming high carbon-cycle feedbacks (mean + 1 standard deviation); upper panel, range across seven model tunings for medium carbon-cycle settings.

11 Best estimate and likely range of equilibrium warming for seven levels of CO2-equivalent stabilisation from the WG1 AR4 are: 350 ppm, 1.0°C [0.6-1.4]; 450 ppm, 2.1°C [1.4-3.1]; 550 ppm, 2.9°C [1.9-4.4]; 650 ppm, 3.6°C [2.4-5.5]; 750 ppm,4.3°C [2.8-6.4]; 1,000 ppm, 5.5°C [3.7-8.3] and 1,200 ppm, 6.3°C [4.2-9.4].

combined effects of extreme weather and air-pollution events on human health). This expansion of scenario scope and application has brought into focus the wide range of potential future impacts and their associated uncertainties [2.2.5,2.5].

M iti gati on/stabilisati on scenari os

The SRES storylines assume that no specific climate policies will be implemented to reduce greenhouse gas emissions (i.e. mitigation). Projections of global mean warming during the 21st century for the six SRES scenarios using two different approaches reported by the WGIAR4 (Chapter 10) are depicted in the middle and lower panels of Figure TS-4. Even without assuming explicit climate policies, differences between projections of warming for alternative emissions scenarios by the end of the century can exceed 2°C [B2.8].

CCIAV studies assuming mitigated futures are beginning to assess the benefits (through impacts ameliorated or avoided) of climate policy decisions. Stabilisation scenarios are a type of mitigation scenario describing futures in which emissions reductions are undertaken so that greenhouse gas concentrations, radiative forcing or global average temperature changes do not exceed a prescribed limit. There have been very few studies of the impacts of climate change assuming stabilisation. One reason for this is that relatively few AOGCM stabilisation runs have been completed so far, although the situation is rapidly changing [2.4.6].

Greenhouse gas mitigation is expected to reduce global mean warming relative to baseline emissions, which in turn could avoid some adverse impacts of climate change. To indicate the projected effect of mitigation on temperature during the 21st century, and in the absence of more recent, comparable estimates in the WGI AR4, results from the Third Assessment Report using a simple climate model are reproduced in the upper panel of Figure TS-4. These portray the temperature response for four CO2-stabilisation scenarios by three dates in the early (2025), mid (2055), and late (2085) 21st century12 [B2.8].

Large-scale singularities

Very few studies have been conducted on the impacts of large-scale singularities, which are extreme, sometimes irreversible, changes in the Earth system such as an abrupt cessation of the North Atlantic Meridional Overturning Circulation, or rapid global sea-level rise due to Antarctic and/or Greenland ice sheet melting [2.4.7]. Due to incomplete understanding of the underlying mechanisms of these events, or their likelihood, only exploratory studies have been carried out. For example, in terms of exploring the worst-case scenario of abrupt sea-level rise, impact assessments have been conducted for the coastal zone for a 5 m rise, and for a 2.2 m rise by 2100 [2.4.7]. This is the first time these scenarios have been included in any WGII assessment, and the expectation is that many more such studies will become available for assessment in the future.

Probabilistic characterisations

Probabilistic characterisations of future climate and non-climate conditions are increasingly becoming available. A number of studies focused on the climate system have generated probabilistic estimates of climate change, conditional on selected or probabilistic emissions scenarios, the latter being a subject of considerable debate [2.4.8]. Probabilistic futures have been applied in a few CCIAV studies to estimate the risk of exceeding predefined thresholds of impact and the associated timing of such exceedances [2.3.1].

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