Non-climate scenarios

While the CCIAV studies reported in the TAR typically applied one or more climate scenarios, very few applied contemporaneous scenarios of socio-economic, land-use or other environmental changes. Those that did used a range of sources to develop them. In contrast, AR4 studies which include SRES assumptions may now have several estimates, taking into account different storylines. The role of non-climate drivers such as technological change and regional land-use policy is shown in some studies to be more important in determining outcomes than climate change [2.4.6].

Scenarios of CO₂ concentration are required in some studies, as elevated concentrations can affect the acidity of the oceans and the growth and water use of many terrestrial plants. The observed CO₂ concentration in 2005 was about 380 ppm and was projected in the TAR using the Bern-CC model to rise to the following levels by the year 2100 for the SRES marker scenarios – B1: 540 ppm (range 486-681 ppm); A1T: 575 (506-735); B2: 611 (544-769); A1B: 703 (617-918); A2: 836 (735-1,080); A1FI: 958 (824-1,248) ppm. Values similar to these reference levels are commonly adopted in SRES-based impact studies [2.4.6.2]. Moreover, a multi-stressor approach can reveal important regional dependencies between drivers and their impacts (e.g., the 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].