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It is difficult to predict the human choices that will shape our future emissions, and thus what the world might look like in 2100. For example, under the higher emissions scenario (A1F1) atmo- spheric concentrations of carbon dioxide (CO2) reach more than triple pre - industrial levels, or 960 ppm, by 2100. Similarly, a lower emissions scenario (B1) represents a world with high economic growth and mid - century population peak and subsequent decline. In contrast to the A1FIscenario, the B1 scenario includes a shift to less fossil -fuel intensive industries and the introduction of clean and efficient technologies with a resulting peak in emissions of greenhouse gases by 2050, and then a decline. In the B1 scenario, CO2 concentrations reach 550 ppm by 2100, which is about double pre - industrial levels (Nakicenovic et al. 2000). In 2009, the global annual mean concentration of atmospheric CO2 was 386.27 ppm (NOAA /ESRL 2010). If recent emissions growth rates continue, CO2 levels, along with the associated effects of climate change, are very likely to exceed even the highest existing emissions scenarios (Rahmstorf et al. 2007). Global climate /general circulation models (both GCMs) are computer -based models of the climate system developed from weather forecasting models (Goodess 2000) which incorporate interactions MULTI-MODEL AvESAGES AND SSEamn Marcos ron Sunmoe W&Pmwa A2 VI -ih::k i��"W 1l�li I-Al,V1 6.0 AlB B1 5.0 Year 2000 Constant _ Concentrations U 20th century a 4.0 co 3.0 L) // 7 1.0 CD 0.0 —1.0— ° K:'0 Q m Q 1900 2000 2100 Year Figure SPM.5. Solid lines are multi -model global averages of surface warming (relative to 1980 -1999) for the scenarios A2, A18 and 81, shown as continuations of the 20th century simulations. Shading denotes the ±1 standard deviation range of individual model annual averages. The orange line is for the experiment where concentrations were held constant at year 2000 values. The grey bars at right indicate the best estimate (solid line within each bar) and the likely range assessed for the six SRES marker scenarios. The assessment of the best estimate and likely ranges in the grey bars includes the AOGCMs in the left part of the figure, as well as results from a hierarchy of independent models and observational constraints. (Figures 10.4 and 10.29) Figure 1 -3. Multi -model averages and assessed ranges for global surface warming under different emissions scenarios (Source: Nakicenovic et al. 2000). Regardless of which scenario is assessed, significant global warming is expected to occur.