The top band shows the range of estimates across models for emissions under a business-as-usual scenario, i.e. temperature will rise by 5 degree Celsius above the pre-industrial era temperature. The lower band shows a trajectory that could yield a concentration of 450 ppm of CO2 emissions (with a 50 percent change of limiting warming to less than 2 degree Celsius). Greenhouse gas emissions include CO2, CH4, and N2O. Negative emissions (eventually required by the 2 degree Celsius path) imply that the annual rate of emissions is lower than the rate of uptake and storage of carbon through natural processes (for example, plant growth) and engineered processes (for example, growing biofuels and when burning them, sequestering the CO2 underground). The baseline for projection is usually the emission level during pre-industrial era.
Source: WDR 2010: Development and Climate Change, pp.10, figure 5