evidence: The net impacts of these human actions and choices on future greenhouse gas concentrations are fed into models as different "scenarios." For example, the scenario represented by the blue trend line above (IPCC Scenario B1) assumes that humans worldwide will make more sustainable development choices by using a greater range of, and more efficient, technologies for producing energy. In this scenario, carbon emissions are projected to increase from today’s rate of about 9 billion metric tons per year to about 12 billion tons per year in 2040, and then gradually decline again to 1990 levels—5 billion tons per year—by 2100. [...] Because temperature projections depend on the choices people make in the future, climate scientists can’t say which one of the scenarios is more likely to come to pass by the end of the century. These scenarios are estimates, and greenhouse gas concentrations may grow at rates that are higher or lower than the scenarios shown in the graph. If future carbon dioxide emissions follow the same trajectory as they have over the last decade, increasing at a rate of more than 3 percent per year, carbon dioxide levels in the atmosphere would exceed the scenario represented by the red line (IPCC scenario A2) by the end of this century, if not before.