PPM targets can be set based upon what is possible (technically, politically, economically), what is desirable (environmentally, socially), or some combination of the two. In line with the principles of Gas-CAP, we do not presuppose what is possible technically, politically, or economically, and we are not providing links to opinions about the feasibility of various approaches.
The links that follow reflect the best in scientific understanding of the consequences of various levels of ppm. It is up to the user to decide what outcomes and risk levels are acceptable when weighed against the requisite emission reductions.
Finally, it must be pointed out that changes in atmospheric concentrations are determined not only by emissions, but also by complex environmental feedback loops that may have large nonlinear effects. These feedback effects are not reflected in the ppm graph in Gas-CAP and should thus be considered an added risk. More information on feedback loops and abrupt climate change can be found in the links below.