In this thesis various forms of scenario analysis are discussed both to explore 1) how energy system and associated greenhouse gas emissions may develop in the absence of climate policy and 2) how strategies aimed at drastically reducing greenhouse gas emissions may turn out. As uncertainties play a major role in long-term scenarios, the thesis also discusses various forms of dealing with uncertainty in scenario analysis. The thesis finds that global emissions are expected to continue to grow under a wide-range of different assumptions in the absense of climate policy. At the same time, the required reductions for reaching low greenhouse gas concentrations levels consistent with a 2 degree Celsius target are shown to be technically feasible within a likely range of direct costs of 1-2% of GDP. However, this will require immediate action using a broad range of options and worldwide participation in mitigating greenhouse gas emissions. Possible crucial factors in determining the likelihood of achieving ambitious climate targets include assumptions on technology development and the possible links between climate policy and such related issues as the reduction of air pollution.