As a consequence of most building's design lifespan, changing weather conditions driven by climate change, are likely to influence energy demands for heating and cooling, thereby altering greenhouse gas (GHG) emissions due to operation of these systems. A methodology is presented that allows estimation of building lifecycle GHG emissions (both embodied and operational) at the early design stage, accounting for the changing weather conditions produced by climate models (specifically the probabilistic projections provided by the UK Climate Impacts Programme (UKCIP) Weather Generator). Annual heating and cooling demands are estimated from projected future temperatures using a model 'calibrated' to building performance through dynamic thermal modelling of a selection of example years. A worked example assessing lifecycle GHG emissions of a UK mixed-use development is presented. This shows annual GHG emissions clue to space cooling could increase by between 26 and 70% from 2020 to 2080 depending on the future emissions scenario applied. Over the same period, emissions due to heating may decrease by between 12 and 42%, giving an overall net increase in GHG emissions from these systems. Improvements to building cooling systems and reduction to heat gains, particularly lighting, are recommended to reduce risk of increasing GHG emissions over time. (C) 2012 Elsevier B.V. All rights reserved.