Equity and efficiency dimensions of global time flexibility in GHG emission reduction are analyzed with an integrated assessment model. Global time flexibility is justifiable to some extent as found in previous studies by Wigley et al. Nevertheless, it does not necessarily serve as a rationale to delay emission reduction commitment and efforts of developed countries as they suggested. The time flexibility can be saved for developing countries, and it must be so in equity consideration; early reduction by developed countries eases burden of developing countries in both time and emission quantity dimensions. This equity-oriented argument is robust against time and spatial efficiency consideration, since the apparent benefits that might accrue to developed countries from delaying reductions will by no means be transferred to far distant future developing countries for mitigation of and adaptation to climate change. The analysis thus support entry into force of the Kyoto Protocol without participation of key low income developing countries such as China and India with legally binding quantified targets in the First Commitment Period from 2008 to 2012. (C) 2003 Elsevier Science Ltd. All rights reserved.