It will be “physically impossible” to have a detailed deal to tackle climate change by this December’s summit in Copenhagen, UN Framework Convention on Climate Change (UNFCCC) Executive Secretary Yvo de Boer said here Wednesday. However, he hoped, the “Copenhagen (summit) will deliver clarity on key political issues in this debate, that there will be clarity on the extent industrialised countries will reduce their (GHG) emissions, and clarity on what major developing countries (including India) are willing to do to mitigate their emissions.”