NEW DELHI: Redefining the valuation of trees , often felled to make way for road, rail and other projects, the Supreme Court on Thursday said the time had come to calculate environmental cost of trees by taking into account the volume of oxygen they contribute to the atmosphere in their lifetime.A bench of Chief Justice SA Bobde and Justices BR Gavai and Surya Kant said, "Why do authorities, while computing environmental compensation, not take into account the volume of oxygen a tree would release into the atmosphere in its lifetime? Look at the amount of oxygen generated by a single tree in its lifetime. Value of oxygen generated by a tree in its lifetime should be taken into account in determining damage to environment caused by its felling."The CJI-led bench made this observation while dealing with a petition filed by the Association for Protection of Democratic Rights, which has challenged a Calcutta HC decision permitting the West Bengal government to cut 356 trees for construction of five railway overbridges (ROBs) at Kazipara, Ashok Nagar, Habra-I, Habra-II and Bongaon and expansion of NH-112, or Jessore Road.Appearing for the petitioner NGO, advocate Prashant Bhushan said many of these 356 trees identified for felling were heritage trees and were over 80 years old.According to scientists, global warming will lead to increase of temperature by seven degrees Celsius in 50 years, which will wipe out the human race on earth. Trees and increase in green cover are the only means to combat global warming. The state government must consider alternatives, including underpasses and changing alignment of roads, to avoid felling of precious heritage trees," he said.For the Mamata Banerjee government, senior advocate AM Singhvi said ROBs were a necessity as these five stretches had witnessed 700-odd deaths in the last four years."The government, mandated under law to plant twice the number of trees it fells for a project, is planting five times the number of trees. Against a requirement of 700-odd trees, the government is planting 1,780 trees. Human lives are more precious than trees. Look at the huge number of human lives the ROBs will save," he added.CJI Bobde said, "Our experience is that though project proponents and governments undertake afforestation by planting saplings, most die because of neglect. You (state government) either change the alignment or construct underpasses."Panel have been asked to give its report on alternatives to felling of trees for ROBs within five weeks.