The UN has received pledges from 55 countries, accounting for 78 per cent of global emissions, to cut their share of greenhouse gases by 2050.The world body has welcomed this development as an important invigoration to the Copenhagen climate talks while experts said that the current pledges are not sufficient to meet the goal set to limit global temperature rise to two degree Celsius. Some of the world's biggest polluters, the US, China, European Union and India, have reiterated their previous pledges of cutting emissions by 2020.UN's Climate Chief Yvo de Boer said he sees these pledges as clear signals of willingness to move negotiations towards a successful conclusion.