Eight Nobel peace laureates on Wednesday urged Canadian Prime Minister Stephen Harper in an open letter to halt the expansion of the Alberta oil sands. \"Further exploitation of the tar sands will dramatically increase the amount of greenhouse gas emissions being produced in North America,\" said the letter, signed by South Africa\'s Archbishop Desmond Tutu, Iran\'s Shirin Ebadi, Jodi Williams of the United States, and others. \"It will also ultimately make turning the clock back on climate change impossible.\" And so, \"we are calling on you to use your power to halt the expansion of the tar sands, and ensure that Canada moves towards a clean energy future,\" they said to Harper. Acting to curb greenhouse gas emissions linked to climate change, they added, is a \"profoundly moral decision, one that deserves to be placed alongside any other major struggle in human history.\" The letter comes two days after federal police arrested 117 protestors for storming Canada\'s parliament to protest Ottawa\'s support for a $7 billion pipeline to bring oil from Canada\'s tar sands to the US Gulf Coast. The Keystone XL pipeline proposed by TransCanada would begin in Alberta in western Canada and pass through the US states of Montana, South Dakota, Nebraska, Kansas and Oklahoma before ending up at refineries in Texas. A number of environmental and citizen groups are fighting against the 1,700-mile (2,700-kilometer) pipeline because exploiting the unconventional oil sands of Alberta requires energy that produces a large volume of greenhouse gasses. The eight Nobel laureates also wrote to US President Barack Obama three weeks ago asking him to reject a permit for the proposed pipeline, which is to be decided by year\'s end after further reviews and hearings. Canadian Natural Resources Minister Joe Oliver has defended the oil sands and the pipeline, saying the Keystone XL pipeline would generate over 140,000 Canadian jobs and $600 billion in economic activity over the ensuing 25 years. \"Our government will continue to promote Canada, and the oil sands, as a stable and secure source of energy to the world, and will defend Canadian jobs and interests,\" Oliver said.