IANS: US President Barack Obama has warned that the effects of climate change were already being felt in national parks across the country. Speaking from Yosemite National Park in California on Saturday, one of the most popular natural sanctuaries, Obama said that climate change is the "greatest challenge" facing nature reserves, EFE news reported. "Make no mistake, climate change is no longer just a threat, it is a reality," said Obama from the Sentinel Bridge, which faces one of the highest waterfalls...