Kenyan president Uhuru Kenyatta (3rd L) welcomes Pope Francis (C) as he arrives a the state house of Nairobi on November 25, 2015 as part of his visit in Kenya. Pope Francis said the world was facing a "grave environmental crisis" as he arrived in Kenya on Wednesday on a landmark Africa trip just days before a crucial UN summit aimed at curbing climate change. He also warned of the need to tackle poverty as a key driver of conflict and violence as he kicked off a landmark Africa trip fraught with security concerns and urged Kenya's leaders to work with "transparency" to ensure a fair distribution of national resources as criticism grows over runaway graft in this east African country. AFP PHOTO / JOHN MUCHUCHA