These two young siblings appear to cling on to a tree for dear life and even hold hands as they learn the ropes of climbing, on June 25, 2013. (Photo by Tim Fitzharris/Minden/Solent)

Growing cities, overuse of fertilizers and factory wastewater have degraded China's water supplies to the extent that half the nation's rivers and lakes are severely polluted. China aims to spend $850 billion to improve filthy water supplies over the next decade, but even such huge outlays may do little to reverse damage caused by decades of pollution and overuse in Beijing's push for rapid economic growth. Photo: Fishermen row a boat in the algae-filled Chaohu Lake in Hefei, Anhui province, June 19, 2009. (Photo by Jianan Yu/Reuters)

Thor, Virginia Tech College of Engineering, Robotics and Mechanisms Laboratory, tries to connect the hose to a firehose connection during the hose task at the Homestead-Miami Speedway, Friday, December 20, 2013, in Homestead, Fla. (Photo by Alan Diaz/AP Photo)

A young oriental small-clawed otter at the zoo in Neumuenster, Germany, March 5, 2014. The six young otters (one male, five female) were born on December 7, 2013. (Photo by Maja Hitij/EPA)

Dark clouds bearing down on the city on April 17, 2011 in Foshan, Guangdong Province of China. According to flood control authorities on Monday, gales as strong as 45.5 meters per second, accompanied by hailstorm, cloudburst and strong wind battered cities including Guangzhou, Foshan, Zhaoqing and Dongguan of south China's Guangdong Province on Sunday, has killed at least 17 people and injured 118. (Photo by ChinaFotoPress/Getty Images)

His majesty King Jigme Khesar Namgyel Wangchuck (L), 31, smiles at his bride during the purification marriage ceremony to Queen Jetsun Pema, 21, in the historical Punakha Dzong on October 13, 2011 in Punakha, Bhutan. The Dzong is the same venue that hosted the King's historic coronation ceremony in 2008. The Oxford-educated king is popular in the country and the ceremony will be followed by celebration in the capital and countryside. (Photo by Paula Bronstein/Getty Images)

Thailand's King Bhumibol Adulyadej in serious portrait January 01, 1960. Thailand's Royal Palace said on Thursday, October 13, 2016, that Thailand's King Bhumibol Adulyadej, the world's longest-reigning monarch, has died at age 88. (Photo by John Dominis/The LIFE Picture Collection/Getty Images)

Pakistani civil society activists release lanterns into the sky in Lahore on December 15, 2016, as they pay tribute to victims on the second anniversary of an attack on The Army Public School in the city of Peshawar. Taliban militants attacked on Pakistani Army Public School in Peshawar, killing 151 people mostly children on December 16, 2014. (Photo by Arif Ali/AFP Photo)