There is no single cause for the drastic shrinkage of Lake Chad: natural fluctuations as part of a long term cycle, heavy human use through the pumping of water for irrigation and deforestation have all been cited. Climate change has also been given as a major reason for why Lake Chad has been reduced from 25,000 square kilometers in 1963 to 1,300 square kilometers today. As a symbol of global warming, however, Lake Chad is seen in the context of Africa being the continent most vulnerable to climate change, facing rising temperatures and increased drought. This despite Africa being the least responsible for global greenhouse emissions, which cause global warming. The consequences of Lake Chad drying up are dire. The lake is estimated to provide a lifeline to nearly 30 million people in Nigeria, Cameroon, Chad and Niger. It has led to a massive reduction in fishing, which many people relied on for income and has caused tensions at community level between different land and water users and regionally between countries competing for the resources of the lake. The shrinkage has a negative impact on, among other things, large scale irrigation schemes in Nigeria. The lake serves as a critical, strategic area for global biodiversity, home to 120 species of fish as well supporting 372 bird species. The countries within the region are among the poorest countries in the world. Based on the 2007/2008 UNDP Human Development Index (HDI) for 177 countries the LCBC countries rank amongst the lowest globally.