International climate scientists are more certain than ever that humans are responsible for global warming, rising sea levels and extreme weather events, according to a leaked draft report by an influential panel of experts, Reuters reported. The early draft, which is still subject to change before a final version is released in late 2013, showed that a rise in global average temperatures since pre-industrial times was set to exceed 2 degrees Celsius by 2100, and may reach 4.8 Celsius. \"It is extremely likely that human activities have caused more than half of the observed increase in global average surface temperatures since the 1950s,\" the Intergovernmental Panel on Climate Change (IPCC) draft report said. The IPCC\'s previous report, in 2007, said it was at least 90 percent certain that human activities, led by burning fossil fuels, were the cause of rising temperatures.