The draft summary of the report all but dismissed recent claims of a slowdown in the pace of warming, which has seized upon by climate-change sceptics.
"It is extremely likely that human influence on climate caused more than half of the observed increase in global average surface temperature from 1951 to 2010," The New York Times quoted a section of the leaked report as saying.
"There is high confidence that this has warmed the ocean, melted snow and ice, raised global mean sea level and changed some climate extremes in the second half of the 20th century.
"
The IPCC is made up of several hundred scientists worldwide who won the Nobel Peace Prize in 2007 along with former US vice-president and noted environmental campaigner Al Gore.
The scientists' findings will be included in the next major United Nations report on climate change.
The leaked summary said the IPCC believed a worst case scenario could see sea levels rising by as much as three feet (90 centimeters) by the year 2100.