By Valerie Volcovici

WASHINGTON, Jan 20 (Reuters) - Last year's global average temperature was the hottest ever by the widest margin on record, two U.S. government agencies said on Wednesday, adding to pressure for deep greenhouse gas emissions cuts scientists say are needed to arrest warming that is disrupting the global climate.

Data from U.S. space agency NASA and the National Oceanic and Atmospheric Administration showed that in 2015, the average temperature across global land and ocean surfaces was 1.62 degrees Fahrenheit (0.90 Celsius) above the 20th century average, surpassing 2014's previous record by 0.29 F (0.16 C).

This was the fourth time a global temperature record has been set this century, the agencies said in a summary of their annual report.

Courtesy Lamont Doherty Earth Observatory

"2015 was remarkable even in the context of the larger, long-term warming trend," said Gavin Schmidt, director of NASA's Goddard Institute for Space Studies.

The sharp increase in 2015 was driven in part by El Niño, a natural weather cycle in the Pacific that warms the ocean surface every two to seven years. But scientists say human activities - notably burning fossil fuels - were the main driver behind the rise.

"We would not have seen the record warming without the long-term trend," Schmidt said.

The latest El Niño started in late 2015 and will last until spring 2016. It is among the strongest ever recorded but Schmidt and others say the weather phenomenon played just a supporting role in the earth's temperature rise.

Courtesy Lamont Doherty Earth Observatory

The 2015 data underscores the urgency of cutting greenhouse gas emissions if the world is to hold temperature increases to well below 2 degrees C, the target agreed to by more than 190 countries at climate talks in Paris last December.

Schmidt said the fact that the world is now halfway to the U.N. goal has led many scientists to argue that even that target is too high and more stringent goals are needed.

The summary can be found here: http://www.ncdc.noaa.gov/sotc/summary-info/global/201512