The World Meteorological Organisation (WMO) said it expects the average surface temperature for the full year to stand 0.45 degrees higher than normal.
The hottest year on record was 1998, when the average temperature was 0.55 degrees higher.
The agency, which collects data from government forecasters worldwide, bases its figures on an average temperature of 14 degrees centigrade.
It said the three hottest years since accurate records began in 1861 have all been in the past six years.
“The rhythm of temperature increases is accelerating,” said WMO deputy secretary-general Michel Jarraud.
This summer, much of Europe was struck by a heatwave which saw temperatures exceed 40 degrees.
Alpine glaciers melted faster and forests in several countries were destroyed by fire, WMO said.
India and Pakistan were hit by a heat wave in May and June, when 1,500 people died as temperatures soared above 50 degrees.
The western US continued to suffer from drought, and wildfires in California burned 74,130 acres of land in October.
In the southern hemisphere springtime, Australia logged a record September temperature of 43.1 degrees.
Over the 2002-3 winter, North America received its tenth-lowest recorded snow fall, although the north-eastern US was battered by a record snowstorm in February, the agency said.
Other parts of the world also faced extreme winter weather. January temperatures in north-eastern Russia dropped to minus 45 degrees, while Mongolia was also gripped by an exceptionally harsh winter for the third year running, devastating livestock.
As winter hit the southern hemisphere, 200 people died in Peru when temperatures fell to minus 20 degrees.
“You cannot attribute this to any single cause,” Jarraud said. “It’s about the very complex interaction between all the elements that make up the very complex machine that is the earth.”
In the Atlantic Ocean, 16 separate storms developed this year, well above the 1948-96 average of 9.8. Hurricane Isabel, which battered North Carolina, was one of the strongest on record. Hurricane Fabian was the most destructive to hit Bermuda in 75 years.
“By definition, exceptional events are exceptional, so they don’t occur very often,” said Jarraud. “But global warming is likely to lead to more frequent extraordinary events and greater intensity of these events.”
Swiss Reinsurance, which backs insurance companies against major claims and analyses the effect of disasters, said 20,000 people were killed by natural catastrophes in 2003.
Swiss Re said its preliminary findings showed that disasters caused total losses of E53 billion. Insurers paid out E12 billion, it said.