The U.N. weather agency says carbon dioxide levels in the atmosphere reached a record high in 2013.
The World Meteorological Organization says the heat-trapping gas blamed for global warming was at global concentrations of 396 parts per million last year.
That is an increase of 2.9 ppm from the previous year, which the Geneva-based agency reported Tuesday was the biggest year-to-year change in three decades.
WMO Secretary-General Michel Jarraud said "we know without any doubt that our climate is changing and our weather is becoming more extreme."
The report also finds the rate of ocean acidification, which comes from added carbon absorbed by oceans, "appears unprecedented at least over the last 300 million years."