Greenhouse gas levels in the atmosphere have reached their highest levels since pre-industrial times, reports the World Meteorological Organization.

These gases, which warm the Earth’s surface, continued to build up in 2009 – the latest year studied – despite the economic slowdown, the United Nations weather agency said Wednesday in its latest Greenhouse Gas Bulletin.

Emissions of carbon dioxide, the single most important greenhouse gas caused by human activity, increased at a smaller rate last year than in 2008 but still rose 1.6 parts per million – to 386.8 parts per million, according to the Geneva-based agency. They jumped 38% since 1750, when they averaged 280 parts per million, due largely to the burning of fossil fuels, deforestation and changes in land use.

The findings will be discussed at a two-week U.N. Climate Change Conference that is slated to begin in Cancun, Mexico, on Monday.