Record high levels of greenhouse gases causing climate change will linger in the atmosphere for decades to come, even if the world manages to stop emissions output today, the United Nations' weather agency said this week.

Carbon dioxide accounted for 80 percent of the increase and human activities, such as the use of fossil fuels and agriculture, were the major causes of the emissions rise, the World Meteorological Organization (WMO) said in its annual Greenhouse Gas Bulletin.

"The atmospheric burden of greenhouse gases due to human activities has yet again reached record levels since pre-industrial time," said WMO Secretary-General Michel Jarraud.

"Even if we managed to halt our greenhouse gas emissions today, and this is far from the case, they would continue to linger in the atmosphere for decades to come and so continue to affect the delicate balance of our living planet and our climate," he said.

The report adds to a number of warnings that time is running out to act on climate change and prevent worsening extreme weather as the Earth's temperature rises.

BP data earlier this year showed global carbon dioxide emissions grew at their fastest rate since 1969 last year, as countries rebounded from economic recession.

In 2010, countries agreed in Cancun, Mexico, that deep emissions cuts were needed to hold an increase in global average temperature below 2 degrees Celsius above pre-industrial levels, a threshold beyond which scientists say risks even more extreme weather, crop failure and major floods.

Delegates from nearly 200 countries will meet in South Africa next week for a U.N. summit but only modest steps towards a broader climate deal are seen as likely.

HOTTING UP

The WMO said greenhouse gas concentrations in the atmosphere increased by 1.4 percent last year from 2009 and 29 percent since 1990.

The WMO measured the overall amount of greenhouse gases in the atmosphere, based on monitoring stations in more than 50 countries, including natural emissions and absorption processes - so-called sources and sinks - as well as emissions caused by human activity.

Three of the most dangerous gases, carbon dioxide, methane and nitrous oxide, were more prevalent in the atmosphere in 2010 than at any time since the industrial revolution.

Between 2009 and 2010, the amount of CO2 in the atmosphere increased by 2.3 parts per million, higher than the average for both the 1990s (1.5 parts per million) and the past decade (2.0 parts per million), the report said.

The second biggest greenhouse gas, methane, has been growing in the past five years after levelling off between 2000 and 2006, for reasons that are not fully understood.

The third biggest greenhouse gas is nitrous oxide, which can trap almost 300 times as much heat as carbon dioxide. Its main human source is the use of nitrogen-based fertilisers, which the report said had "profoundly affected the global nitrogen cycle".

The impact of fertiliser use is so marked that more nitrous oxide is detected in the northern hemisphere, where more fertiliser is used, than in the south.

The WMO data showed no pause in the growth of greenhouse gases, and more work needs to be done to help understand which policies would have the most effect, the report's authors said.

So far, the clearest discernable impact of policies was a decrease in chlorofluorocarbons, or CFCs, which were banned because they caused depletion of the ozone layer.

But hydrofluorocarbons, which have replaced CFCs, are also potent greenhouse gases and their abundance in the atmosphere, while still small, is rising at a rapid rate. (Additional reporting by Nina Chestney; Editing by Janet Lawrence)