GENEVA — Heat-trapping greenhouse gases in the atmosphere reached a record high in 2005 and are still increasing, the U.N. weather agency said.

The measurements coordinated by the World Meteorological Organization show that the global average concentrations of carbon dioxide, or CO2, and nitrous oxide, or N2O, reached record levels last year and are expected to increase even further this year, said Geir Braathen, a climate specialist at the Geneva-based agency.

"There is no sign that N2O and CO2 are starting to level off," Braathen said Friday at the global body's European headquarters. "It looks like it will just continue like this for the foreseeable future."

The concentration of carbon dioxide rose by about 0.5 percent last year to reach 379.1 parts per million, according to the agency. Nitrous oxide has totaled 319.2 parts per billion, which is 0.19 percent higher than in 2004. Levels of methane, another so-called greenhouse gas, remained stable since last year, Braathen said.

Water vapor is the most common greenhouse gas, followed by CO2, N2O — produced by natural sources as well as fertilizers, tree burning and industry — and methane — produced by wetlands and other natural and human processes. There is 35.4 percent more carbon dioxide since the late 18th century primarily because of human burning for fossil fuels, the WMO statement said.

Scientists say that carbon dioxide and other gases primarily from fossil fuel-burning trap heat in the atmosphere and have warmed the Earth's surface an average 1 degree in the past century.

A report this past week by British government warned that global warming would devastate the world economy on the scale of the world wars and the Great Depression if left unchecked.

It said such warming could have effects such as melting glaciers, rising sea levels, declining crop yields, drinking water shortages, higher death tolls from malnutrition and heat stress, and widespread outbreaks of malaria and dengue fever. Developing countries often would be the hardest hit.

The U.N. agency said it also has concluded that "greenhouse gases are some of the major drivers behind global warming and climate change."

Braathen said power plants, automobiles, ships and airplanes using coal, oil or gas were contributing to the rise in carbon dioxide emissions.

"The increase in CO2 is linked to the burning of fossil fuels," he said.

WMO said it based its findings on readings from 44 countries that were collected in Japan.

The agency's findings come just ahead of the second meeting of the countries that adhered to the Kyoto Protocol — aimed at capping greenhouse gas emissions and staving off global warming — to be held in Nairobi, Kenya, Nov. 6-17. Under the 1997 Kyoto accord, 35 industrialized nations have committed to reducing emissions by an average 5 percent below 1990 levels by 2012. The United States, the biggest emitter, rejects the agreement.

Braathen said it would take time until the protocol, which has been in effect since last year only, leads to a reduction of greenhouse gas emissions and that countries need to do more.

"To really make CO2 level off, we need more drastic measures than are in the Kyoto Protocol today," he said.

On Monday, the U.N. climate treaty secretariat also reported that global greenhouse gas emissions are on the rise, with increased values from 34 industrialized nations between 2000 and 2004. In the United States, source of two-fifths of the industrialized world's greenhouse gases, emissions grew by 1.3 percent in that period, and by almost 16 percent between 1990 and 2004, the U.N. said.