GENEVA, Sept 9 (Reuters) - Atmospheric volumes of greenhouse gas hit a record in 2013 as carbon dioxide concentrations grew at the fastest rate since reliable global records began, the World Meteorological Organization said on Tuesday.

"We know without any doubt that our climate is changing and our weather is becoming more extreme due to human activities such as the burning of fossil fuels," said WMO Secretary-General Michel Jarraud in a statement accompanying the WMO's annual Greenhouse Gas Bulletin.

"Past, present and future CO2 emissions will have a cumulative impact on both global warming and ocean acidification. The laws of physics are non-negotiable," Jarraud said. "We are running out of time."

The volume of carbon dioxide, or CO2, the primary greenhouse gas emitted by human activities, was 396.0 parts per million (ppm) in 2013, 2.9 ppm higher than in 2012, the largest year-to-year increase since 1984, when reliable global records began.

The second most important greenhouse gas, methane, continued to grow at a similar rate to the last five years, reaching a global average of 1824 parts per billion (ppb). The other main contributor, nitrous oxide, reached 325.9 ppb, growing at a rate comparable to the average over the past decade.

The world has the knowledge and tools to keep global warming within 2 degrees Celsius (3.6 Fahrenheit), a U.N. goal set in 2010, Jarraud said, which would "give our planet a chance and... our children and grandchildren a future".

"Pleading ignorance can no longer be an excuse for not acting." (Reporting by Tom Miles, editing by Stephanie Nebehay)