As activists are mobilizing for what they hope will be "the largest climate march in history," the World Meteorological Organization has issued another warning that runaway greenhouse gas emissions show "we are running out of time" to act on the climate crisis.

In its latest annual Greenhouse Gas Bulletin (pdf), which was released Tuesday, the WMO states that atmospheric levels of CO2 — the single most important anthropogenic greenhouse gas in the atmosphere — are at record levels.

In 2013, levels of CO2 were 396 parts per million (ppm), which is 142 percent of the pre-industrial level. The Bulletin warns that the world's global annual average is on track to surpass 400 ppm of CO2 in 2015 or 2016.

Former NASA climatologist James Hansen has stated, "If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced ... to at most 350 ppm."

Methane, another important greenhouse gas, reached a record high in 2013 as well, surging to 253% of its pre-industrial level, according to the Bulletin.

This year's Bulletin is the first to include a section on ocean acidification, the rate of which "appears unprecedented at least over the last 300 million years." Acting as a carbon sink, the ocean absorbs roughly one-fourth of human-caused CO2 emissions, the WMO states. That absorption makes it more acidic, which can have a detrimental effect on marine life with shells because "calcification declines with increased acidification."

“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,” stated WMO Secretary-General Michel Jarraud.

"We have the knowledge and we have the tools for action to try keep temperature increases within 2°C to give our planet a chance and to give our children and grandchildren a future. Pleading ignorance can no longer be an excuse for not acting,” he continued.

“The Greenhouse Gas Bulletin shows that, far from falling, the concentration of carbon dioxide in the atmosphere actually increased last year at the fastest rate for nearly 30 years. We must reverse this trend by cutting emissions of CO2 and other greenhouse gases across the board,” he added. “We are running out of time.”