The level of carbon dioxide in the atmosphere rose by a record rate last year, reaching levels that have in the past been associated with global temperatures 2 to 3 degrees warmer and sea levels 10 to 20 metres higher than present, according to the World Meteorological Organization.
The WMO released its annual Greenhouse Gas Bulletin on Monday, showing CO2 levels surged by 3.3 parts per million in 2016 to an average level of 403.3 ppm. Such a level is comparable to conditions 3 to 5 million years ago.
Last year's increase was driven by human activities, such as burning of fossil fuels, and the strong El Nino event in the Pacific.
"From the most-recent high resolution reconstructions from ice cores, it is possible to observe that changes in CO2 have never been as fast as in the past 150 years, and the natural ice-age changes in CO2 have always preceded corresponding temperature changes," the bulletin said, noting the rate of increase was almost 100 times faster than at the end of the last ice age.
The release of the annual data comes just a week before the 23rd UN Climate Change Conference (COP23) convenes in Bonn, Germany.
The meeting will provide an update on progress of the Paris Climate accord in 2015 and identify gaps between national pledges to cut emissions and the target of keeping global warming to less than 2 degrees above pre-industrial levels. President Donald Trump has vowed to pull the US out of the agreement although any exit cannot take place for several years.
The Turnbull government is due to complete its review of climate policies by the end of 2017. Rising emissions from most sectors - including a resurgence in carbon releases from land clearing - are likely to show Australia will have to do much more to meet its Paris promise of reducing 2005 levels of pollution by 26-28 per cent by 2030.
The WMO noted current CO2 levels were last reached during the mid-Pliocene era. At that time, Greenland and West Antarctic ice sheets had melted, lifting sea levels by as much as 20 metres from current levels.
Last week, scientists led by researchers at the University of Melbourne used updated estimates of Antarctic ice sheet melting to show sea-level rises by the end of century may be 55 per cent higher than forecast by the last assessment report of the Intergovernmental Panel on Climate Change (IPCC).
The WMO said CO2 levels are now 45 per cent higher than the rate in 1750. The 3.3 ppm increase last year was about 50 per cent faster than the average rate of 2.21 ppm over the past decade.
Methane, a more potent greenhouse gas than CO2, had increased even faster since 1750, rising 157 per cent since then to 1853 parts per billion in the atmosphere.
It increased 9 ppb last year, and while less than the increase in 2015, the rise was much higher than the average 6.8 ppb annual increase over the past decade.
The added greenhouse gas levels mean the so-called radiative forcing - the warming effect on the climate - has risen 40 per cent since 1990, according to the US National Oceanic and Atmospheric Administration (NOAA). Last year alone, the forcing rose 2.5 per cent.
"Without rapid cuts in CO2 and other greenhouse gas emissions, we will be heading for dangerous temperature increases by the end of this century, well above the target set by the Paris climate change agreement," WMO Secretary-General Petteri Taalas??? said in a statement. "Future generations will inherit a much more inhospitable planet, " he said.
Global temperature records were broken each year from 2014 to 2016, as the El Nino event combined with background warming.
With conditions tilting towards the opposite pattern in the Pacific - a La Nina - 2017 is likely to be marginally cooler than last year but still be second or third warmest on record, NOAA estimated last month.