NEW DELHI: As there is no let up in climate-damaging human activities including burning of fossil fuels across the world, the monthly global average concentration of carbon dioxide in the atmosphere had exceeded the threshold limit of 400 parts per million in March 2015 for the first time in recorded history.“This threshold is of symbolic and scientific significance and reinforces evidence that the burning of fossil fuels and other human activities are responsible for the continuing increase in heat-trapping greenhouse gases”, said the World Meteorological Organization (WMO) in a statement on Thursday.The WMO referred to the monthly global average of carbon dioxide concentration in the atmosphere, data of which was released by the US National Oceanic and Atmospheric Administration.Accumulation of carbon dioxide is quite serious as the CO2 remains in the atmosphere for hundreds of years. Its lifespan in the oceans is even longer.It is the single most important greenhouse gas emitted by human activities. It was responsible for 85% of the increase in radiative forcing – the warming effect on our climate - over the decade 2002-2012. The CO2 absorption by the oceans causes ocean acidification."The latest finding highlights the relentless rise of concentrations of greenhouse gases in the atmosphere. All of the northern hemisphere monitoring stations contributing to the World Meteorological Organization (WMO) Global Atmosphere Watch network passed the threshold of 400 PPM monthly average atmospheric CO2 concentrations in April 2014 during the seasonal maximum. This maximum occurs early in the northern hemisphere spring before vegetation growth absorbs CO2", said the WMO.The NOAA’s report from its Global Greenhouse Gas Reference Network , which constitutes substantial part of the GAW observational network, issued on Wednesday marks the first time that the global monthly average within their network – including stations in the southern hemisphere – has crossed 400 PPM, with a March value of 400.8 PPM.“The WMO’s Greenhouse Gas Bulletin showed that between 1990 and 2013 there was a 34% increase in radiative forcing – the warming effect on our climate – because of long-lived greenhouse gases such as carbon dioxide (CO2), methane and nitrous oxide.“In 2013, concentration of CO2 in the atmosphere was 142% of the pre-industrial era (1750), and of methane and nitrous oxide 253% and 121%, respectively”, said the WMO.