London: Atmospheric carbon dioxide (CO2) is for the first time expected to exceed 400 parts per million for 2016 as a result of an extra boost from El Nino, finds a new study. El Nino warms and dries tropical ecosystems, reduces their uptake of carbon as well as exacerbates forest fires. The rising trend in CO2 has been the result of human activity and has now been worsened by this year's extreme weather. The alarming levels will remain high even if human emissions begin to decline, the researchers said. "The atmospheric carbon dioxide concentration is rising year-on-year due to human emissions, but this year it is getting an extra boost due to the recent El Nino event -- changes in the sea-surface temperature of the tropical Pacific Ocean," said lead author Richard Betts, professor from the University of Exeter in Britain. "Since human emissions are now 25 per cent greater than in the last big El Nino in 1997/98, this all adds up to a record CO2 rise this year," Betts added. "Studying how these natural cycles interact with human influences is an important part of climate science. Making and testing predictions like this helps us build our understanding and further develop climate models," noted Chris Jones, from the Met Office Hadley Centre in Britain. For the study, published in the journal Nature Climate Change, the team used a seasonal climate forecast model and statistical relationship with sea temperatures. The average concentration in 2016 is forecast to be 404.45 +-0.53 parts per million, dropping to 401.48 +- 0.53 in September before resuming their ongoing rise next year.