By Tim Radford
LONDON, 15 June, 2016 – With a little help from that natural cyclic phenomenon El Niño and a lot of help from humans, carbon dioxide concentrations in the atmosphere will exceed 400 parts per million (ppm) not just for the rest of the year, but probably for a lifetime.
And as carbon dioxide levels rise, so will temperatures. Scientists say that summers 50 years from now will be far hotter than any experienced by anybody now.
The record CO2 level is yet another notch in the seemingly inexorable rise of the greenhouse gas that, for most of human history, oscillated around 285 ppm.
In 1958, a scientist recording atmospheric concentrations at the top of a mountain in Hawaii recorded levels of 315 ppm and, very slowly, climate scientists began to worry about the possibility of global climate change as a consequence of global warming from the increasing combustion of fossil fuels.
That is because carbon dioxide and other greenhouse gases are transparent to solar radiation, but trap infra-red waves as they re-radiate from the planet’s rocks and ocean.
Last year, global average temperatures were measured at 1°C higher than historic averages, and carbon dioxide levels briefly went over the 400ppm mark. Also last year, at the UN climate change conference,195 nations pledged to do their best to keep long-term temperature rise to below 2°C.
Richard Betts, climate impacts expert at the UK Met Office and the University of Exeter, and colleagues report in Nature Climate Change that the average concentration of CO2 during 2016 will be 404.45 parts per million.
Last month, it reached 407 ppm, and may drop to 401.48 ppm in September before rising again. Concentrations vary with the changing seasons, but human and other natural cycles can affect the total.
“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 Niño event – changes in the sea-surface temperature of the tropical Pacific Ocean,” Professor Betts says.
“This warms and dries tropical ecosystems, reducing their uptake of carbon, and exacerbating forest fires. Since human emissions are now 25% greater than in the last big El Niño in 1997/98, this all adds up to a record CO2 rise this year.”
Plants absorb CO2 in summer, and release it in autumn and winter. Although one hemisphere’s winter is the other’s summer, the land areas of the hemispheres are uneven. So in an ordinary year, some point out, concentrations could fall below the 400ppm figure.
“However, we predict that this will not happen now, because the recent El Niño has warmed and dried tropical ecosystems and driven forest fires, adding to the CO2 rise,” Professor Betts says.
And if humans go on burning fossil fuels as they have been doing for a century, Flavio Lehner, a climatology expert at the National Centre for Atmospheric Research in the US, and colleagues report in Climatic Change that there is a greater than 90% chance of summers between 2061 and 2080 being the hottest on record in large parts of the Americas, central Europe, Asia and Africa.
Such research is confirmatory. Scientists have repeatedly warned that hotter summers – sometimes dangerously hotter – are on the way.
The difference this time is that the scientists can say with confidence that if emissions are reduced, the probability of record summer temperatures drops to 41%.
“Extremely hot summers always pose a challenge to society,” Dr Lehner says. “They can increase the risk for health issues, and can also damage crops and deepen droughts. Such summers are a true test of our adaptability to rising temperatures.” – Climate News Network