Now for the carbon dioxide forecast: levels of this gas in the atmosphere will rise by 2.5 parts per million to average 408 ppm in 2017.
And the monthly average could exceed 410 ppm for the first time during this year’s peak in May (CO2 levels rise and fall each year with seasonal changes in plant growth). The precise forecast is 409.86 plus or minus 0.61 ppm.
It is just four years since the peak level of CO2 first exceeded the troubling milestone of 400 ppm. If its concentration keeps rising at this rate, it will double compared with pre-industrial times well before the end of the century.
The prediction of a 2.5 ppm rise this year is the first ever official CO2 forecast by the UK’s Met Office. It was actually made last November, but the weather organisation has only just made it public.
“We were able to successfully forecast the record CO2 rise that we saw last year,” says Richard Betts, who leads research into climate impacts at the Met Office Hadley Centre. “Now we’re getting happier with the method, we are going to start to do it as a routine forecast every year.”
The forecast is specifically for Mauna Loa in Hawaii, where CO2 levels have been monitored since the 1950s, providing plenty of fodder for forecasters. Levels at other sites can differ slightly.
CO2 is the main greenhouse gas responsible for warming the planet. Prior to the industrial age, levels in the atmosphere were around 280 ppm – and had remained below 300 ppm for at least 800,000 years. Now they have shot up to more than 400 ppm.
Faster and faster
Not only is the CO2 concentration rising, it is rising ever faster. Levels at Mauna Loa have increased by an average of 2.1 ppm per year over the past decade, up from 0.7 ppm in the 1950s. But there are big variations from year to year.
In particular, there are larger increases during El Niño years, during which warm surface waters in the Pacific Ocean affect weather worldwide. Levels of CO2 rose 2.9 ppm during the strong El Niño of 1997-98, for instance. This is thought to happen because more of the gas is released from tropical forests during hot, dry years, not least because there are more wildfires.
The team’s method combines data on human CO2 emissions, predictions of Pacific sea surface temperatures over the coming year and the observed link between sea temperatures and CO2 levels.
The predicted increase of 2.5 ppm in 2017 is well below the record 3.4 ppm rise last year, but still significantly more than the annual average of 2.1 ppm over the past decade. There have been only half a dozen years with annual increases of 2.5 ppm or more, the first being 1998.
The continuing rapid rise in atmospheric levels might appear to be at odds with estimates that human emissions from energy and industry have barely grown in the last three years, largely thanks to a decline in coal use in China. However, these estimates don’t include emissions from land-use change, such as the clearing of forests.
In addition, around half of all human emissions are soaked up by the land and seas. So if they absorb less CO2, the rate at which atmospheric levels are rising can increase even if human emissions remain constant.
The current lull in growth of fossil fuel emissions is probably only temporary, many experts believe. And to limit global warming, human emissions from all sources need to fall to below zero.