Methane levels stopped rising about 20 years ago. It was a major milestone in the fight against climate change and global warming as methane is the second most potent greenhouse gas. But now those levels are on the rise, threatening to jeopardize the aims of the 2015 Paris climate agreement.

Steps to limit rising global temperatures to just 2°C above pre-industrial levels call for a concerted effort from the international community. From raising awareness to investing in fossil-free power, a degree of progress has been made. But with concerns about the growth of coal-fired electricity generation in China and India, and the decision by the US to withdraw from the accord, there is no room for complacency. Now, the best efforts of those working to stick to the Paris target may have been dealt an unfortunate blow.

While everyone was focused on how to combat carbon dioxide, atmospheric methane levels – which had declined and stabilized – began to creep up at an alarming rate. And the most vexing part of this scenario is that it is unclear what’s responsible for the methane increase.

The fall and rise of methane

In 1984, which is the first year with reliable data on methane levels, 1645 parts per billion (ppb) of the gas was detected in the atmosphere. Scientists measure its concentration by looking at the average mole fraction of methane in the remote marine boundary layer of the Earth’s atmosphere.

Between 1984 and 2006 it rose at a moderate rate. But by 2017 it had shot up to an annual global mean of 1850 ppb, marking an increase of about 75 ppb in the 2007-2017 period.

While that growth has been experienced worldwide, it has shown up in more concentrated levels in the tropics and in northern mid-latitudes. Explanations for the increase are not immediately obvious. According to a peer-reviewed paper, published in Global Biogeochemical Cycles: “The possible causes of the change include increasing emissions, with changing relative proportions of source inputs, or a decline in methane destruction, or both.”

That statement needs a little unpacking. In short, the causes of the methane increase are not fully understood. It could be that more methane is being released into the atmosphere – which begs the question where is it coming from. Or the atmosphere’s inherent ability to clean itself may have been compromised – but why, and by what? One of the report’s authors, Euan Nisbet, an Earth scientist at Royal Holloway, University of London, summed it up by saying: “Something that is very, very worrying.”

Cause and effect

Cows, sheep and goats have a number of things in common. Being some of the most commonly raised livestock, there are a lot of them – around 1.4 billioncows, more than 1 billion sheep and more than 950 million goats.

Something else they have in common is that they are all ruminants. That means they have a complex digestive tract in which hard-to-digest food – such as grass and other cellulose materials – ferments, is regurgitated and redigested. The digestion of cellulose produces, among other things, methane.

Global meat consumption is on the rise. Consequently, the populations of animals raised for meat production have increased, leading to an unavoidable boom in methane production. An upward trend in plant-based eating may take a little of the heat out of the meat industry’s growth, and its problematic gassy byproducts, but will it be enough? It’s not terribly likely.