Greenhouse gas emissions and air pollution have changed the composition of the atmosphere, and thereby initiated global warming and reduced air quality. Our editorial board members note the need for a deeper understanding of atmospheric fluxes and processes to tackle climate and human health issues.
The atmosphere is a vital life source: it is the air we breathe and a protective shield from harmful radiation. However, increasing levels of greenhouse gases are warming our atmosphere at an alarming rate and pollution is threatening air quality. Reducing and removing greenhouse gases requires a good grasp of their existing sources and sinks, and a comprehensive understanding of the net benefits of different strategies. Similarly, both natural and anthropogenic aerosols affect the atmospheric energy balance, air quality, and human health. Here, our editorial board members discuss some of the paths forward.
Joshua Dean: Time to reconcile methane inventories
Methane is a potent greenhouse gas driving climate change, second only to carbon dioxide. Unlike carbon dioxide, however, methane is naturally removed from the atmosphere quite quickly and lasts on average about 10 years before it is oxidized. This means if we stopped methane emissions, the remaining methane in the atmosphere would eventually be removed, which would slow climate change. The opposite is also true: sustained growth in atmospheric methane concentrations requires substantial growth in emissions. And grow they have; atmospheric methane concentrations have increased by ~0.5% per year since 20101.
Tellingly, during the height of the COVID-19 pandemic in 2020, growth in atmospheric carbon dioxide concentrations slowed while methane concentrations grew faster than ever. Roughly half the global methane emissions are from anthropogenic sources (mainly agriculture, waste, and the oil and gas industry), the other half from natural ecosystems (mainly wetlands). The sooner we reduce anthropogenic methane emissions, the greater the impact will be in the fight against climate change.
It is not clear what is driving the increase in atmospheric methane concentrations. We have a good handle on the overall numbers of the global inventory of methane sources and sinks, but uncertainty surrounds individual methane sources. If we add up all the observations of methane emissions from individual sources and upscale them for the whole globe (a bottom-up approach), we get values more than 25% higher than direct measurements of the whole methane inventory using atmospheric measurements (a top-down approach). While we can address anthropogenic emissions directly, it is much harder to reduce natural methane emissions. Thus, we must reconcile our top-down and bottom-up methane inventories to understand where methane emissions are coming from, which emissions we can directly prevent and which we cannot, and therefore constrain the future trajectory of atmospheric methane concentrations.
One key place where top-down versus bottom-up methane inventories disagree is in the oil and gas industry. Oil and gas bottom-up methane inventories, derived from industry reporting, underestimate methane emissions by 20–70%2,3. Natural gas, comprised primarily of fossil methane and an efficient energy source, is sometimes touted as a bridging fuel as we move towards a low-carbon future. This same fossil methane has also been targeted as a potential source of hydrogen to fuel a future hydrogen economy. But emissions of methane from the energy sector are currently far too high to offset the potential benefits of using it to generate energy. Perhaps renewable natural gas, generated from the decomposition of waste products in the agriculture and food industry, could play a role in a low-carbon future but only if methane emissions to the atmosphere are negligible.