The findings published today in the journal Science Advances suggests that the decline of global sulphur emissions as the result of clean air policies, coupled with the warming and fertilization effects of carbon dioxide emissions lifts a lid on wetland methane production resulting in increased emissions.

The resulting additional future release of 20-34 million tonnes of methane each year from natural wetlands would mean targets to reduce human-caused emissions need to be more stringent than currently set out in the Global Methane Pledge.

Methane, which is one of the most potent greenhouse gases in trapping heat in the atmosphere, is produced in wetlands around the world. Sulphur (in the form of sulphate) has a very specific effect in natural wetlands that reduces methane emissions, while CO₂ increases methane production by increasing growth in plants that make the food for methane-producing microbes.

Professor Vincent Gauci from the University of Birmingham and a senior author of the study said:

“Well-meaning policies aimed at reducing atmospheric sulphur appear to be having the unintended consequence of lifting this sulphur ‘lid’ on wetland methane production. This coupled with increased CO₂ means we have a double whammy effect that pushes emissions much higher.

“How has this happened? Put simply, sulphur provides the conditions for one set of bacteria to outmuscle another set of microbes that produce methane when they compete over the limited food available in wetlands. Under the conditions of acid rain sulphur pollution during the past century, this was enough to reduce wetland methane emissions by up to 8%.

“Now that clean air policies have been introduced, the unfortunate consequence of reducing sulphur deposition, which does have important and welcome effects for the world’s ecosystems, is that we will need to work much harder than we thought to stay within the safe climate limits set out in the Paris agreement.”

More than 150 nations signed up to the Global Methane Pledge at COP26 in Glasgow, which seeks to reduce human-caused emissions of methane by 30% on a 2020 baseline, by 2030.

The study is the latest to implicate reductions in atmospheric sulphur in driving warming at a faster rate than anticipated. In 2020 shipping pollution controls were introduced to reduce emissions of sulphur dioxide and fine particles that are harmful to human health. This reduction in atmospheric sulphur over the oceans has been implicated in larger warming that expected in what has come to be known as ‘termination shock’.

Lead author of the paper Lu Shen of Peking University said:

“Our study points to the complexity of the climate system. Representation of these complex biogeochemical interactions has not previously been well integrated into estimates of future methane emissions. We show that it is essential to consider these feedbacks to get a true understanding of the likely future of this important greenhouse gas.”

The two grains are critical for food security in India, the second most populous country in the world and home to a quarter of all undernourished people globally.

“We wanted to understand the impact of India’s coal electricity emissions on its agriculture because there might be real trade-offs between meeting growing electricity demand with coal generation and maintaining food security,” said Kirat Singh, a PhD student in environment and resources in the Doerr School of Sustainability and lead author of the Feb. 3 study in Proceedings of the National Academy of Sciences.

Clean air and food security

Past studies have sought to quantify overlooked costs of burning coal for electricity by estimating the number of deaths linked to resulting pollution. Government agencies and other organizations use these figures — and estimates of the economic value of statistical life — to understand the costs and benefits of various economic development strategies and environmental regulations.

Until now, however, estimates of crop damages specifically tied to coal-fired power stations — which supply more than 70% of electricity in India — have been lacking despite more than a decade of research showing that air pollutants such as ozone, sulfur dioxide, and nitrogen dioxide hurt crop yields.

“Crop productivity is incredibly important to India’s food security and economic prospects,” said senior study author David Lobell, the Benjamin M. Page Professor in the Doerr School of Sustainability’s Earth System Science Department. “We’ve known that improved air quality could help agriculture, but this study is the first to drill down to a specific sector and measure the potential benefits of reducing emissions.”

Crop damage concentrated in key regions and seasons

For the new study, the authors estimated rice and wheat crop losses linked to emissions of nitrogen dioxide, or NO₂, from coal power stations. They used a statistical model that combines daily records of wind direction and electricity generation at 144 power stations in India and satellite-measured nitrogen dioxide levels over cropland.

The authors found coal power plants affected NO₂ concentrations above cropland up to 100 kilometers, or roughly 62 miles, away. Eliminating coal emissions from all farmland within this range during key growing seasons (January-February and September-October) could boost the value of rice output across India by approximately $420 million per year and of wheat output by $400 million per year, according to the study.

“This study underscores the importance of looking at environmental issues under a systems lens,” said study co-author Inês Azevedo, a professor of energy science and engineering in the Doerr School of Sustainability. “Any policy focused on reducing emissions from coal power plants in India will be ignoring a crucial part of the problem if it does not consider the damages from air pollution to agriculture.”

In some states with high levels of coal-fired electricity generation, such as Chhattisgarh, coal emissions account for as much as 13-19% of the region’s nitrogen dioxide pollution, depending on the season. Elsewhere, like Uttar Pradesh, coal emissions contribute only about 3-5% of NO₂ pollution. Other common sources of the gas, which results from burning fossil fuels, include vehicle exhaust and industry.

Broad benefits from emission cuts

The analysis reveals that the value of lost crop output is almost always lower than the mortality damage caused by any given coal power station. But the intensity of crop damage per gigawatt-hour of electricity generated can often be higher. At 58 of the 144 power stations studied, rice damage per gigawatt-hour exceeded mortality damage. Wheat damage per gigawatt-hour exceeded mortality damage at 35 power stations.

“It’s rare to find a single thing — in this case, reducing coal emissions — that would help agriculture so quickly and so much,” said Lobell, who is also the Gloria and Richard Kushel Director of Stanford’s Center on Food Security and the Environment.

The researchers found little overlap among the stations associated with the largest crop losses and those associated with the highest mortality. This means benefits from possible emission reductions in the future could be more significant and widely distributed than previously understood. According to the authors, the results highlight “the importance of considering crop losses alongside health impacts when regulating coal electricity emissions in India.”

“Well-targeted policies to cut emissions could deliver thousands of dollars of increased crop output for each clean gigawatt-hour, in addition to all the climate and human health benefits,” said Singh.

Lobell is also the William Wrigley Senior Fellow at the Stanford Woods Institute for the Environment and a senior fellow at the Freeman Spogli Institute (FSI) and at the Stanford Institute for Economic Policy Research (SIEPR).

Azevedo is also a professor (by courtesy) of civil and environmental engineering, a joint department of the Doerr School of Sustainability and Stanford School of Engineering. She is also a senior fellow at the Stanford Woods Institute for the Environment and at the Precourt Institute for Energy.

“Some own the land for recreational purposes, some own it because they want to produce timber, and some are a combination of both. If one landowner controls invasive species but their neighbor does not for some reason, that could be problematic because the bioinvasion will spread over space and time,” said study author Shadi Atallah, associate professor in the Department of Agricultural and Consumer Economics, part of the College of Agricultural, Consumer and Environmental Sciences at Illinois.

Currently, conservation cost-share programs reimburse forest landowners for up to 75% of the cost of controlling invasive species. However, this is not necessarily the most efficient use of funds, Atallah noted.

“When you’re incentivizing someone to do something, you would like to know they weren’t going to do it without the incentive, or you have wasted your money. Would it make sense to change who qualifies for these payments in a way that optimizes the effect?”

Atallah employed game theory to explore these questions, using the example of glossy buckthorn in U.S. Eastern white pine forests. Glossy buckthorn is a fast-growing shrub that is exotic and invasive in North America. It can form a dense, persistent layer in forest understories, interfering with the growth of young pine trees and affecting wildlife habitat.

In a previous study, Atallah surveyed Maine and New Hampshire family forest landowners to understand how their willingness to control glossy buckthorn was motivated by the shrub’s effects on timber, trail recreation, and wildlife viewing. For the current study, he developed a theoretical modeling framework that estimated how each type of landowner would act and how their neighbor would respond, given various circumstances over a period of time.

“The model focuses on two cases: one is the recreationist, who likes to hike and enjoy the land, but they don’t like that the invasive shrub will block their path and interfere with wildlife viewing. The other is the timber producer, who gets a reduced amount of timber if the invasive shrub prevents white pine from growing to maturity,” he explained.

Atallah also considered the connectivity between forests and how the bioinvasion spreads to surrounding areas.

“Some invasive species are mostly spread by short distance dispersal, such as squirrels or other small mammals, while other species are spread through long-distance dispersal, such as birds and waterfowl, and the spread can be either fast or slow. Another factor is where the bioinvasion first appears and whether it makes sense to prioritize control at the starting point,” he said.

He found that in most scenarios, the recreationist would not control the invasive shrub without cost-share payment, while the timber owner would choose to control regardless of subsidies.

“Only in the case of fast long-distance dispersal does control get so expensive that nobody can do it without support. This is consistent with what we are currently doing; everybody qualifies for it. However, in all other cases — slow long-distance dispersal, and slow or fast short-distance dispersal — it would make sense to pay only the recreationists, who act as sources of bioinvasion spread when they refrain from control.”

Atallah noted that this may not seem fair; however, the timber landowners would still benefit even if only the recreationists are subsidized, because the spread is contained earlier and to a larger extent.

“We find the cost of control for the timber producers becomes lower once you have cleaned up the neighboring forestlands. Instead of partially reimbursing both types of landowners, the money could be used to cover 75% of control for the recreationist, and as a result, the timber owner would have less invasive species to deal with and control would be less expensive.”

The study provides one example of different landowner motivations, but this doesn’t mean the same dynamic always plays out, Atallah stated.

“My conclusion is that we should provide the subsidy to the weakest link, which is the entity that wouldn’t act otherwise and ends up acting as the source of the externality. In this case it’s the recreationist, but you could imagine a situation where it was the other way around. The model is adaptable to any given scenario,” he said.

“The idea is to consider any two different motivations to own the resource and how that could lead to different decisions in terms of controlling bioinvasion spread, and thinking about prioritizing those who would not control without subsidies.”

Thin-film solar cells require little energy and material to produce and therefore have a very small environmental footprint. In addition to the well-known and market-leading silicon solar cells, there are also thin-film solar cells, e.g. based on copper, indium, gallium and selenium, known as CIGS cells. CIGS thin films can even be applied to flexible substrates.

Now, experts from HZB and Humboldt University Berlin, have developed a new tandem solar cell that combines a bottom cell made of CIGS with a top cell based on perovskite. By improving the contact layers between the top and bottom cells, they were able to increase the efficiency to 24.6 %. This is the current world record, as certified by the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany.

This record cell was the result of a successful team effort: the top cell was fabricated by TU Berlin master’s student Thede Mehlhop under the supervision of Stefan Gall. The perovskite absorber layer was produced in the joint laboratory of HZB and Humboldt University of Berlin. The CIGS sub-cell and contact layers were fabricated by HZB researcher Guillermo Farias Basulto. He also used the high-performance cluster system KOALA, which enables the deposition of perovskites and contact layers in vacuum at HZB.

‘At HZB, we have highly specialised laboratories and experts who are top performers in their fields. With this world record tandem cell, they have once again shown how fruitfully they work together,’ says Prof. Rutger Schlatmann, spokesman for the Solar Energy Department at HZB.

The record announced today is not the first world record at HZB: HZB teams have already achieved world record values for tandem solar cells several times, most recently for silicon-perovskite tandem solar cells, but also with the combination CIGS-perovskite.

We are confident that CIGS-perovskite tandem cells can achieve much higher efficiencies, probably more than 30%,” says Prof. Rutger Schlatmann.