Test of 400 options shows we can save money while limiting climate change

If a group of scientists announced that reducing emissions of some pollutants would prevent global warming, it wouldn’t make headlines—we’ve been hearing that for years when the pollutant is carbon dioxide. However, if they added that those reduced emissions would also prevent millions of premature deaths per year and increase annual crop yields by tens to hundreds of millions of tons, you would probably take notice. But the part that will really blow your mind—and what might make some people reconsider their stance—is that all of this could be done at a profit.

A large group of scientists identified 14 emissions reduction measures—out of around 400 considered—that primarily reduce ozone and black carbon (BC; think soot) using existing technology. The study was authored by Drew Shindell, of NASA Goddard and Columbia University, who had 23 coauthors from a total of 13 different institutions around the world (from countries including the US, UK, Italy, Austria, Thailand, and Kenya). The group concluded that the economic benefits of improved air quality and diminished global warming exceed the typical costs of these 14 approaches.

Carbon dioxide (CO₂) has been the focus of most climate change studies and is one of the most significant greenhouse gasses because of its long lifespan in the atmosphere. Ozone and BC don't stay in the air as long, but they cause both warming and decreased air quality, which directly impacts human health and agriculture productivity. Because they remain in the atmosphere for only a few weeks, control measures that target them would quickly produce noticeable improvements in the climate and air quality.

The authors started out by evaluating existing technologies that improve air quality, and ranked them according to how large an impact they had on climate change. They discovered that the top 14 measures were able to produce about 90 percent of the total possible reductions that could be achieved using all measures combined.

Of the 14 emissions reduction measures, half of them target methane (CH₄), an ozone precursor. These targeted areas including coal mining, oil and gas production, long-distance gas transmission, municipal waste and landfills, wastewater, livestock manure, and rice paddies. The remaining seven target BC emissions through technological approaches that reduce incomplete combustion, which can occur when there isn’t enough oxygen to burn all the fuel, usually due to inefficient technology. These inefficiencies show up in diesel vehicles, biomass stoves, brick kilns, and coke ovens. BC can also be targeted by regulations banning agricultural waste burning, the elimination of high-emitting vehicles, and adoption modern cooking and heating methods.

By pairing these emissions reduction approaches with a complex climate model, the researchers found that the predicted global warming by 2030 would be reduced by approximately 0.5°C, which would limit the increase to about 1.2°C from the 1890-1910 preindustrial mean temperature. The current international limit for global temperature rise is 2°C, which the authors here say requires both significant CO₂ and CH₄ + BC emissions reductions—neither alone will prevent us from passing that limit.

In addition to the decreased global warming potential, reduction of ozone and BC in the atmosphere—or more specifically the troposphere, the lowest level of the atmosphere—would  have other benefits. It should improve global food crop yields (including wheat, rice, maize, and soy) and prevent premature deaths due to air pollution. The authors' calculations showed that approximately 53 million metric tons of crops could be saved (valued at around $8.2 billion) and 0.7-4.7 million deaths could be avoided each year in 2030 and beyond. These measures would also result in improved indoor air quality, which is a bit harder to calculate due to limited data; the authors estimate an additional 373 thousand lives a year in India and China would be saved.

One important note to these results: there would be further crop yield benefits from the limited climate change, but this study didn’t consider these indirect results. The agricultural benefits calculated here resulted directly from reduced levels of ozone and BC in the troposphere.

Now for the cost of all this: they estimated that 2030 emissions could be reduced by 110 teragrams (Tg, 10¹² grams) of CH₄ with costs below $1500 per metric ton of CH₄. Accepting a somewhat lower target, 90 Tg, would cost even less: $250 per metric ton. The economic benefit of reducing these emissions, on the other hand, is conservatively about $1100 per metric ton of CH₄ and potentially as high as $5000 per metric ton. Most of the reductions, then, produce benefits outweighing the costs.

The BC improvements, mainly in brick kilns and clean-burning stoves, actually lead to net cost savings through efficiency improvements. Regulations on vehicle emissions and agricultural waste burning would require political capital rather than actual money. The total benefits of BC improvements work out to around $5.4 trillion, mainly health-related, so a significant profit is also likely here.

Finally, the authors identified the regions that would benefit the most from these measures. While the avoided warming is fairly well spread out across the globe, central and northern Asia, southern Africa, and the Mediterranean in particular would be helped. The benefits in these areas come in large part from reduced albedo forcing on snow and desert areas thanks to lower BC levels. Albedo refers to the reflection of light from the earth’s surface; bright surfaces reflect more light while darker surfaces (such as those covered in BC particles) absorb more sunlight. With the higher albedo, snow and ice would last longer into the year in these areas.

Developing nations in Asia and Africa would avoid the most premature deaths due to cleaner air. If measuring increased crops by total metric tonnage gains, China, India, and the US would benefit the most here, but the Middle East would see the greatest improvement by percentage (due to large potential ozone reductions).

This study, unlike most climate change studies that emphasize reduction of CO₂ emissions, demonstrates that a small number of air-quality improvement measures could also have a large impact on global warming. These techniques, all of which could be implemented with current technology, provide practical benefits for food crops and human health that outweigh the costs in most cases, and simultaneously slow the rate of climate change. If policy makers know that following this plan makes economic sense, it might have a better chance of being enacted—and we might be able to limit climate change.

Science, 2012. DOI: 10.1126/science.1210026 (About DOIs)

Listing image by Photograph by nasa.gov