Global Warming Mitigation: Smoke and Not Mirrors

The UN proposes a way to delay dangerous warming by decades.

If you’ve read this recent U.S. National Academy of Sciences report, you’d have an idea of how little time we may have to avoid passing the 2-degree Celsius (3.6-degree Fahrenheit) warming threshold set by international groups (see here [pdf] and here) to avoid dangerous climate change.

Our best estimate is that we’ll cross that 2-degree threshold if greenhouse gases like carbon dioxide (CO2) exceed 450 parts per million. Without aggressive emissions-slashing measures, that could happen in 30 years or less. Given the current political scene, one might conclude it’s hopeless.

Not so fast, says a new report by the UN Environment Programme and the World Meteorological Organization. Building on a 2009 paper by Mario Molina of the University of California, San Diego, the report claims it’s possible to reduce warming by about 0.5 degrees Celsius in 2050 and potentially delay dangerous warming by a decade or two even if CO2 continues to rise over that time. How? By implementing existing, relatively inexpensive air-pollution controls — in the process we’d clean the air, save lives, and improve agricultural production. A potential win-win, no-regrets strategy, even for climate refudiaters, if there ever was one.

Targeting Two Pollutants: Ozone and Soot

Lower atmospheric ozone: Sometimes referred to as the “bad” ozone to distinguish it from the “good” stratospheric kind, tropospheric or lower atmospheric ozone is a toxic product of photochemical smog which plagues cities throughout the world. The “photochemical” description means it’s produced by chemical reactions in the presence of sunlight. The chemicals involved, known as ozone precursors, include methane, carbon monoxide, a wide class of gaseous or volatile organic compounds (VOCs), and nitrogen oxides.

This bad ozone is not limited to urban areas — estimates indicate its concentrations in the lower atmosphere have increased by as much as 30 percent since pre-industrial times. And ozone is a greenhouse gas in addition to being toxic, so slowing photochemical smog would reduce greenhouse warming.

How? Actually, we’ve been doing that for decades to improve air quality. Much of the precursor emissions come from burning fossil fuels, wood, and other biomass. Technologies like the catalytic converter in cars have been paying huge dividends in the United States: addressing U.S. air pollution has prevented more than 350,000 premature deaths.

Black carbon: Take a look at campfire smoke or the exhaust of an old diesel truck. Those blackish-gray plumes are loaded with tiny particles of carbon — a k a soot. If you live in a city like New York, you can get your very own sample of the stuff: open your window for a couple days, then wipe the sill with a white towel.

No surprise: this stuff’s not good for you. It’s bad enough when smoke get in your eyes. In your lungs those tiny particles can do major damage (e.g., lung and heart disease).

It can also play havoc with climate. Because it’s black, it absorbs sunlight, heating the atmosphere and contributing to global warming. When deposited on ice, it heats, and hastens melting.

Today’s major soot-producing culprits include diesel engines and in large parts of the developing world cook stoves and biomass burning. Like ozone precursors, technologies for reducing black carbon emissions exist and are commonly used in the United States and other developed cities.

Why Not Jump on It?

So it’s no-brainer: let’s aggressively extend existing technologies to developing economies, and expand them in developed countries. “Full implementation of the identified measures,” the report estimates, “could avoid 2.4 million premature deaths … and the loss of …1–4 percent, of the global production of maize, rice, soybean and wheat each year.” The added benefit? A significant slowing of global warming and thus more time to transition to a low-carbon economy. Plus, while it takes a decade or so for the climate to respond to decreases in CO2 emissions, the response to emission reductions in these traditional air pollutants will be almost immediate.

But there’s a catch. Look at the horizon on a smoggy day: you’ll see a milky haze caused by tiny particles. Particles produced by the same photochemical reactions that produce lower atmospheric ozone. The particles are not all that different from black carbon with one important exception: black carbon absorbs sunlight; the hazy particles scatter sunlight and in so doing reflect some back to space.

From the perspective of climate, black carbon acts like absorbing smoke while hazy particles act like mirrors. The smoke heats the atmosphere; the mirror cools it. And there’s the rub. If air-pollution controls reduce ozone, we get both an air-quality and climate benefit (less warming). But if such controls also reduce haze, and they likely will, we get an air quality benefit but with less of a climate benefit.

Which is not to say we shouldn’t improve air quality — it will save lives. We just need to recognize that the win-win, no-brainer strategy of slowing global warming with air-pollution controls may not buy us as much time as we think.