Tiny aerosol particles, big global impacts
Laura Nielsen for Frontier Scientists
Tiny particles suspended in the air, present in the air we breathe and in the highest reaches of the atmosphere, are called aerosols. And those aerosols, though relatively short-lived, have a huge impact on global climate change. In fact, much of the atmospheric warming observed since 1976 in the Arctic, and elsewhere, can be attributed to aerosols.
Minuscule dark sooty aerosol particles called black carbon absorb solar radiation. While carbon dioxide lingers in the air for roughly 100 years, airborne particles like black carbon may have a lifespan of only days or weeks before they drift back to ground or are carried down by precipitation. Still, their impact is evident. According to black carbon specialist Tami Bond of the University of Illinois, in one-to-two weeks, one pound of black carbon absorbs 650 times as much energy as one pound of carbon dioxide gas will absorb over the span of 100 years. That makes black carbon a short-lived climate forcer.
While black carbon is often human-created, the majority of aerosols which enter the atmosphere have natural causes. Wildfires create smoke, while volcanic eruptions spew forth gasses and sulfate particles. Drought promotes dust clouds, and desert sandstorms fling sand and mineral dust into the atmosphere. Even the ocean is a source of aerosols, since spray from waves sends salt particles into the air. Yet it's beneficial to focus on the anthropogenic (human-caused) aerosols injected into the atmosphere, because they are not part of natural cycles. If we can understand our role in creating them and how they impact our planet's climate, then we can change our behavior to better our environment.
Fossil fuel combustion is one anthropogenic creator of polluting particulate matter. Biomass burning to clear land or dispose of agricultural waste is another. Dust is sent aloft by overgrazing, deforestation, and excessive irrigation. Smog and other air pollution hazes the air over major cities. All of these aerosols travel swiftly through the atmosphere, and can influence places far from where they were released.
While carbon dioxide is the largest man-made contributor to global warming, the second largest contributor is black carbon. Black carbon is essentially minuscule soot, tiny dark particles suspended aloft in the atmosphere. It is created by engines burning fossil fuels like diesel, stoves burning biofuels like wood or dung, power plants burning coal, and certain industrial processes. This tiny demon has a a strong warming influence on the atmosphere. It also promotes and worsens human respiratory and cardiovascular diseases.
The impact of sunlight-absorbing black carbon on climate is complex. Functionally, the tiny dark particles absorb solar radiation and hold heat near the Earth. An old diesel truck traveling 60 miles would emit about one ounce of black carbon. One ounce absorbs as much sunlight as does an entire dark tennis court, says Bond. When black carbon falls on snow or ice it reduces the ice's albedo, or reflectiveness. Instead of bouncing the sun's rays back into space the darker ice absorbs solar radiation, heats, and melts. Enough black carbon deposited on a glacier's surface speeds glacial melting. Yet the aerosol particles can also promote cloud formation or change cloud reflectivity. Clouds are amazingly versatile and have either a cooling or warming effect. To add to the complexity, sources which emit black carbon often emit other particles; sometimes those co-emitted polluting particles have a cooling effect.
Take a look at other polluting aerosols. Fossil fuel combustion (burning coal and oil) produces sulfates. Those sulfates scatter solar rays, and thus have a cooling effect. Yet in North America and Europe we have successfully cut down on the amount of anthropogenic sulfates in the air by heightening emission standards. Fewer sulfates means a lessened cooling effect, and so the impact of solar-ray absorbing black carbon is felt more strongly. Methane matters too. Methane is a greenhouse gas more potent than carbon dioxide. It's a major component of natural gas, and humans boost its release through mining activities, creating decomposing garbage, and manure. It also escapes when ancient matter decays below frozen lakes or in thawing permafrost, and it's escaping into the atmosphere in increasing amounts as temperatures in the Arctic warm. When it reaches the tropics, where ozone sits near ground level, atmospheric methane is an important precursor to ground-level ozone. Ozone itself is a greenhouse gas, but it also creates smog, threatens human health, and damages crops.
Polluting aerosols promote temperature increases. *"Aerosols, particularly black carbon, can alter reflectivity by depositing a layer of dark residue on ice and other bright surfaces. In the Arctic especially, aerosols from wildfires and industrial pollution are likely hastening the melting of ice." In the Arctic, we see their effects in the diminishing summer ice cover, which creates a positive feedback loop of warming Arctic temperatures and can be linked to more severe weather events in much of the Northern Hemisphere.
Climate scientist Drew Shindell of NASA's Goddard Institute for Space Studies suggests implementing 14 techniques to reduce the emission of black carbon and methane. Shindell states: **"We've shown that implementing specific practical emissions reductions chosen to maximize climate benefits would also have important 'win-win' benefits for human health and agriculture." His study, which utilizes models run on supercomputers, indicates that implementing those 14 attainable emission control strategies could slow average global warming 0.9 ºF by 2050, increase global agricultural yields, and save hundreds of thousands of lives every year. Continuing to add black carbon and methane to the atmosphere will exacerbate climate change, while enacting the proposed control strategies could half the temperature increase we face in the next 40 years.
We already know how to reduce harmful man-made emissions. Controlling diesel emissions with particle filters, using cleaner-burning stoves and boilers, reducing or banning the burning of agricultural waste, and upgrading industrial brick kilns and coke ovens will all cut down amounts of black carbon in the atmosphere. Meanwhile, methane emissions can be reduced if leaks are fixed along long-distance pipelines, and wastewater treatment plants are updated. Methane from coal mines, oil works, and natural gas facilities can be captured, as well as that escaping from city landfills and farm manure. Steps like these, attainable and realistic, reduce climate impact by anthropogenic emissions, improve human health and living conditions, and boost crop yields.
Those steps alone will not be enough to stem the forward march of human-caused global climate change. We still need to take action to reduce carbon dioxide emissions. Carbon dioxide lingers in the atmosphere for a century, driving temperatures up. On the other hand, pollutants like black carbon and ozone-creating methane inflict powerful changes swiftly, then circulate out of the atmosphere in a matter of weeks. Taking steps to control these pollutants now can measurably slow global warming in the short term. According to Shindell, ***"We will have very little leverage over climate in the next couple of decades if we're just looking at carbon dioxide. If we want to try to stop the Arctic summer sea ice from melting completely over the next few decades, we're much better off looking at aerosols and ozone." And we can do it now. Right now. Shindell says: ****"I found it remarkable that for incomplete combustion, which gives you black carbon, a group of just nine measures was able to pull down the emissions by about 70 to 80 percent. And all of the technologies already exist. There’s no technological barrier whatsoever to reducing black carbon."
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
National Aeronautics and Space Administration http://www.nasa.gov/mission_pages/Glory/about/index.html
***'Aerosols May Drive a Significant Portion of Arctic Warming'
National Aeronautics and Space Administration http://www.nasa.gov/topics/earth/features/warming_aerosols.html
*'Aerosols: Tiny Particles, Big Impact'
National Aeronautics and Space Administration http://earthobservatory.nasa.gov/Features/Aerosols/page3.php
**'Arctic summer wind shift could affect sea ice loss and U.S./European weather, says NOAA-led study'
National Oceanic and Atmospheric Administration http://www.noaanews.noaa.gov/stories2012/20121010_arcticwinds.html
'Black carbon larger cause of climate change than previously assessed'
National Oceanic and Atmospheric Administration http://researchmatters.noaa.gov/news/Pages/soot.aspx
'Black Carbon's Day on the Hill'
What on Earth, a NASA Blog http://blogs.nasa.gov/cm/blog/whatonearth/posts/post_1270497028672.html
****'Cleaning the Air Would Limit Short-Term Climate Warming'
National Aeronautics and Space Administration http://www.nasa.gov/topics/earth/features/cleanair-warming.html