Nav: Home

Current climate models underestimate warming by black carbon aerosol

November 19, 2018

Soot belches out of diesel engines, rises from wood- and dung-burning cookstoves and shoots out of oil refinery stacks. According to recent research, air pollution, including soot, is linked to heart disease, some cancers and, in the United States, as many as 150,000 cases of diabetes every year.

Beyond its impact on health, soot, known as black carbon by atmospheric scientists, is a powerful global warming agent. It absorbs sunlight and traps heat in the atmosphere in magnitude second only to the notorious carbon dioxide. Recent commentaries in the journal Proceedings of the National Academy of Sciences called the absence of consensus on soot's light absorption magnitude "one of the grand challenges in atmospheric climate science."

Rajan Chakrabarty, assistant professor in the School of Engineering & Applied Science at Washington University in St. Louis, and William R. Heinson, a National Science Foundation postdoctoral fellow in Chakrabarty's lab, took on that challenge and discovered something new about soot, or rather, a new law that describes its ability to absorb light: the law of light absorption. With it, scientists will be able to better understand soot's role in climate change.

The research has been selected as an "Editors' Suggestion" published online Nov. 19 in the prestigious journal Physical Review Letters.

Because of its ability to absorb sunlight and directly heat the surrounding air, climate scientists incorporate soot into their models -- computational systems which try to replicate conditions of the real world -- and then predict future warming trends. Scientists use real world observations to program their models.

But there hasn't been a consensus on how to incorporate soot's light absorption into these models. They treat it over-simplistically, using a sphere to represent a pure, black carbon aerosol.

"But nature is funny, it has its own ways to add complexity," Chakrabarty said. "By mass, 80 percent of all black carbon you find is always mixed. It's not perfect, like the models treat it."

The particles are mixed, or coated, with organic aerosols that are co-emitted with soot from a combustion system. It turns out, black carbon absorbs more light when it is coated with these organic materials, but the magnitude of absorption enhancement varies non-linearly depending on how much coating is present.

Chakrabarty and Heinson wanted to figure out a universal relationship between the amount of coating and the ability of soot to absorb light.

First, they created simulated particles that looked just like those found in nature, with varying degrees of organic coating. Then, using techniques borrowed from Chakrabarty's work with fractals, the team went through exacting calculations, measuring light absorption in particles bit-by-bit.

Reaction from a pioneer in black carbon research

"This paper strengthens the science and affirms the role of black carbon as a significant forcer of climate change," said Mark Z. Jacobson, professor of civil and environmental engineering and director of the Atmosphere/Energy Program at Stanford University.

"This paper shows, with a combination of data and highly-detailed modeling, that not only are the strong climate effects previously found supported, but also that they may be even underestimated in some of the previous studies because many previous studies did not account for the actual shapes of black carbon particles and the resulting mixtures."

When they plotted the absorption magnitudes against the percentage of organic coating, the result was what mathematicians and scientists call a "universal power law." This means that, as the amount of coating increases, soot's light absorption goes up by a proportionately relative amount.

(The length and area of a square are related by a universal power law: If you double the length of the sides of a square, the area increases by four. It does not matter what the initial length of the side was, the relationship will always hold.)

They then turned to work done by different research groups who measured ambient soot light absorption across the globe, from Houston to London to Beijing. Chakrabarty and Heinson again plotted absorption enhancements against the percentage of coating.

The result was a universal power law with the same one-third ratio as was found in their simulated experiments.

With so many differing values for light absorption enhancement in soot, Chakrabarty said that the climate modelers are confused. "What on earth do we do? How do we account for the reality in our models?

"Now you have order in chaos and a law," he said. "And now you can apply it in a computationally inexpensive manner."

Their findings also point to the fact that warming due to black carbon could have been underestimated by climate models. Assuming spherical shape for these particles and not properly accounting for light absorption enhancement could result in significantly lower heating estimates.

Rahul Zaveri, senior scientist and developer of the comprehensive aerosol model MOSAIC at Pacific Northwest National Laboratory, calls the findings a significant and timely advancement.

"I am particularly excited about the mathematical elegance and extreme computational efficiency of the new parameterization," he said, "which can be quite readily implemented in climate models once the companion parameterization for light scattering by coated black carbon particles is developed."
-end-
The School of Engineering & Applied Science at Washington University in St. Louis focuses intellectual efforts through a new convergence paradigm and builds on strengths, particularly as applied to medicine and health, energy and environment, entrepreneurship and security. With 94 tenured/tenure-track and 28 additional full-time faculty, 1,300 undergraduate students, 1,200 graduate students and 20,000 alumni, we are working to leverage our partnerships with academic and industry partners -- across disciplines and across the world -- to contribute to solving the greatest global challenges of the 21st century.

This work was supported by the U.S. National Science Foundation (AGS-1455215, CBET-1511964, and AGS-PRF-1624814), and the National Aeronautics and Space Administration Radiation Sciences Program (NNX15AI66G). Funding for collecting data during the 2010 CARES field campaign in California was provided by the Atmospheric Radiation Measurement (ARM) Program sponsored by the US Department of Energy (DOE), Office of Biological and Environmental Research (OBER).

Washington University in St. Louis

Related Climate Change Articles:

Mapping the path of climate change
Predicting a major transition, such as climate change, is extremely difficult, but the probabilistic framework developed by the authors is the first step in identifying the path between a shift in two environmental states.
Small change for climate change: Time to increase research funding to save the world
A new study shows that there is a huge disproportion in the level of funding for social science research into the greatest challenge in combating global warming -- how to get individuals and societies to overcome ingrained human habits to make the changes necessary to mitigate climate change.
Sub-national 'climate clubs' could offer key to combating climate change
'Climate clubs' offering membership for sub-national states, in addition to just countries, could speed up progress towards a globally harmonized climate change policy, which in turn offers a way to achieve stronger climate policies in all countries.
Review of Chinese atmospheric science research over the past 70 years: Climate and climate change
Over the past 70 years since the foundation of the People's Republic of China, Chinese scientists have made great contributions to various fields in the research of atmospheric sciences, which attracted worldwide attention.
A CERN for climate change
In a Perspective article appearing in this week's Proceedings of the National Academy of Sciences, Tim Palmer (Oxford University), and Bjorn Stevens (Max Planck Society), critically reflect on the present state of Earth system modelling.
Fairy-wrens change breeding habits to cope with climate change
Warmer temperatures linked to climate change are having a big impact on the breeding habits of one of Australia's most recognisable bird species, according to researchers at The Australian National University (ANU).
Believing in climate change doesn't mean you are preparing for climate change, study finds
Notre Dame researchers found that although coastal homeowners may perceive a worsening of climate change-related hazards, these attitudes are largely unrelated to a homeowner's expectations of actual home damage.
Older forests resist change -- climate change, that is
Older forests in eastern North America are less vulnerable to climate change than younger forests, particularly for carbon storage, timber production, and biodiversity, new research finds.
Could climate change cause infertility?
A number of plant and animal species could find it increasingly difficult to reproduce if climate change worsens and global temperatures become more extreme -- a stark warning highlighted by new scientific research.
Predicting climate change
Thomas Crowther, ETH Zurich identifies long-disappeared forests available for restoration across the world.
More Climate Change News and Climate Change Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Making Amends
What makes a true apology? What does it mean to make amends for past mistakes? This hour, TED speakers explore how repairing the wrongs of the past is the first step toward healing for the future. Guests include historian and preservationist Brent Leggs, law professor Martha Minow, librarian Dawn Wacek, and playwright V (formerly Eve Ensler).
Now Playing: Science for the People

#566 Is Your Gut Leaking?
This week we're busting the human gut wide open with Dr. Alessio Fasano from the Center for Celiac Research and Treatment at Massachusetts General Hospital. Join host Anika Hazra for our discussion separating fact from fiction on the controversial topic of leaky gut syndrome. We cover everything from what causes a leaky gut to interpreting the results of a gut microbiome test! Related links: Center for Celiac Research and Treatment website and their YouTube channel
Now Playing: Radiolab

The Flag and the Fury
How do you actually make change in the world? For 126 years, Mississippi has had the Confederate battle flag on their state flag, and they were the last state in the nation where that emblem remained "officially" flying.  A few days ago, that flag came down. A few days before that, it coming down would have seemed impossible. We dive into the story behind this de-flagging: a journey involving a clash of histories, designs, families, and even cheerleading. This show is a collaboration with OSM Audio. Kiese Laymon's memoir Heavy is here. And the Hospitality Flag webpage is here.