A Closer Look at Dust

                    A wide plume of dust blowing off the Saharan Desert toward the Canary Islands.  Credit: NASA Earth Observatory

Each summer, sandstorms lift millions of tons of dust from the Sahara, carrying plumes of it off the West Coast of Africa and over the Atlantic Ocean. Eric Wilcox, a researcher at NASA’s Goddard Space Flight Center, has been using data from NASA satellites to examine the impact such storms can have on rainfall patterns. Wilcox has a new paper about his findings in Geophysical Research Letters; Nature Geoscience also highlighted it in a recent issue. As a result, we sat down with Wilcox to discuss his new findings and some little-known details about dust.

WhatOnEarth: How does dust affect the atmosphere?

Wilcox: Well, we know that dust isn’t just a passive particle floating around. We know it can either absorb or scatter sunlight. Dust outbreaks surely cool the planet by reducing the amount of sunlight that reaches the surface. Likewise, we know dust can warm the atmosphere, although the magnitude is still uncertain.

WhatOnEarth: What causes a particle to absorb rather than scatter light?

Wilcox: It has to do with the color of the particle and, to some degree, the shape. We call this property the single scattering albedo, which is the probability that a photon of light will scatter versus getting absorbed. If the scatter is very high—say 0.99—then we’re confident that 99 out of 100 photons will scatter. If it’s lower, say 0.85, that means there’s a 15 percent chance that the proton will be absorbed.

WhatOnEarth: Where does Saharan dust fit in?

Wilcox: Some Saharan dust is quite bright and some is much darker. It depends on where the sand is coming from and what its mineralogy is like.

WhatOnEarth: Why does it matter if dust is absorbing light?

Wilcox: We’ve found that dust outbreaks, along with other factors, seem to be shifting tropical precipitation (which typically occurs in a narrow band where winds from the northern and southern hemispheres come together) northward by about four or five degrees, which is about 240 to 280 miles at the equator.

WhatOnEarth: Really? What does dust have to do with precipitation?

Wilcox: The main pathway for dust off the Sahara is usually well north of the band of tropical Atlantic rain storms. However, dust storms coincide  with a strong warming of the lower atmosphere, so the atmospheric circulation over the ocean responds to that warming by shifting wind and rainfall patterns northward during the summer. The rainfall responds to the passage of a dust storm even if the dust does not mix with the rain. 

WhatOnEarth: Will the upcoming Glory mission help you study this phenomenon? I know it has an instrument that will measure aerosols such as  dust? 

Wilcox: Definitely. Over bright reflective surfaces such as deserts — where it has been nearly impossible to distinguish aerosols from the surface — we’re at the point that any new information will be helpful. 

WhatOnEarth: What’s the significance of a northward migration of rainfall during dusty periods?

Wilcox: Certainly people local to the area have an interest in understanding how dust affects their rainfall patterns. The finding also lends support to an idea from one of my colleagues—Bill Lau—who studies the elevated heat pump.The idea is that aerosols from dust storms and air pollution actually affect monsoons. Space Flight Center.

Image Details: The lead image was acquired by the MODIS Land Rapid Response Team at NASA’s Goddard Space Flight Center in 2004.

— Adam Voiland, NASA’s Earth Science News Team

Same Words But Different Meanings

Earth scientists milling around the lobby during coffee breaks at this year’s AGU had something unusual to mull over this year.  A phalanx of colorful posters, created by a visual communicator who describes herself as a note taker on steroids, adorned the lobby of the Moscone Center. Snippets from the illustrated notes offer a fascinating look into some of the brainstorming sessions that have taken place about communicating climate science. AGU intstalled the posters at a fitting time: it’s been a disorienting month for climate scientists who have watched seemingly specious charges of scientific malpractice become a major news item.

One of the posters — called Communicating with Congress (and Everybody Else) — brainstorms some of the pitfalls that make communicating climate science such a challenge. High on the list: jargon. Scientists use such a specialized language that it can be difficult for non-scientists — even for those of us who cover the topic regularly — to distill the meaning from certain scientific presentations or articles. Complicating matters more, there are some words that have distinctly different meanings to scientists and the public. The poster highlighted a handful of them. I’ve taken the liberty of elaborating upon and defining a few of them below. 

Did you know the difference?  Have any good examples to add to the list?

The Public: Spray cans that dispense a liquid mist, many of which damage Earth’s ozone layer.

Scientists: A suspension of any solid or liquid droplet in the atmosphere. Includes dust, soot, pollen, sea salt, sulfates and more. More details about aerosols. 

The Public: Harmful material that leaks from nuclear material and is used to battle cancer.
Scientists: Energy that comes from a source and travels through some material or space. Includes electromagnetic radiation such as radio waves, infrared light, visible light, ultraviolet light, and X-rays.  More details about
electromagnetic radiation.

The Public: Something over Antarctica that protects against cancer-causing light waves. 
Scientists: A molecule containing three oxygen atoms that functions as a harmful air pollutant near the surface, a greenhouse gas in the upper troposphere, and a buffer against ultraviolet radiation in the stratosphere. 
More details about ozone.

The Public: Willful manipulation of facts to suit political ideology.
Scientists: A term used to describe a statistical sample in which members of the sample are not equally likely to be chosen. Also a term used to describe the difference between an
estimator’s expectation and the true value of the parameter being estimated. For some scientific analyses, a certain degree of bias can actually be beneficial.

–Adam Voiland, NASA’s Earth Science News Team

Science at the Intersection of Air Quality and Climate Change

Smog over New York. Credit: NASA

The atmosphere is a stew of gases and particles. Some affect climate. Others degrade air quality and threaten human health. Some do both. Some do neither. Many of them interact with and affect one other.

Ozone, for example, causes respiratory problems near the surface, but also functions as a greenhouse gas. Black carbon aerosol particles do the same, and also contribute to heart disease.

Other pollutants — notably sulfates and nitrates—create health problems but simultaneously reflect incoming sunlight and cool the climate. Some, like nitrogen oxide, are precursors to ozone, but also affect the abundance of the light- scattering pollutants that cool the climate.

All of this adds up to a question that keeps some climatologists up at night: Is it possible to reduce emissions of toxic air pollutants in a way that will mitigate global warming, or at least not make it worse?

For example, reducing black carbon has the potential to improve health and reduce global temperatures by as much as a degree. On the other hand, reducing sulfates—which industries often emit along with black carbon—could negate any reduction in warming that pollution controls might produce. (These are just a few examples from the dozens of gases and particles that scientists have to factor in tabulating Earth’s energy budget.)

Almut Arneth, a researcher from Lund University in Sweden, and colleagues, including NASA climatologist Nadine Unger, considered the question recently in a “perspectives” piece in Science. You can read the full paper here (though you may need to brush up on your atmospheric chemistry to understand the details). Unger and her coauthors sum the complicated situation up this way:

“Given the toxicity of pollutants, the question is not whether ever stricter air pollution controls will be implemented, but when and where. The jury is out on whether air pollution control will accelerate or mitigate climate change. Still, the studies available to date mostly suggest that air pollution control will accelerate warming in the coming decades.”

If that’s correct, not only do we have a bigger climate problem on our hands than we may have thought, but some will surely misinterpret the finding by concluding that we ought to continue polluting—or even ramp up the emission of certain pollutants with geoengineering—to stave off climate change, a point that NASA climatologist Gavin Schmidt made recently on RealClimate.

–Adam Voiland, NASA’s Earth Science News Team

Smelling the Air in Kanpur

Winter haze piles up against the Himalayas above the Indo-Gangetic Plain.      (Credit: Earth Observatory)

“When the plane was about 30 minutes from touchdown, we could start to smell the air,” said David Giles. “It was shocking.”

Giles — a young scientist at NASA’s Goddard Space Flight Center — was en route to Kanpur, a large Indian industrial city on the banks of the Ganges river. Dust and soot tend to hover over the region, which is sandwiched between the sharp edge of the Tibetan Plateau to the north and the highlands of the Deccan Plateau to the south.

There’s so much soot in the air that satellites can routinely see a cloud of haze blanketing the region.

The bowl-like Indo-Gangetic Plain is second only to some parts of China for having the heaviest load of air pollution in the world. In the spring, when dust blows in from the deserts to the West, aerosols from factories, buses and trucks, and fires are especially heavy. So much so, in fact, that NASA researchers suggested recently that dust and soot may be driving the retreat of Himalayan glaciers by altering the monsoon.

Giles was in Kanpur to man one of NASA’s AERONET stations in the region as part of the ongoing TIGERZ campaign. He spent 17 days in Kanpur hauling the instrument around and getting harassed by local police officers, the occasional herd of roaming sheep and dust storms. In between all that, he spent the bulk of his time collecting measurements to determine whether dust and soot can glom onto one another to create new types of hybrid aerosols.

They do, he found, a seemingly mundane point but one that’s of considerable interest to the scientists trying to sort out how these two types of aerosols affect the climate. He presented his results in detail this week to colleagues at the American Geophysical Union fall meeting in San Francisco.

I nabbed him after his talk in the afternoon, to have a beer and talk through his travels. I asked him what was the most memorable part of the trip to India. “Well, it was unbelievably hot,” he said with a laugh. “Temperatures routinely hit 105 degrees.”

And how was the air? “You’d get used to it after a while,” said Giles, “but, at first, in the taxi, we were holding our sleeves over our mouths just to avoid breathing the stuff.”

Giles and colleagues using sun photometers to measure aerosols from a rooftop in Kanpur.  (Credit: Giles)

–Adam Voiland, NASA’s Earth Science News Team

How Do Global Soot Models Measure Up?

A image from a simulation that shows the spread of black carbon aerosols in Asia. Areas where the air was thick with
the pollution particles are white, while lower concentrations are transparent purple. (Credit:
Earth Observatory)

As NASA atmospheric scientist Eric Wilcox recently told Time magazine, emerging evidence suggests that a short-lived type of air pollution called black carbon—known popularly as soot—can exacerbate global warming by absorbing incoming solar radiation.

Yet pinning down precisely how much the black carbon exacerbates warming is no easy task, research conducted by Goddard Institute for Space Studies climatologist Dorothy Koch suggests. The study, published in Atmospheric Chemistry and Physics tracked how the predictions from 17 global black carbon models compared with actual measurements collected by airplane, satellite, and ground-based sensors. It shows, among other things, that models generally underestimate black carbon’s warming effect on climate.

Koch tested all the models in three ways. In the simplest of the three, she compared the models’ predictions to the amount of black carbon measured at the surface, finding that they matched real life reasonably well.

Her second test compared the models’ predictions to black carbon measurements made higher in the atmosphere using airplanes, and the results were much less clear cut. Though the models usually had too much black carbon over pollution sources, most had too little over remote regions such as the Arctic.

Koch’s final and most important test looked at how much solar radiation black carbon actually absorbs, an indicator of the amount of warming the particles actually produce. Again, the results were mixed. The models were largely accurate over North America and Europe, but were not for areas that have high levels of black carbon such as Central Africa, Southeast Asia, and the Amazon.

In a write-up on the Goddard Institute for Space Studies web site, Koch summarizes her findings this way:

We concluded from this study that most models have enough black carbon at ground level in polluted regions, too much in the atmosphere above source regions, but not enough in the Arctic where black carbon may play an important role in contributing to Arctic warming and ice/snow melt. The models’ soot generally does not absorb enough sunlight and therefore these models would underestimate black carbon heating effects. This probably results from underestimating the absorbing properties of the particles rather than the amount (mass) of black carbon.

Wondering how climate modelers can continue to close the gap between model predictions and reality? Koch put forward some advice on how to fine-tune the next generation of aerosols models. Her top three:

1) Account for mixing between black carbon and other components of the atmosphere,
2) Incorporate better measurements of particle size and source amount in some regions.
3) Continue to mine ongoing satellite and field campaigns for data about black carbon.

You can read more GISS science briefs and NASA news stories about black carbon here, here, and here.

–Adam Voiland, NASA’s Earth Science News Team