Shedding light on global dimming

Shedding light on global dimming

Dalhousie University Researchers Track Pathways of Pollution
May 1, 2006
In the summer of 2004, Dalhousie University physicist Thomas Duck spent most of his time in a cow pasture in Chebogue Point, on Nova Scotia’s southern tip. Duck set up a portable observatory known as the Canadian Atmosphere Biosphere Observations Trailer (CABOT) to investigate one of the biggest issues emerging in climate change: global dimming.

Global dimming is the gradual reduction of solar radiation or sunlight that reaches Earth. Researchers believe air pollution—specifically, the increased presence of aerosols in the atmosphere—is blocking sunlight by bouncing it back into space, instead of allowing it to reach Earth. Researchers believe global dimming plays a role in climate change by affecting global temperatures.

Using transportable LIDAR (Light Detection and Ranging), a kind of laser radar installed in the observatory, Duck measured aerosols in the atmosphere. Aerosols play a central role in air quality because they can enter people’s lungs and affect respiratory health.

CABOT’s portability means that Duck and his students can take their measurements anywhere. Inside the observatory, they use a telescope, LIDAR, and an instrumentation room full of computers to measure aerosols and other molecules at different altitudes.

LIDAR works by sending out pulses of light into the air, which get scattered back to the observatory by aerosols and other particulates in the atmosphere. The time it takes for the light to return enables the researchers to measure the distance and wavelength of the molecules. These measurements allow scientists to track the source of the air pollution, and plot its transport pathways.

“When we have a lot of aerosol in the atmosphere, cloud droplets tend to be smaller and more numerous,” explains Duck. That makes the droplets highly reflective. The more aerosol particles in the atmosphere, such as those contained in condensation trails from aircraft, the more they change the optical properties of clouds that block sunlight. That means the higher the aerosol level, the less sunlight gets through the clouds to warm up Earth at ground level.

“Aerosols change the reflectivity of the clouds, and this has a significant impact on climate change,” says Duck. “All the aerosol put into the atmosphere in recent years, whether by pollution or forest fires, is to some degree blocking out the sun’s light.”

Duck was also able to identify smoke from forest fires raging in Alaska and the Yukon that had travelled all the way to Halifax. Using mathematical models and satellite data, Duck and his team identified a smoke plume that extended up to eight kilometres into the atmosphere and tracked it, day by day, back to its source in Alaska. It was the first time the Dalhousie team had used the CABOT observatory, which they had just finished building in 2004. “It was very exciting,” Duck says. “And it all worked.”

The measurements that team members took, along with their ability to trace the forest fire smoke, will help them achieve their goal of tracking the trajectory of contaminants that leave North America and contribute to pollution in Europe. Tracking the smoke plume, which eventually dispersed over the ocean, “helps us understand the whole transport problem a little bit better, as we can pinpoint sources and see what they look like from afar,” explains Duck.

This summer, the team plans to travel to the West Coast to track air pollution flowing into North America and its effect on climate change. “Developing countries are burning more and more fossil fuels,” Duck says. “We want to track what’s coming into North America.”



Nova Scotia is often referred to, at least by climatologists, as “the tailpipe of North America,” says Thomas Duck, professor of physics and atmospheric science at Dalhousie University in Halifax. That’s because the Atlantic provinces are located downwind of chief industrial and population centres that produce major pollution.

“We’re very interested in understanding the pathways of pollution coming here,” Duck says. By identifying pollution sources and pathways, policy-makers will be better equipped to devise ways to decrease the production of the pollution that is contributing to climate change. “There’s no way you can really alter transport pathways. All you can do is try to limit emissions.”

Duck hopes that by measuring and plotting transport pathways, he will also be able to work with the Meteorological Services of Canada and Nova Scotia, using their pollution forecast models to help interpret the Dalhousie team’s measurements. By watching for activity occurring outside of the models, the team will be able to uncover and learn more about physical phenomena that may affect pollution pathways. Their research may also contribute to improved pollution forecasts specifically aimed at people with poor or compromised respiratory health.

“What we’re interested in doing is using all of our data sets and combining them to understand the extent of the pollution transport problem, provincially and also across the Maritimes,” Duck says.


At the Polar Environment Atmospheric Research Laboratory (PEARL) at the Eureka weather station on Ellesmere Island in Nunavut, researchers are monitoring dramatic changes in the Arctic’s climate. Dalhousie professor Thomas Duck works with his colleagues at the Canadian Network for the Detection of Atmospheric Change (CANDAC) to measure and chart these changes.

“In some parts of the Arctic, the temperatures have warmed an average of four degrees Celsius over the last 30 years. This has contributed to a huge shrinkage of the ice cap,” says Duck.

To put the change in perspective, the difference in temperatures between our current age and an ice age is, on average, about six degrees, Duck says. “So when we’re talking about a four-degree change, this is enormous.”

Created as an informal network of researchers, CANDAC’s focus is on collaborative research, facilities, and training, says James Drummond, a University of Toronto researcher who is CANDAC’s principal investigator. Its current project is to reactivate the research station at Eureka as a “whole atmosphere” research station addressing issues of ozone, air quality, and climate change.

Drummond says that Duck’s radar, by monitoring the clouds and aerosols, helps to increase scientists’ understanding of the energy flows in the atmosphere and how they cause temperature changes at the surface.

Learn More

Visit the Atmospheric Optics Laboratory.