Report from the On-board Scientist: Aerosols, Volcanoes and Global Dimming

Volcanic ash from Mt. Eyjafjallajökull in Iceland is disrupting air travel. Could it also disrupt the climate? “Ahh,” say the skeptics, “that should solve the global warming issue for awhile.” Any excuse for business as usual. But the fact is that this eruption, and the aerosols they disperse into the atmosphere, are small by comparison to previous events. Any global dimming, and associated temperature decrease, they produce is a short-term adjustment to the continuously increasing global temperature. Aerosols do, in fact, counteract the warming from increased greenhouse gasses, but they are short lived and they can never completely stop the warming process. We review aerosols, global dimming, and nuclear winter. The measurement program on Ocean Watch is discussed.

What is an Aerosol?

Very simply, an aerosol in the atmosphere is any particle small enough to be kept aloft by air currents. Aerosols include water droplets (clouds are aerosols), ash, dust, salt crystals from ocean spray, and smoke of any kind. The smoke from the burning of tropical rain forests is a major source of aerosols as is automobile exhaust. Aerosols largely come from man-made sources from many countries and also some natural sources. African dust and its impact on climate and the health of the West Indies (including corals) is of growing concern.

In general, aerosols are continuously falling to back to Earth. How fast they fall depends on their size, shape, and density. Nevertheless, when compared to other processes, such as increasing CO₂ or orbit changes, their impact on climate, weather, or human activities is short-term (days to months) and is generally localized near major sources rather than being spread globally.

Some major volcanic aerosol emissions, such as Mt. Pinatubo, are injected high into the stratosphere and are carried around the world. But this is not the norm. The effect of aerosols on climate change is highly uncertain compared to what we know about the effect of “greenhouse gasses”. The ability of global climate computer models to describe or predict climate change is limited by our ability to define aerosol effects.

Mt. Eyjafjallajökull, Iceland: today’s volcano

On about April 14, Mt. Eyjafjallajökull, Iceland, began a major eruption. The volcano had been venting several weeks prior to the eruption. Extreme Icelandic cold caused the hot emission to cool into a gritty ash plume which was carried by high winds right over Europe (see figure below). The eruption is far from finished, and at this writing has kept much of Europe land-bound. New mini-eruptions raise concerns about longer-term damage to world air travel and trade.

As volcanic eruptions go, Mt. Eyjafjallajökull is not a big event. It pales in comparison to past climate-cooling eruptions. Eyjafjallajökull’s ash has reached a height of 55,000 feet, according to press reports. By contrast, ash from the 1991 Mt. Pinatubo eruption, one of the biggest in the 20th century, reached 78,740 feet.

Overall, Pinatubo, which is in the Philippines, ejected 14 to 26 million metric tons of sulfur dioxide that produced a significant global cooling effect for a few years. Following the eruption, this temporary cooling also slowed sea level rise rates temporarily. Nevertheless, in an April 18 news article (Headland SatNews) it was reported “Modern Europe has never seen such a travel disruption. Air space across a swath from Britain to Ukraine was closed and set to stay that way until Sunday or Monday in some countries, affecting airports from New Zealand to San Francisco. Millions of passengers have had plans foiled or delayed. Activity in the volcano at the heart of this increased early Saturday, and showed no sign of abating. ‘There doesn’t seem to be an end in sight,’ Icelandic geologist Magnus Tumi Gudmundsson said on Saturday.”

Climate Change, Global Dimming and Nuclear Winter

The setting sun in Los Angeles turns different shades of orange because the aerosols, from traffic mainly, filter incoming sunlight. Refer to Figure 2 and imagine the atmosphere as a dirty window. The solar radiation that strikes the Earth’s atmosphere (Referred to as the “top of the atmosphere radiation,” or TOA.) is measured by satellites and solar observatories and is well known.

As the radiation penetrates the atmosphere it interacts with air molecules, ozone, and aerosols. Some of the radiation passes through without interaction; this is the sometimes orange sun you see in a clear sky. Some of the sunlight interacts with air molecules and is scattered in different directions. (Think of light as a stream of individual particles, called photons, and think of collisions between the photons and aerosol and air molecules as a game of billiards.) Some of the scattered photons are sent back to space. Therefore, aerosols reduce the heating effect of solar radiation and, in a small way, help to cool our climate and offset the warming effects of greenhouse gasses (CO₂, methane, ozone).

On the other hand, aerosols can add heat to the atmosphere which partially offsets the cooling effect. As the Earth heats up from the sun, it radiates heat back to space. Aerosols absorb some of the heat radiation and reduce the amount of heat radiation escaping out to space. This is the same heat-blocking effect attributed to greenhouse gasses, and in this way aerosols can have a heating effect on global climate.

Nevertheless, the net effect of aerosols is to reduce the rate of global warming from greenhouse gasses. Does this mean we should all go build fires and drive our cars? No, because the offset that aerosols make on all of all these activities is smaller than the impact those activities make on global warming. Models and data now show that aerosols reduce the increase in global temperature by a factor of approximately 50% (there is uncertainty in the actual amount). So, they slow down the process but do not stop it. And they create pollution and effect health at the same time.

Global dimming is a term used for the reduction of sunlight as it passes through the atmosphere. When aerosols are concentrated the sunlight is reduced, heat reaching the surface is reduced, and global warming is reduced.

The TOA (top of the atmosphere) radiation (Figure 2) is scattered and absorbed during its passage through the atmosphere. The fraction of sunlight reaching the Earth’s surface depends on two things: the aerosol loading and the angle of the sun. As the sun nears the horizon the beam cuts through more and more atmosphere thickness and the impact of the aerosol increases. This explains why the setting sun turns different colors in a smoggy or hazy sky.

A numerical measure of the aerosol loading factor is the “optical depth.” Optical depth is a logarithmic measure like the Richter scale for earthquake intensity and pH for acidity. The total optical depth is related to the total reduction in sunlight reaching the Earth’s surface. When the sun altitude is 60° above the horizon and the optical depth is 0.2, roughly 82% of incoming radiation reaches the ground. Alternately, when the optical depth is 1.0, as under thin cloud, only 13% reaches the ground.

The total optical depth is the sum of effects from ozone, air molecules, and aerosols. When all other contributions are removed from the measure the resulting number is the “aerosol optical depth,” which is the dominant term for solar radiation. As aerosol optical depth is increased due to particles from anthropogenic sources, the atmosphere becomes less clear. This is easily observed by eye as a haziness in the atmosphere. The sky looks whiter rather than sky-blue, and visibility is reduced, i.e. distant objects can no longer be seen.

Excessive aerosol optical depths are found around and downwind from major urban areas. For example the Ohio River Valley, the northeast U.S. coast, Los Angeles, Phoenix, Mexico City, and Beijing are regions with high optical depths over five-hundred to one-thousand mile distances. These plumes of aerosol are released near ground level where they are often effected by local weather. For instance, aerosol concentrations at low levels and are drastically reduced from rainfall (hence the clear skies following a rain storm).

Figure 5 is a graph of optical depth in recent history showing volcano events in 1982 and in 1991. The increase in optical depth is apparent and the graph shows that the effects of major events last only a couple of years. Of interest is the fact that the optical depth since 1981 and especially after 1991 has been dropping by a considerable amount. The decrease is attributed to increased clean air legislation in North America and Europe. Reducing aerosol optical depths means increased global warming.

Will Mt. Eyjafjallajökull significantly reduce global warming?

No.

The Union of Concerned Scientists recently published an opinion on how the Iceland volcano might affect climate change. The headline was “Iceland Volcano Eruption Too Small to Have Significant Climate Effect, Science Group Says.”

“Even if a volcanic eruption were big enough to temporarily cool the planet, heat-trapping carbon dioxide from burning fossil fuels and destroying rain forests (burning and decomposition) would still pose a significant threat, says UCS climate scientist Brenda Ekwurzel. ‘Unlike volcanic ash that will leave the atmosphere within a few months or years, carbon dioxide remains there for decades and even centuries,’ Ekwurzel said. ‘Overloading the atmosphere with carbon dioxide has put us on the path toward a long-term warming trend, so we really can’t pin our hopes on occasional volcanic eruptions to solve the problem.’

“The short-term cooling effects of the Mt. Pinatubo eruption are long gone, and global warming is continuing unabated, she said. ‘In fact, we just experienced the hottest decade on record.’”

Aerosol Measurements from Ocean Watch

The Around the Americas voyage is collaborating with the Joint Institute for the Study of the Atmosphere and Ocean of the University of Washington to support a NASA project to measure the sun’s brightness at sea level and to record the aerosol optical depth along the coast of the Americas. These data complement ongoing observations from a network of continental and island stations. The observation network is called Aeronet. All of these measurements together combine to provide validation points for long-term NASA satellite measurements of optical depth on the worldwide scale.

NASA is using satellites to measure optical depth by a different method: by measuring the radiation reflected back to space. NASA operates a number of satellites to take images of the Earth, and then use the images to estimate the brightness of the Earth-atmosphere. This brightness, as measured from space, is related to the optical depth that a sun photometer measures directly at the surface.

The particular value of the sun photometry measurements from the Ocean Watch AtA voyage is that they are made near the coast where satellite measurements area difficult and where validation of the satellite data is particularly important.

Michael Reynolds, Ph.D., RMR Company