Particulates, Aerosols, And Climate: The More Important Story

Guest opinion: Dr. Tim Ball [1]

The article “US Senate Considering Albedo Modification Geoengineering Proposal” appeared as I finished this article. It commented on the plan to introduce particles into the atmosphere to increase the reflective capability of the Earth’s atmosphere known as the albedo. Most responses correctly identified it as unwise. Isn’t it already happening? Jet contrails from commercial airline flights reduce the amount of sunlight that makes it to the surface, as this satellite image shows:

The Senate proposal is not new. In 2009, John Holdren pushed the same idea as a Daily Mail story headlined “Obama may fire pollution particles into stratosphere to deflect sun’s heat in desperate bid to tackle global warming” (Figure 1).

The controversial experiment was touted yesterday as a possible last resort to help cool the Earth’s air by the president’s new science advisor John Holdren.

‘It’s got to be looked at. We don’t have the luxury of taking any approach off the table,’ said Mr Holdren, Director of the White House Office of Science and Technology.

Figure 1, Original caption: “Sunscreen: Could its rays be deflected as a last resort to beat global warming?”

Actions without thought or concern for the consequences are the pattern as political agendas ignore facts or logic. Adding particulates to the atmosphere is another dangerous and unnecessary proposal.

Issues about particulates all relate to a lack of data or knowledge about their role in the atmosphere and their effect on climate and climate change. The challenge is underscored because changing albedo is just one component of their role.

First, there is the issue of the difference between aerosols and particulates. The IPCC only makes the distinction between Working Groups. In the Physical Science Basis Report of Working Group I (WG-I) they only refer to aerosols. The Impact, Adaptation, and Vulnerability Report of Working Group II (WG-II) refers to particulates. The WG-I Glossary only lists a definition of aerosols.

A suspension of airborne solid or liquid particles, with a typical size between a few nanometres and 10 μm that reside in the atmosphere for at least several hours. For convenience the term aerosol, which includes both the particles and the suspending gas, is often used in this report in its plural form to mean aerosol particles. Aerosols may be of either natural or anthropogenic origin. Aerosols may influence climate in several ways: directly through scattering and absorbing radiation (see Aerosol–radiation interaction) and indirectly by acting as cloud condensation nuclei or ice nuclei, modifying the optical properties and lifetime of clouds (see Aero- sol–cloud interaction).

The use of the term particulates in WG-II is apparently related to the health impacts. This is primarily a function of particle size and the ability to enter the lungs.

The atmosphere is composed mostly of gases, but also contains liquid and solid matter in the form of particles. It is usual to distinguish these particles according to their size, chemical composition, water content and fall velocity into atmospheric aerosol particles, cloud particles and falling hydrometeors. Despite their small mass or volume fraction, particles in the atmosphere strongly influence the transfer of radiant energy and the spatial distribution of latent heating through the atmosphere, thereby influencing the weather and climate.

Wikipedia offers a compromise definition.

Atmospheric particulate matter – also known as particulate matter (PM) or particulates – are microscopic solid or liquid matter suspended in the Earth’s atmosphere. The term aerosol commonly refers to the particulate/air mixture, as opposed to the particulate matter alone.

Either way, the IPCC acknowledges the importance of aerosols.

Aerosol particles interact with solar radiation through absorption and scattering and, to a lesser extent with terrestrial radiation through absorption, scattering and emission.

An early attempt to classify aerosols by size and therefore their effect is shown in Figure 2.

Figure 2; Source: Encyclopedia of Climatology

The problem is each particle reacts differently depending on its size, shape, molecular structure and the angle of incidence of the solar radiation among other things. The reactions are physical and chemical, and they change all the time. For example, a cloud has H2O as a gas, liquid and solid, as well as dust particles of varying sizes and all are constantly changing in volume and form. What happens to the reactive properties of a cloud when water vapor surrounds one of those dust particles (condensation nuclei) and becomes a water droplet? What happens when convective motion takes the cloud above the freezing level and ice crystals form?

Particle differences are important as they affect shortwave (SWR) incoming solar radiation and long-wave outgoing radiation (LWR) as the IPCC acknowledge in Figure 3 from AR5

Figure 3.

When determining anthropogenic global warming or climate change, it is inadequate to say that you can work out the net effect at the top or bottom of the cloud or the atmosphere. Effectively isolating AGW to a single cause forces us to know the effects of change of every single variable. However, as the IPCC note,

Owing to inter-annual variability, long-term trends in aerosols from natural sources are more difficult to identify (Mahowald et al., 2010).

So they are forced to conclude,

Thus, confidence is low for global satellite derived AOD (Aerosol optical depth) trends over these relatively short time periods.

You can’t identify the human influence if you don’t know the natural. In a good summary of the problems associated with aerosols NASA concludes;

Scientists have much to learn about the way aerosols affect regional and global climate. We have yet to accurately quantify the relative impacts on climate of natural aerosols and those of human origin. Moreover, we do not know in what regions of the planet the amount of atmospheric aerosol is increasing, is diminishing, and is remaining roughly constant. Overall, we are even unsure whether aerosols are warming or cooling our planet.

Wow, warming or cooling is unknown?

Contrary to popular understanding, virtually all numbers used in climate studies are estimates. Look at the values assigned to different components of the energy flow diagram based on Trenberth’s original (Figure 4). The values for “Absorbed by Atmosphere” is 67 Wm². Others provide a different value. Figure 5 is a recent work of WG-I Co-chairs of the IPCC Report, Martin Wild and Norman Loeb. It shows a value of 79 Wm² for atmospheric absorption, but this is with a range of estimates from 74 to 91 Wm².

Figure 4

The role and impact of aerosols in the atmosphere is large. 107 Wm² reflected and 67 Wm² absorbed is 174 Wm² is the crude estimate of the total of 342 Wm² incoming solar radiation that either doesn’t heat the Earth or indirectly heats the atmosphere. Only a small variation in these variables causes energy balance variations that swamp those attributed to human produced CO2. Apparently there is no value for the amount of long wave absorbed by aerosols in the atmosphere. Is it part of the Back Radiation?

Original Caption: Best estimates of the global mean energy balance components together with their uncertainty ranges, representing present day climate. Surface estimates based on the analyses presented in this study. TOA estimates from Loeb et al. (2009). Units Wm-2 (From Wild et al., submitted)

Figure 5.

The values given in Figures 4 and 5 are for energy flows, but what is not explained is the amount of aerosols. The IPCC only examines human sources of aerosols as their mandate dictates, but there is a massive and constantly varying volume of materials in the atmosphere. There are very few estimates of the actual amount of atmospheric material. Mitchell (1973) estimated the total amount of dust, smoke and other particles as approximately 40 million tons. In 1970 Hubert Lamb published an important article, “Volcanic dust in the atmosphere; with a chronology and assessment of its meteorological significance.” From this he evolved a Dust Veil Index (DVI), a quantification of changes in atmospheric composition and its impact on the Earth’s energy balance.

It appears that some of the AGW proponents realize the DVI is important. As Bob Tisdale reported apparently, Michael Mann saw it as an opportunity to sway the statistics and data on global warming. Simple theory says particulates reduce sunlight reaching the ground. The reality is we have little idea how the DVI varies over time or how aerosols affect temperature as Steve Goddard discussed around the Mauna Loa data in Figure 6.

Figure 6

Compare this with the latest data plot in Figure 7. The word “Apparent” is significant.

Major issues not generally considered are how the changing atmospheric content alters the percentages of effects on incoming solar and outgoing long wave radiation. There are three major effects when radiation strikes the aerosol, absorption, reflection, and scattering. Any changes in the numbers and form of the aerosol will create a different response; for example, the phase change of H2O between gas, liquid and solid.

The effect of change, such as an increase in the DVI, will differ depending on the trend of temperature at the time. There is a study of a historical example of what happens when a singular event exacerbates cooling.

In 1992 we organized a conference in Ottawa to analyze the climate impacts of the Indonesian volcano Tambora. It was the largest eruption in historic times and considered the cause of the history-changing record cold year of 1816, the “The Year Without a Summer.” John Eddy presented the keynote paper.

Eddy identified the cooling associated with the lack of sunspots from 1790 to 1830 known as the Dalton Minimum. This meant global temperatures were falling before volcanic cooling was added in 1815. The cooling due to the volcanic dust injected into the atmosphere amplified a cooling trend. What would be the difference if the global trend was warming?

One factor not considered is the impact of changes on the frequencies of sunlight in the visible spectrum. Whether the solar radiation is absorbed, reflected, or scattered is primarily determined by the relationship between the wavelength of the spectrum and the size of the particle. The sky is blue because the size of the most prominent molecules in the atmosphere is the same as the wavelength of blue light. Change the size of the particles in the atmosphere and the sky colour changes as evidenced by red skies in the lower atmosphere with a low sun angle.

Much volcanic ash that reaches high altitudes is sulfur. There it becomes condensation nuclei that create yellow water droplets, which filter out the yellow portion of the sunlight. I witnessed the effect while driving across the Canadian Prairies in the fall of 1992. The eruption was in 1991, but it takes a year for the global distribution of the high altitude effect. Most crops were still unripened in the first week of September because the yellow portion of the spectrum is critical. This is why you need special neon tubes to grow plants. Farmers resorted to adding a desiccant to dry out the plant to facilitate harvesting.

Wind speed is another weather variable that receives inadequate attention. It is a major determinant of the amount of dust in the atmosphere. Deserts are the windiest climate regions and therefore contribute a great deal of atmospheric dust. Similarly, evaporation increases with wind speed over the ocean thus increasing salt particles in the atmosphere.

Aerosol effects are generally measured by comparing observations of reflected and transmitted sunlight between satellite sensors and ground sensors. The few observations available produce columnar data, which are then used in models to simulate what they think is happening. The Global Earth Observation and Monitoring GEOMON started in 2006 underscored the inadequacies.

The wider question is,

“What are the global trends of atmospheric composition from ground-based and satellite observations assimilated in modelling studies, and what key measurements should be added for reducing uncertainties on surface emissions and atmospheric processes?”

Many factors cause climate change, but only a few are considered in the current scientific debate and most are based on estimated or inadequate data. The role of aerosols in the atmosphere are little known, measured or understood.

The proposal to add particulates to offset warming is the environmental equivalent of adding to the debt to get out of debt, only worse. Despite this, politicians demonstrate their lack of knowledge of the science by proposing to play God. Maybe they should wait until there is enough space debris to block the sun and cause cooling.

[1] This article is a composite of two published at the Friends of Science web page and on my web page.