A Sector-by-Sector look at Climate Change

This is a four-part class offered by Arlington Continuing Education. Benjamin Brown-Steiner was the instructor

Emissions: Session I (11/6/2019)

Greenhouse gas emissions by sector (across the entire united states)

• 29\% from transportation
• 28\% from electric power generation
• 22\% from industry
• 12\% from commercial and residential uses
• 9\% from agriculture

In Massachusetts, the sector contributors to GHGs are

• 42\% from transportation
• 19\% from electric power generation
• 15\% from residential uses
• 9\% from commercial uses

Some terms to know:

• RF. Radiative forcing. The difference between incoming solar radiation and outgoing terrestrial radiation. RF is measured in watts per square meter (w/m²). You can think of this as one watt of illumination applied to a square meter. An incandescent Christmas tree light bulb emits around a watt.
• GWP. Global warming potential.
• Gt. gigaton. One billion tons.
• Chemical Lifetime. How long a chemical lasts, before breaking down in the atmosphere or environment.
• Burden. The amount of a chemical released into the atmosphere that remains there.
• Loss. The amount of a chemical released into the atmosphere that breaks down or is absorbed.
• CO2eq. Carbon dioxide equivalent. This is the warming potential of some GHG, expressed as the ratio of the warming potential of CO₂. For example, a CO2eq of 2 means "twice as much warming potential as the same amount of carbon dioxide".
• tCO2e. Total $\mathrm{CO}_2$ equivalent.

Greenhouse gases include CO₂ (carbon dioxide), CH₄ (methane), N₂O (nitrogen dioxide), and NO_x (other compounds made up of nitrogen and oxygen).

Emissions are anything released into the atmosphere (or more generally, anything released into the environment). They come from cars, agriculture, heating, burning wood, cow flatulence, and other sources.

CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) are another category of GHGs. These have largely been phased out, but there are still small amounts of CFC and HCFC emissions.

Water vapor changes atmospheric radiative forcing, but only to a very small degree.

Different GHGs have different warming effects, and the warming effects change in different ways over time. For example, methane emissions have a significant short-term warming effect. This decreases over time as the methane reacts with other compounds in the atmosphere and breaks down. For ease of comparison, GHGs are often normalized, based on the ratio of CO₂ warming potential. CO₂'s warming potential is defined as 1, and other gases are scaled according to that baseline.

Chemical emissions into the environment form a dynamic system. Some of the compounds remain in the environment ("burden"), and some of them are absorbed or broken down ("loss").

Inert gases have longer lifetimes. Volatile gases have short lifetimes. For example, OH^- is a hydroxide molecule, which carries a negative electrical charge and is very reactive. It doesn't last long.

Gases have a lifetime which is the amount of the gas in the atmosphere, divided by its loss rate. For example, there is 750 Gt of CO₂ in the atmosphere and CO₂'s rate of loss is 215 Gt/year. That gives a lifetime of 750 / 215 = 3.5 years.

We watched a video called "Denial 101x" given by lecturer Gavin Cawley, and available through EDx. https://www.youtube.com/watch?v=FlIJTJWP6PQ. Mr. Cawley used a jelly bean jar to illustrate the concepts of burden, loss, and lifetime.

Plants absorb 123 Gt of CO₂ per year, and emit 119 Gt/year. This system is very close to equilibrium.

The atmosphere moves slowly, so there is an adjustment time that follows a change in emissions. If we were to stop producing GHGs today, it would take 50--200 years for the climate to return to early industrial levels.

Radiative forcing (RF) is the difference between incoming solar radiation and outgoing terrestrial radiation. An RF of 0 indicates balance, where neither warming nor cooling takes place. RF > 0 indicates warming. RF < 0 indicates cooling.

There are several ways that methane can break down in the atmosphere, and one can represent the sequences of reactions as a tree. CO₂ is a potential end-product of methane decomposition.

CO2eq is "carbon dioxide equivalent". It's the warming effect of a GHG, scaled to the warming effect of carbon dioxide. For example, a CO₂ equivalent of 10 means "ten times the warming potential of carbon dioxide". CO₂ equivalence varies over time (because the warming potential of other GHGs varies over time).

tCO2e is total CO₂ equivalent.

Aerosols are small suspended particles. The particles can be solid, liquid, or a mixture of both. Examples include dust, pollen, and soot. Aerosols are larger and heavier than gases, and have short lifetimes.

Black carbon is another name for soot. Almost every country that regulates air quality has soot regulations.

Some emissions counteract the effects of each other. Black Carbon has a warming effect, while sulfates have a cooling effect. In the quantities they typically appear, the two tend to cancel each other out.

A gigaton (Gt) is a billion tons. This is approximately the mass of all land mammals on earth.

A typical US person is responsible for around 20 metrics tons of GHG emissions/year.

There are natural reservoirs of carbon in the environment. There's carbon in rocks, deep ocean water, and fossil fuels. Fluxes represent movement of carbon from one type of reservoir to another.

Homework: fill in reservoir and flux diagram.

Halocarbons were used for propellants, air conditioning, and foam insulation. Since the 1990s, their use has been ramped down to very low levels. However, we see evidence that some locations in China have started using them again.

A fair amount of NO₂ emissions come from agriculture. Some crops take it out of the atmosphere. Fertilizer production also removes a lot.

NO_x comes from burning fossil fuels. It affects the climate, but air quality is a bigger concern with regard to NO_x emissions.

There are natural and anthropogenic sources of aerosols. Vegetation is both an emitter and an absorber. For example, pine trees tend to emit certain types. Aerosols are often soaked up by precipitation, or they settle on the ground.