The Carbon Cycle

What is the boiling point for $CO_2$?

Low boiling point

The boiling/condensation temperature for $CO_2$ is -109 F = -78 C = 195 K far below room temperature.

  • $\Rightarrow$ $CO_2$ molecules are only weakly attracted to each other.
  • $\Rightarrow$ Air temperatures on Earth never dip below that boiling point.
  • $\Rightarrow$ No condensation--No $CO_2$ 'dew' on the grass.
  • $\Rightarrow$ There is no $CO_2$ 'rain' to rapidly remove our carbon dioxide emissions from the atmosphere.

Are there any other processes that remove $CO_2$ from the atmosphere??

Carbon cycle

The diagram below uses units of "Petagrams"=$10^{15}$ grams of Carbon.

Another common unit is Gigatonnes: $$\text{1 Giga Tonne}=10^9 \text{metric tonnes}\times\frac{1000\text{ kg}}{\text{tonne}}\times\frac{1000\text{ grams}}{\text{kg}}=10^{15}\text{ grams}.$$

United States Carbon Cycle Science Program

  • Black number - Predating humans
  • Red numbers - human influence

Carbon flows (Pg / year)

$CO_2$ in Earth's atmosphere: Pre-industrial $CO_2$ levels, 280 ppm $\approx$ 589 Pg.

Human impact: +4 Pg / year

Human impacts that increase atmospheric $CO_2$

  • Burning fossil fuels: +7.8; 

  • Land use change: +1.1; 

  • Increased soil respiration + increased fire: +11.6


Human impacts that decrease atmospheric $CO_2$

  • Increased ocean absorption: -2.3; $\Rightarrow$ Ocean acidification… 

  • Increased photosynthesis / plant growth due to higher $CO_2$ levels: -14.1

Another view


Global Carbon Project

How long does $CO_2$ hang around?

IPCC calculation

IPCC = Intergovernmental Panel on Climate Change

  • Scientists / Diplomats / Business representatives
  • Convoked by the UN and the WMO
  • Studies CC and issues reports of their consensus.

Ocean acidification

Lucky for our atmosphere, a large portion of the $CO_2$ we emit goes into the ocean!

Any side effects??

The first thing to know about ocean life is that the ocean has a lot of $CO_3^{2-}$ (Carbonate) ions in it. So marine life has evolved to use this as a raw material and build strong exo-skeletons by combining carbonate with calcium = Calcium carbonate, which is what most "seashells" are made out of.

Bill GraceyAn abalone shell has irridescent nacre on the inside made from blocks of aragonite ($CaCO_3$) and on the outside calcite (also $CaCO_3$).

When $CO_2$ dissolves in water.. $$\begineq CO_2+H_2O &\to& H_2CO_3\text{ (Carbonic acid)}\\ H_2CO_3&\to&H^++HCO_3^-\text{(things can stop here...)}\\ H^++CO_3^{2-}&\to&HCO_3^-\\ \color{blue}{\overline{CO_2+H_2O+CO_3^{2-}}}&\to&\color{blue}{\overline{2 HCO_3^-}} \endeq$$ The net effect is depletion of carbonate ions.

Environmental effects:

  • Contributes to coral "bleaching" (along with increased water temperatures and damage from boat anchors and bottom fishing...)
  • Weakens the calcium-carbonate skeletons / shells which many marine animals depend on. (These particular images are made in currently unnaturally high levels of acid, to speed up the process, but still illustrate potential long-term damage.)

    GHGs and GWPs

    We can compare other GHGs to $CO_2$ in terms of their "Global Warming Potential":

    • The GWP of $CO_2$ is, by definition, 1.
    • Methane has a GWP of 28 to 36 over a period of 100 years. Methane typically stays in the atmosphere for about a decade.
    • This means that 1 ton of methane emitted today would warm the atmosphere over the next 100 years as much as 28-36 tons of $CO_2$ emitted today.
    • $N_2O$ and CFCs?

    GWP${}_\text{100}$ by weight, compared to $CO_2$

    pre-sustainability.com

    • 2025 - all air conditioning equipment must have a GWP of $\lt;$700, and not cause damage to the ozone layer. R-454B has a GWP of 467. R-32 (GWP of 675) is also legal.