Greenhouse gases, effects and CO2-equivalent calculation

(a) Greenhouse effect

Short-wave solar radiation passes through the atmosphere and warms the earth. The earth re-emits long-wave infrared radiation, and greenhouse gases (GHGs) absorb part of it and send it back, keeping the earth about 33 $^\circ$C warmer (average 15 $^\circ$C instead of $-18\ ^\circ$C). This natural effect is essential for life; the problem is the enhanced greenhouse effect due to rising GHG levels.

   Sun --short-wave--> |  atmosphere (CO2, CH4, N2O, H2O)
                       |   ^   absorbs and re-radiates
   Earth --long-wave IR---+----> part escapes to space

Main GHGs and causes of increase:

Atmospheric $\text{CO}_2$ has risen from about 280 ppm before industrialisation to above 420 ppm today because of fossil fuels and deforestation.

Effects of global warming: rise in global temperature, melting of glaciers (serious for Himalayan glaciers and rivers) and polar ice, sea-level rise, more floods, droughts and extreme weather, glacial lake outburst floods (GLOF), threat to food production, spread of vector-borne diseases, loss of biodiversity, ocean acidification.

(b) Numerical

Reaction: $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$, mass ratio $44/12$.

Carbon in coal $= 0.75\times1\ \text{t} = 0.75$ t.

$$m_{\text{CO}_2} = 0.75\times\frac{44}{12} = 2.75\ \text{t}$$

Methane CO$_2$-equivalent:

$$\text{CO}_2\text{e} = 100\ \text{t}\times28 = 2800\ \text{t CO}_2\text{e}$$

Answer: 1 t of coal gives 2.75 t of CO$_2$; 100 t of methane is equal to 2800 t CO$_2$e.