Explain the molecular mechanism by which carbon dioxide acts as a greenhouse gas.

Discuss the significance of two greenhouse gases, other than carbon dioxide, in causing global warming or climate change.

Any three of:

IR/long wavelength/low frequency radiation radiated/emitted by the Earth’s «surface absorbed in the bonds»

bond length/C=O changes

OR

«asymmetric» stretching of bonds

OR

bond angle/OCO changes

polarity/dipole «moment» changes

OR

dipole «moment» created «when molecule absorbs IR»

«some of» energy is then re-radiated towards «the surface of the» Earth

Do not accept terms such as “reflect” OR “bounced” OR “trapped”.

[3 marks]

Any two of:

H₂O AND «relatively» greater abundance/stable concentration/less effective at absorbing radiation/lower GWP so not much overall effect on global warming/climate change

CH₄/N₂O/CFCs/SF₆/O₃/HCFCs AND more effective «than CO₂» at absorbing radiation/higher GWP so could contribute to global warming/climate change

PFCs/SF₆/NF₃/Some CFCs AND have very long life in atmosphere so could contribute «in the future» to global warming/climate change

Accept names or formulas.

Accept two different gases with the same effect for [2].

Award [1 max] for identifying the names/formulas of two greenhouse gases.

Accept “greenhouse factor” for “GWP” but not just “greenhouse effect”.

For M3, do not allow “CFC” alone as only some have long lifetimes (eg, CFC-115, CFC-113).

[2 marks]