Test Week 4: The fundamentals of climate

Question 1

If we emit a pulse of carbon dioxide into the atmosphere, approximately how much of it would effectively remain in the atmosphere in 60 years time?

a None of it

b All of it

c Around 45%

d Around 10%

Answer 1

C
From the graphs on Week 4, Lecture 1, Slide 4, it is about 45%. Note that it might not be the same molecules that were emitted, because of the large exhanges between the atmosphere, oceans and land (Week 4, Lecture 1, Slide 6) but the atmospheric concentrations will remain 45% higher than they would have been, had the pulse not occured.

Question 1

If the global-mean surface temperature rise due to human activity is (“to a 50% likelihood”) to be kept below 1.5 deg C, then what cumulative emission of CO2 (in Gigatonnes (Gt) CO2) is allowable, and approximately what proportion of this emission do we have left?

a About 2495 GtCO2 and we have already emitted this amount

b About 2875 GtCO2 and the remaining proportion is about 13% of this

c About 2495 GtCO2 and the remaining proportion is about 15% of this

d About 3725 GtCO2 and the remaining proportion is about 33% of this

Answer 1

B
From the left-hand pie chart on Week 4 Lecture 1 Slide 16, the total amount of cumulative emissions that give a 50% likelihood of remaining within 1.5 deg C is 2495+380 = 2875 GtCO2. Hence the fraction remaining of the total is 380/2875 = 0.13, or 13%.

Question 2

The global warming potenial (GWP(100)) for an emission of nitrous oxide is almost 10 times higher than that of methane because

a Nitrous oxide is a heavier molecule than methane

b Nitrous oxide has a longer lifetime than methane

c Nitrous oxide has a longer lifetime and a higher radiative efficiency than methane

d Nitrous oxide has a higher radiative efficiency than methane

Answer 2

c

From the table on Week 4, Lecture 2, Slide 8, the GWP(100) of nitrous oxide (270) is much larger than methane’s (30) because it is both longer lived (lifetime of 110 years, compared to methane’s 12 years), and it has a higher radiative efficiency (0.003 W m-2 ppb-1 compared to 0.0005 W m-2 ppb-1).

Nitrous oxide is a heavier molecule (molecular weight 48) than methane (molecular weight 16), which means there are fewer molecules of nitrous oxide in a kg than in a kg of methane. But because the GWP, by definition, compares the effect of equal mass emissions (e.g.,1 kg) of gases, the effect of these different molecular weights is already taken into account in the GWP value.

Question 3

On Week 4 Lecture 2 Slide 11, Brazil’s emissions of methane are calculated to be 34% of ts total CO2-equivalent emissions. If the methane GWP(100) value from IPCC (2021) on Slide 10 is used instead, what would this value be, if all of Brazil’s other emissions remain unchanged?

a It would remain at 34%

b It would increase to 38%

c It would increase to 41%

d It would decrease to 32%

Answer 3

B
If the 0.014x106 kt of methane emissions given in the red box on Slide 11 are used with the GWP(100) updated from 25 to 30 (as given on Slide 10) then the CO2-equivalent emissions of methane would now be 30x0.014x106 kt = 0.42 million kt of CO2-equivalent. This would increase the total to 1.11 million kt of CO2-equivalent and so the methane fraction would be 0.42/1.11 = 0.38 or about 38%.