Question 19M.2.HL.TZ2.7
| Date | May 2019 | Marks available | [Maximum mark: 7] | Reference code | 19M.2.HL.TZ2.7 |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Calculate, Explain, Show that, Suggest | Question number | 7 | Adapted from | N/A |
The average temperature of ocean surface water is 289 K. Oceans behave as black bodies.
Show that the intensity radiated by the oceans is about 400 W m-2.
[1]
5.67 × 10−8 × 2894
OR
= 396«W m−2» ✔
«≈ 400 W m−2»
This was well answered with candidates scoring the mark for either a correct substitution or an answer given to at least one more sf than the show that value. Some candidates used 298 rather than 289.
Explain why some of this radiation is returned to the oceans from the atmosphere.
[3]
«most of the radiation emitted by the oceans is in the» infrared ✔
«this radiation is» absorbed by greenhouse gases/named greenhouse gas in the atmosphere ✔
«the gases» reradiate/re-emit ✔
partly back towards oceans/in all directions/awareness that radiation in other directions is also present ✔
For many this was a well-rehearsed answer which succinctly scored full marks. For others too many vague terms were used. There was much talk about energy being trapped or reflected and the ozone layer was often included. The word ‘albedo’ was often written down with no indication of what it means and ‘the albedo effect also featured.
The intensity in (b) returned to the oceans is 330 W m-2. The intensity of the solar radiation incident on the oceans is 170 W m-2.
Calculate the additional intensity that must be lost by the oceans so that the water temperature remains constant.
[2]
water loses 396 − 330/66 «W m −2» ✔
extra intensity that must be lost is «170 − 66» = 104 ≈ 100 «W m−2» ✔
OR
absorbed by water 330 + 170/500 «W m−2»✔
extra intensity that must be lost is «500 − 396» = 104 ≈ 100 «W m−2» ✔
This was well-answered, a very straightforward 2 marks.
Suggest a mechanism by which the additional intensity can be lost.
[1]
conduction to the air above
OR
«mainly» evaporation
OR
melting ice at the poles
OR
reflection of sunlight off the surface of the ocean ✔
Do not accept convection or radiation.
Many candidates didn’t understand this question and thought that the answer needed to be some form of human activity that would reduce global temperature rise.