• Atmospheric/tri-cellular circulation (including Hadley, Ferrel & polar cells) creates patterns of climate that determine dominant vegetation types 1 mark

• Low pressure due to intense heating/high insolation at the equator causes rising moist air in the tropics, creating high precipitation giving rise to rainforests 1 mark

• Moving polewards (at high altitude), air cools, becomes denser and sinks forming a high-pressure zone/descending dry air (20–30° latitude/tropics) creates water-limiting/arid conditions giving rise to deserts 1 mark

• Some air continues towards the poles to equalize temperature difference/atmosphere transfers heat from (sub-)tropics to mid-latitudes giving rise to temperate biomes 1 mark

• Descending/dry air (high latitude/polar regions) creates water-limiting conditions in tundra 1 mark

4 marks max

Human impacts on the atmosphere affecting terrestrial biome productivity

• Increased greenhouse gas emissions (e.g. $\text{CO}_2$, $\text{CH}_4$, $\text{N}_2\text{O}$) from burning of fossil fuels/deforestation enhance the greenhouse effect 1 mark

• This leads to global warming/climate change which can:

  • Shift biome boundaries polewards/to higher altitudes 1 mark
  • Increase frequency/intensity of extreme weather events (e.g. droughts, floods) 1 mark
  • Disrupt phenological events/timings (e.g. earlier bud burst) 1 mark

• Increased temperatures can increase productivity in some biomes (e.g. tundra, boreal forests) but decrease productivity in others (e.g. deserts, tropical rainforests) 1 mark

• Increased atmospheric $\text{CO}_2$ can increase photosynthetic rates/productivity (CO$_2$ fertilization effect) 1 mark

Maximum 6 marks if no reference to both warming and increased $CO_2$

7 marks max

Answers may include:

• understanding concepts and terminology of biodiversity; conservation strategies; tropical biomes; rainforests; swamps; coral reefs; hotspots; LEDCs & MEDCs; international v national conservation bodies; environmental value systems; productivity; carbon sinks; global warming; unsustainable exploitation; resources of tropical ecosystems; medicines; indigenous cultures; endangered species; Red List, etc.;
• breadth in addressing and linking threats to biodiversity with different societies, ecosystems, conservation strategies; the global dependence upon and significance of tropical biomes compared with others; implications of economic development in tropical regions with challenges of conservation; different EVS perspectives on reasons for conservation, etc.;
• examples of tropical biomes/ecosystems; their local societies; conservation efforts; reasons/needs for conservation, etc.;
• balanced analysis evaluating the extent to which conservation efforts are of particular importance for tropical biomes along with relevant limitations and counterarguments, etc.;
• a conclusion that is consistent with, and supported by, analysis and examples given e.g. “although the loss of any species may be considered of equal significance from an ecocentric point of view, the high productivity and biodiversity that characterize tropical biomes, along with the limited facility of local societies, which are often less developed, make tropical biomes a priority for international conservation efforts”;

Marks	Level descriptor
0	The response does not reach a standard described by the descriptors below and is not relevant to the question.
1–3	The response contains:• minimal evidence of knowledge and understanding of ESS issues or concepts • fragmented knowledge statements poorly linked to the context of the question • some appropriate use of ESS terminology • no examples where required, or examples with insufficient explanation/relevance • superficial analysis that amounts to no more than a list of facts/ideas • judgments/conclusions that are vague or not supported by evidence/argument.
4–6	The response contains: • some evidence of sound knowledge and understanding of ESS issues and concepts • knowledge statements effectively linked to the context of the question • largely appropriate use of ESS terminology • some use of relevant examples where required, but with limited explanation • clear analysis that shows a degree of balance • some clear judgments/conclusions, supported by limited evidence/arguments.
7–9	The response contains: • substantial evidence of sound knowledge and understanding of ESS issues and concepts • a wide breadth of knowledge statements effectively linked with each other, and to the context of the question • consistently appropriate and precise use of ESS terminology • effective use of pertinent, well-explained examples, where required, showing some originality • thorough, well-balanced, insightful analysis • explicit judgments/conclusions that are well-supported by evidence/arguments and that include some critical reflection.