1. 1
   Air bubbles trapped in an ice core drilled at Vostok in Antarctica record the composition of the atmosphere over the past 420,000 years, and the ratio of hydrogen isotopes in the ice records past temperature. In each cold glacial period, carbon dioxide concentration fell to about 180 ppm and temperature was about 8 °C lower than today. In each warm interglacial period, carbon dioxide rose to about 280–300 ppm. Carbon dioxide never exceeded 300 ppm in the whole record. Direct measurements show that atmospheric carbon dioxide is now about 420 ppm, and methane has risen from about 700 ppb before the industrial revolution to about 1,900 ppb.

   (a)

   Which statement best describes the relationship between carbon dioxide concentration and temperature in the ice-core record?

   [1 mark]

   • AA negative correlation: temperature was highest when carbon dioxide was lowest
   • BA positive correlation: temperature and carbon dioxide rose and fell together
   • CNo correlation, because carbon dioxide stayed between 180 and 300 ppm
   • DA proven causal link, because the two variables changed at the same times

   (b)

   Which human activity is a major anthropogenic source of the extra methane in the atmosphere?

   [1 mark]

   • ARearing ruminant livestock such as cattle
   • BPlanting new deciduous woodland
   • CGenerating electricity with wind turbines
   • DGrowth of phytoplankton in the oceans

   (c)

   Explain why the ice-core data alone do not prove that carbon dioxide causes global temperatures to rise.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Emperor penguins breed on landfast ice (sea ice attached to the Antarctic coast). Eggs are laid in the southern winter, and chicks need until about December to grow waterproof adult feathers. In 2016, storms broke up the landfast ice at the Halley Bay colony, then one of the largest in the world, in October. Almost no chicks survived that year, breeding failed again in the next two years, and most adults moved elsewhere. In the Arctic, walruses rest on floating sea ice between dives to feed on clams on the shallow continental shelf. As summer sea ice has retreated north beyond the shelf, tens of thousands of walruses now come ashore on crowded beaches in late summer.

   (a)

   Why did the early break-up of the landfast ice cause the chicks at Halley Bay to die?

   [1 mark]

   • AThe adults could no longer find fish to feed them
   • BThe eggs had not yet been laid when the ice broke up
   • CThe melting ice made the sea too warm for the chicks
   • DThe chicks had not yet grown waterproof feathers and could not survive in the sea

   (b)

   Which is the most likely direct consequence for walruses of the retreat of summer sea ice beyond the continental shelf?

   [1 mark]

   • AWalruses can feed more easily because clams are found in deeper water
   • BWalruses switch to feeding on emperor penguins
   • CWalruses must rest on land far from feeding areas, so they spend more energy travelling to feed
   • DWalruses no longer need to rest between dives

   (c)

   Explain how the loss of sea ice and snow acts as a positive feedback in global warming.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Birds were surveyed on the slopes of Mount Karimui in the tropical mountains of New Guinea in 1965 and again, using the same methods along the same routes, in 2012. For the 20 bird species recorded in both surveys, the mean upper limit of their elevation range rose from 1,850 m to 2,000 m. Fifteen species moved upslope, three showed no change and two moved slightly downslope. Over the same period the mean annual temperature at the site rose by 0.5 °C. The region's forest was not cleared or logged between the two surveys.

   (a)

   Calculate the mean rate of upslope movement of the upper range limit in metres per decade, and explain why bird species move upslope as the climate warms.

   [3 marks]

   (b)

   Evaluate the conclusion that climate change caused the upslope shift in these bird species.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A region of boreal forest (taiga) in northern Canada has been monitored for 40 years. Over this time, mean summer temperature rose by 1.5 °C and mean winter snowfall fell by 25%. The area burnt per decade tripled, and fires increasingly burned into the deep organic soil layer that had built up over centuries. Satellite images show forest browning, with reduced summer leaf growth. Net carbon exchange changed from an uptake of about 40 g of carbon per square metre per year to a loss of about 20 g of carbon per square metre per year. The national government plans to offset its emissions and is choosing between planting fast-growing non-native conifers in large plantations on cleared land, and restoring drained peatlands and letting native forest regenerate naturally.

   (a)

   Using the data, explain how climate change could push this boreal forest past a tipping point.

   [6 marks]

   (b)

   Discuss the two approaches the government is considering as ways of sequestering carbon.

   [6 marks]

   Total for question 4: 12 marks

End of questions