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Question 1

Human interference

Answer 1

• A balanced carbon cycle is important in sustaining other systems.
• It plays a key role in regulating the Earth’s global temperature and climate by controlling the amount of COz in the atmosphere, which then affects the hydrological cycle.
• Ecosystem development and agriculture depend on the carbon cycle.
• Carbon stores and fluxes involve natural processes that have helped regulate the carbon cycle and atmospheric CO, levels for millions of years.
• However, the system is being increasingly altered by anthropogenic actions.

Question 2

Synoptic themes:
Players, futures and certainties

Answer 2

Humans have not created more carbon on Earth, but have depleted or enhanced some stores, and speeded up some fluxes. Atmospheric carbon has become a major focus for decision makers because of the role of COz and CH4 as greenhouse gases. Human interference has consequences for the future climate, ecosystems and food supply.

Question 3

The greenhouse effect

Answer 3

the Earth has a natural temperature-control system that relies on greenhouse gases. The concentration of atmospheric carbon (carbon dioxide and methane) strongly influences the natural greenhouse effect (Figure 4.7 and Table 4.6).
The Earth’s climate is driven by incoming shortwave solar radiation:

Question 4

The Earth’s climate is driven by incoming shortwave solar radiation:

Answer 4

• approximately 31 per cent is reflected by clouds, aerosols and gases in the atmosphere and by the land surface
• the remaining 69 per cent is absorbed; almost 50 per cent is absorbed at the Earth’s surface, especially by oceans
• 69 per cent of this surface absorption is re-radiated to space as longwave radiation
• however, a large proportion of this longwave radiation emitted by the surface is re-radiated back to the surface by clouds and greenhouse gases (Figure 4.7); this ‘trapping’ of longwave radiation in the atmosphere is what gives a life-supporting average of 15°C, the ‘natural greenhouse effect.

Question 5

earth dealing with climate change in the past

Answer 5

In the Earth’s past, the carbon cycle has responded to natural climate change driven by variations in the Earth’s orbit affecting solar energy. In the Pleistocene era, the northern hemisphere summers cooled and the last Ice Age slowed down the carbon cycle. Increased phytoplankton growth increased the amount of carbon that the ocean took out of the atmosphere.
As an example of positive feedback, the drop in atmospheric carbon then caused additional cooling.
At the end of the last Ice Age, temperatures rose as did atmospheric COz.

Question 6

diagram of the greenhouse effect

Answer 6

Question 7

The Anthropocene

Answer 7

The current geological era, the Holocene, is often called the Anthropocene because of the profound changes to the Earth caused by humans. The natural greenhouse effect has become enhanced; CO, has increased in volume by 40 per cent in the last 300 years.

Question 8

Constant levels of atmospheric CO, help to maintain stable global average temperatures

Answer 8

• Fast carbon cycling is thought to have been relatively balanced before the Industrial Revolution, which started in the eighteenth century. It functioned in a
  ‘steady state system’.
• The slow carbon cycle, volcanism and sedimentation, have been fairly constant over the last few centuries, although erosion and river fluxes have been modified by changes in land use.
• Natural exchange fluxes between the slow and fast domains of the carbon cycle were relatively small, at under 0.3 PgC yr 1. Evidence from ice cores shows relatively small variations of atmospheric CO, until the late nineteenth century, despite small emissions over the last millennia from land-use changes caused by human activity.

Question 9

Greenhouse gas increases raise temperatures, which in turn affect

Answer 9

precipitation patterns. The temperature at any place depends on the input of solar radiation.
Average figures may hide important seasonal differences and also changes over longer climatic periods. Maps and graphs showing anomalies from the average may help.

Question 10

Atmosphere, plants and soils

Answer 10

The carbon cycle relies on ocean and terrestrial photosynthesis.
This section focuses on the role of
photosynthesis in regulating the composition of the atmosphere, and how soil health and ecosystem productivity is influenced by stored carbon.

Question 11

Photosynthesis and the atmosphere

Answer 11

• Photosynthetic organisms play an essential role in helping to keep CO, levels relatively constant, thereby helping to regulate Earth’s average temperature.
• There are distinct spatial patterns in plant productivity and carbon density (carbon storage)

Question 12

Skills focus: Interpreting maps

Answer 12

The specification requires you to practise the geographical skills of analysing maps. Use Figures 4.8 and 4.9, showing global temperature and average precipitation distribution between 1960 and 1990, to practise your skills. Use the acronym PEA:
* Pattern: describe the big patterns before any details.
* Evidence: refer to specific geographical areas and place
* Analysis: suggest a range of reasons.
Focus on physical factors only: solar input, albedo, latitude, continentality, role of ocean currents and altitude.

Question 13

Climate and nutrients are the main controls on NPP, which is a measure of the size of carbon sink. Highest productivity occurs:

Answer 13

• On land: in areas that are warm and wet. The amount of water available limits primary production; for example, deserts and dry shrub lands have little biomass above ground, although their huge extent nonetheless means a significant store. Forests store the largest amount of carbon collectively. Tundra has the least spatial extent but has the highest density of carbon storage in its permafrost.
• In the oceans: in shallower water, allowing higher photosynthesis, and in places receiving high nutrient inputs.

Question 14

The rank order of rates of NPP per hectare is:

Answer 14

estuaries, swamps and marshes, tropical rainforests, and temperate rainforests. However, when NPP is multiplied by ecosystem extent, the rank order changes to: open oceans, tropical rainforests, savannahs, and tropical seasonal forests.

Question 15

Ecosystems have varied in their role as a

Answer 15

sink or source of carbon, as summarised in Table 4.8 (page 90).
Regrowth of forests from past land clearance, discussed in Chapter 6, can increase the carbon sink, but the result of anthropogenic activity on the land globally has increased net carbon fluxes to the atmosphere.

Question 16

Key concept: CO2 fertilisation

Answer 16

Anthropogenic rises in CO, should speed up the rate of photosynthesis, and hence NPP, by 63 per cent by 2100.
However, plant growth is limited by nutrient availability (nitrogen and phosphorus), needed in order to utilise CO2. As a result, the IPCC estimates extra growth rates of only 20 per cent in tropical rainforests, savannahs, boreal forests and tundra.

Question 17

Changes in the carbon storage of ecosystems
eras:

Answer 17

Before the nineteenth century
Nineteenth and early twentieth centuries
Mid-twentieth century
2015 onwards

Question 18

Before the nineteenth century

Answer 18

Plants as a net sink/source of CO2: Sink

Data from IPCC, MEA and other sources:
Until the eighteenth century human disturbance was localised

Question 19

Nineteenth and early twentieth centuries

Answer 19

Plants as a net sink/source of CO2:
Source

Data from IPCC, MEA and other sources:
Globalising scale of degradation and destruction of ecosystems: deforestation, desertification, soil erosion, resource extraction and urbanisation

Question 20

Mid-twentieth century

Answer 20

Plants as a net sink/source of CO2
Sink

Data from IPCC, MEA and other sources:
Despite carbon loss from land-use changes, net carbon sequestration because of afforestation and reforestation in North America, Europe and China, as well as improved agricultural practices

Question 21

2015 onwards

Answer 21

Plants as a net sink/source of CO2:
An increasing source?

Data from IPCC, MEA and other sources
Warmer temperatures trigger faster decomposition and recycling of carbon in dead plants and soils, fluxing more carbon back into the atmosphere

Question 22

Soil health

Answer 22

Soil health depends on the amount of organic carbon stored in the soil. This depends on its inputs (plant and animal residues and nutrients) and outputs (decomposition, erosion and use in plant and animal productivity). Figure 4.10 illustrates the stores and fluxes in soil nutrient cycling, while Figure 4.11 shows how these depend on the climate.
Carbon is the main component of soil organic matter and helps give soil its water-retention capacity, its structure and its fertility. This is in contrast to ‘active’ soil carbon found in topsoil. Organic carbon is concentrated in the surface soil layer as easily eroded small particles, so soil erosion is a major threat to carbon storage and soil health.

Question 23

nutrient and carbon cycling in soils diagram

Answer 23

Question 24

Key concept: Soil carbon balance

Answer 24

If plant residue is added to the soil at a faster rate than soil organisms convert it to CO2, carbon will gradually be removed from the atmosphere and sequestered in the soil.

Question 25

Fossil fuel combustion

Answer 25

Fossil fuels have been burnt at increasing rates since the start of the Industrial Revolution. They continue to be the primary energy source driving modern civilization.
Without human interference, the carbon in fossil fuels would flux very slowly into the atmosphere through volcanic activity. Fossil fuel combustion shifts this flux from slow to fast carbon cycling.
About half of the extra emissions of CO2 since 1750 have remained in the atmosphere. The rest has been fuxed from the atmosphere into the stores of oceans, ecosystems and soils shown in Figure 4.1 (page 79).
IPCC modelling suggests:

Question 26

IPCC modelling suggests:

Answer 26

• increased fluxes to the biological store
• increased soil storage in high latitudes, only limiter by nitrogen availability
• loss of storage in unfreezing permafrost and in the Southern Ocean and North Atlantic because of warming.

This already changed balance of carbon pathways and stores has varying implications for the climate, ecosystems and hydrological cycle.

Question 27

Key concept: Climate forcing

Answer 27

Climate forcing means the causes, or drivers, of climate change.
Currently the most important driver is fossil fuel combustion.

Question 28

Key concept: Climate forcing

Answer 28

The impacts of fossil fuel combustion on climate are at global and regional scales. The 2014 IPCC report, aimed at policy makers, explicitly linked greenhouse gas concentrations to fossil fuel emissions, rising global temperatures and sea levels, as shown in Table 4.9
Global warming and the alteration of ocean temperatures and salinity levels could affect the thermohaline current by slowing or reversing the North Atlantic Drift (NAD), also called the Gulf Stream. This happened 20,000 years ago when temperatures dropped.
The NAD keeps UK temperatures 5°C higher than they would otherwise be in winter.

Question 29

Climate predictions
Global

Answer 29

On average, the Earth will become warmer, hence more evaporation and precipitation
Sudden shifts in weather patterns
More extreme, intense and frequent events: floods, droughts
Rising mean sea level

Question 30

Climate predictions
Regional

Answer 30

Some regions will become warmer and drier, others wetter
Some regions will have less snow, more rain
Storm surges may increase

Question 31

Key concept: Positive feedback

Answer 31

Global warming creates ice melt, and permafrost thawing releases trapped methane. Drying forests and warming oceans emit CO2. Increased greenhouse gases mean increased warming.

Question 32

Implications for ecosystems

Answer 32

• Ecosystems are valued for the services they provide for the planet as well as for humans, helping regulate carbon and hydrological cycles.
• By the end of the century, global warming and its impacts may be the dominant direct driver of changes in these services and in biodiversity.
• Already at risk are species with low population numbers, limited climatic ranges and restricted or patchy habitats.
• There is increasing evidence of changes in the distribution and geographical
  ranges of species, their population size and timings of reproduction and migration.
• Marine organisms are threatened with progressively lower oxygen levels and high rates and magnitudes of ocean acidification, as well as rising temperatures, which may alter the foundation of the food chain: plankton growth.
• Impacts on coastal ecosystems and low-lying areas at risk from sea level rise will continue for centuries, even if the global mean temperature stabilises.
• Although more species will be negatively affected by climate change, there may be some that benefit.
• Cool, moist regions such as the UK could provide habitats for additional species, while in hotter, arid regions species diversity may decline.
• The two biomes most at risk immediately are Arctic and coral ecosystems, as shown in Figure 4.12.

Question 33

Implications for the hydrological cycle

Answer 33

Figure 4.13 shows how the hydrological cycle’s flows and stores are vulnerable to global warming, which may:
* increase evaporation rates and hence trigger more moisture circulating throughout the cycle, rather than storage in oceans, and intense precipitation events

• change precipitation type, as in the northern hemisphere where spring snow cover has decreased in extent; earlier springs mean earlier peaks in snowmelt and resulting river flows and flows into oceanic stores
• increase surface permafrost temperatures, a trend recorded since the early 1980s
• reduce sea ice, ice cap and glacier storage, as in the Arctic already
• change the capacity of terrestrial ecosystems to sequester carbon and store water; an example of the importance of water storage is in the Amazon, where 60 per cent of precipitation originates from evapotranspiration by upwind ecosystems.
  Lastly, the complex El Niño-Southern Oscillation (ENSO) is an important factor in the Earth’s climate system and affects the hydrological cycle. Droughts and floods driven by ENSO may be more intense and increase in frequency because of a warming atmosphere and ocean surface.