Human interference
- 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.
Synoptic themes:
Players, futures and certainties
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.
The greenhouse effect
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:
The Earth’s climate is driven by incoming shortwave solar radiation:
- 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.
earth dealing with climate change in the past
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.
diagram of the greenhouse effect
The Anthropocene
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.
Constant levels of atmospheric CO, help to maintain stable global average temperatures
- 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.
Greenhouse gas increases raise temperatures, which in turn affect
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.
Atmosphere, plants and soils
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.
Photosynthesis and the atmosphere
- 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)
Skills focus: Interpreting maps
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.
Climate and nutrients are the main controls on NPP, which is a measure of the size of carbon sink. Highest productivity occurs:
- 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.
The rank order of rates of NPP per hectare is:
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.
Ecosystems have varied in their role as a
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.
Key concept: CO2 fertilisation
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.
Changes in the carbon storage of ecosystems
eras:
Before the nineteenth century
Nineteenth and early twentieth centuries
Mid-twentieth century
2015 onwards
Before the nineteenth century
Plants as a net sink/source of CO2: Sink
Data from IPCC, MEA and other sources:
Until the eighteenth century human disturbance was localised
Nineteenth and early twentieth centuries
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
Mid-twentieth century
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
2015 onwards
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
Soil health
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.
nutrient and carbon cycling in soils diagram
Key concept: Soil carbon balance
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.
