EQ3: How are the carbon and water cycles linked to the global climate system? GapFill

The demands we place on Earth for resources are  growingequalunchangingshrinking.  Population is rising, meaning that we are increasing demand for food (agricultural land), resources including timber and cement, and water supply.  We chop down forest for land and to provide these goods.

We have already chopped down over  a thirdhalfa quarter90 per cent of the world’s forest and are still chopping down trees, but trees are being planted in some areas.  Deforestation and the conversion of grassland to agriculture decrease the carbon stored as biomass and soil.

As a result, the water and carbon cycles are changing; for example, more run-off from reduced interception, decreases in rainfall due to less evapotranspiration, and increased  fluxchangesequesterraft of carbon into the atmosphere.

Between 2010 and 2015, the countries with the highest rate of deforestation were  FranceSwedenthe UKBrazil, Indonesia and Myanmar as the front runners, followed by several African countries.  Most of this has occurred in  rainforestsgrasslandssavannatundra.  Deforestation occurs for different reasons across the world.  In Brazil, most forest is cleared for cattle ranching, while in Indonesia the main causes are for agriculture (  olive oilcoconut oilpalm oilsunflower oil) and for timber plantations (e.g. for pulpwood).  Almost half of the world’s paper products are made in Asia.

 ReafforestationDeforestationAfforestationClear-cut is when trees are planted in areas where there have not been trees before.  We distinguish this from reafforestation, when trees have been chopped down and are replanted.  A study in 2018 suggested that in the past 35 years tree cover had actually  increaseddecreasedequalledstayed the same.  The majority was due to human action – afforestation in China and Africa, and the growth of trees in mountains, tundra and deserts – because climate change had allowed growth in areas that were once too cold for trees.

Conversion of grassland to  urban areasfarmlandforestwetland has increased the loss of soil carbon, increasing atmospheric CO₂.  The carbon is respired when the soil is ploughed up, as carbon is released to the air and is broken down by decomposers.

Humans might trigger a  tippingboilingtriggerfreezing point in Earth’s climate.  This is a point at which permanent damage could occur.  The point is also called a  boiling pointfluxthresholdmelting point.

Deforestation affects the carbon and water cycles in the following ways:

Water cycle

• Increased surface run-off (less interception)
• Reduced  droughtevaportanspirationphotosynthesisflooding – large-scale deforestation in the Amazon reduces convectional rainfall and affects weather patterns hundreds of miles away. Inland weather systems are dominated by convectional rainfall.
• Over time, river flow decreases and soils dry out
• Increases the risk of  weatheringdroughtforest firesphotosynthesis (and threatens water supply to large cities)

Carbon cycle

• Release of CO₂ into the atmosphere as the plant biomass is  harvestedsequesteredrestoredburned and decomposes
• Reduced soil store of carbon
• Reduced intake of atmospheric CO₂ from  photosynthesisrespirationcombustionbiomass
• Therefore, an overall decrease in carbon biomass in an increase in atmospheric CO₂