Causes of Global Climate Change

Question 1

Natural Cause - Solar Variations

Answer 1

• As the sun ages, the amount of energy it releases increases by about 10% for every 1 billion years
• Solar variations refer to the changes in solar activity over time. One solar variation is known as the sunspot cycle, which describes the cyclic occurrence of sunspot activity
• Sunspot activity occurs in an 11 year cycle with the number of sunspots peaking at approximately five and a half years before declining again (sunspots are huge magnetic storms that occur on the sun’s surface). This extra radiation can increase the amount of insolation reaching Earth
• The cycle has had cases of decreased sunspot activity such as between 1650 and 1715, and the reduced insolation resulted in a cooling effect contributing to Europe’s ‘Little Ice Age’

Question 2

Natural Cause - Volcanoes

Answer 2

• Large volcanic eruptions can influence the global climate due to the large number of gasses (including sulphor dioxide, dust and ash) they throw up into the atmosphere
• Initially the dust and ash can cause a short-termed cooling effect due to their reflective abilities of insolation. The process of this emitted gas reacting in the atmosphere can take many months, resulting in maximum cooling occurring up to a year after the eruption. It may take as long as seven years to disappear completely
• The 1815 eruption of Mount Tambora in Indonesia caused a death toll of 71 000 and released huge amounts of gasses into the atmosphere, causing average global temperatures to drop by 0.4-0.7 degrees
• The amount of CO2 released during eruptions is minor compared to that released by anthropogenic activities, which emits around 130x more each year. However, it still contributes to the enhanced greenhouse effect

Question 3

Anthropogenic Cause - Deforestation

Answer 3

• Agriculture accounts for over 50% of all deforestation, especially within the tropical and temperate forest biomes of southeast Asia
• Agriculture contributes to 80% of the deforestation that has taken place in the tropical rainforest biome
• The rate of deforestation is estimated at 10 million hectares per year between 2015 and 2020
• Native forests are carbon reservoirs (sinks), sequestering and storing large amounts of carbon (one tree can store around one tonne of carbon in its lifetime). Trees sequester CO2 from the atmosphere through photosynthesis
• Deforestation is considered a dual cause of climate change; it releases carbon dioxide into the atmosphere (as plants are burnt), while also reducing the ability of ecosystems to remove it from the atmosphere (due to there being less trees)
• Global loss of tropical forests between 2015-17 averaged almost 5 billion tones of CO2 emitted each year, which equates to 8-10% of annual human emissions
• ‘Slash and burn’ clearing, a popular method is less economically developed countries, involves the burning of vegetation. This biomass burning releases CO2 into the atmosphere, contributing to the enhanced greenhouse effect
• Deforestation can also lead to climate change by causing a change to evapotranspiration levels and albedo - In forested areas, vegetation absorbs more heat than bare soil due to differences in albedo, meaning that forested areas will have higher levels of evapotranspiration of soil and plant moisture, leading to higher levels of rainfall (approximately 50% of the rainfall in a tropical forest is attributed to evapotranspiration). When forests are cleared the reflectivity of the ground increases, meaning less heat is absorbed and therefore less moisture is taken up into the atmosphere. The removal of forests for large-scale commodity farming is estimated to reduce evapotranspiration by up to 27%, and also reducing long-term rainfall at a local and regional scale. For example, in the Australian south-west region where extensive deforestation has occurred since the 1970s, a reduction of rainfall of almost 20% has been seen.

Question 4

Agriculture

Answer 4

• Agricultural activities are estimated to produce around 20% of anthropogenic global greenhouse gas emissions (primarily methane and nitrous oxide, mainly from wet rice cultivation and rearing livestock
• Rice cultivation has the highest yield when fields are flooded, however by cutting off oxygen supply to the soil it causes anerobic decomposition of organic matter (whereby it releases methane)
• The world’s rice production areas have increased by 41% during the last 40 years, with 507 million tonnes produced from 2020-2021
• Rice cultivation contributes 50 to 100 million tonnes of methane per annum (a greenhouse gas 25 times more potent than CO2), and represents 10% of total methane emissions
• Livestock is the largest contributor to agricultural greenhouse gasses, creating nearly 29% of global methane emissions, 9% of CO2 emissions, and 65% of nitrous oxide emissions.
• In Australia, direct livestock emissions account for almost 70% of all greenhouse gas emissions from the agricultural sector and 13% of total national greenhouse gas emissions