What causes climate change?

What causes climate change?

Menu: The causes

(Image: Aaron Greenwood)

Land use

Summary

Fig. 1 (Image: Our World in Data)

The Agricutural Revolution

The transition of many human cultures from hunting and gathering to agriculture began ~12,000 to 15,000 years ago, around the time the last glacial maximum ended. By ~11,750 years ago the global climate began stabilising enough for agriculture to spread. By 9,000 years ago agriculture was common in many places. At the same time, the Earth’s climate was slowly moving into a natural cooling phase. Greenhouse gas emissions from agriculture have been credited with offsetting this very slight cooling, thereby maintaining a relatively stable temperature until the Industrial Revolution. At that point, burning fossils fuels for energy began releasing equally huge quantities of greenhouse gasses into the atmosphere.

The Industrial Revolution also led to industrial-scale agriculture and horticulture. Enabled by science and technology, some 25% of the Earth’s natural landscapes has since been converted into monoculture crops enhanced by fertilisers and protected by pesticides and herbicides engineered to eradicate all competing species. This has resulted in relatively cheap plentiful food with little to no resiliency in the face of climate change. As the IPCC has pointed out, industrial agriculture has simultaneously destroyed the life-supporting ecosystem servicesincluding clean water and a liveable climatenecessary for the planet to remain habitable.

In effect, industrial agriculture is a giant Ponzi scheme that’s now catching up with us, and farmers are the first to feel the economic and social impacts.

Fig. 1: Mouse over anywhere in the graph to show the exact breakdown of figures by country and year. (Graph: Our World in Data)

Burning tropical rainforests for agriculture

From 2018-2017, around 30% of all human-generated CO2 emissions was being absorbed by the world’s land surface area, with tropical forests playing a major role in this ‘carbon sink’. Another 30% is absorbed by the oceans.

Recent research indicates the ability of intact tropical forests to remove CO2 from the atmosphere reached its peak in the 1990s and has since been in decline. Meanwhile, millions of hectares of tropical rainforest continue to be burned specifically to grow meat, soya, and palm oil.

The CO2 ‘fertilisation effect’ isn’t helping

The speeding-up of photosynthesisknown as ‘CO2 fertilisation’is well-known to be an important consequence of higher CO2 concentrations, along with increased water use efficiency. As CO2 in the atmosphere increases, in theory plants don’t lose so much water through their leaves because the number of stoma decreases, so drier conditions shouldn’t have such a large impact. However, fast growing plants including food crops are structurally weaker, making them more prone to higher and hotter winds (increasing evapotranspiration) of the type that commonly occur in Canterbury. Crucially, they are also decreasing in nutritional values. Additionally, orchards and forests may not reach maturity before their tolerance for increasing temperatures is exceeded.

Urban areas

“Warming and extreme heat events due to urbanisation and increased energy consumption are simulated to be as large as the impact of doubled CO2 in some regions.” – McCarthey et al

Approximately 1% of the surface of the Earth is classed as ‘urban’ , ie, cities and infrastructure including roads. The ‘heat-island’ effect of cities has been recognised since the late 1800s and well-studied since then (Fig. 3). On the whole, modern cities create vast areas of surfaces that are impermeable to rain: concrete pavement, bitumen roads, and rooftops. Waste heat from powering buildings adds to the ambient temperatures. Dark bitumen surfaces and concrete retain daytime heat. The end result is that cities  1–3°C  warmer on average—and as much as 12°C warmer in the evening—than surrounding areas.

“Residents of just 100 cities account for 20 percent of humanity’s overall carbon footprint.” – McCarthey et al

Fig. 3. The ‘heat-island’ effect of cities and urban areas.

Our carbon footprint

In terms of how much cities contribute to climate change, it’s not so much the land area or use that contributes, as the activities and consumption of the people that inhabit them. This is our ‘carbon footprint’. Urban dwellers almost exclusively depend upon food grown by industrial agricultural systems and for carbon-intensive manufacturing, buildings and infrastructure manufactured by intensive carbon-emitting processes, and linked and serviced by equally intensive carbon-emitting transport systems.

Our consumer driven society demands cheap, conveniently available food and goods, the latest tech and modern conveniences, and fast easy transport. This drives all aspects of land use including agriculture, mining, urban development and the infrastructure to support these demands. This in turn drives climate change.

Fig. 4. Emissions from household consumption only. These do not include the carbon footprint to build and maintain these cities.

Explainers

Land use :

Due to the albedo effect and the short term cooling effect of evapotranspiration, changes in land use have caused a slight decrease in the average temperature of the troposphere over these areas. This does not mean agriculture is ‘cooling’ the planet; the effect is the same as more trees in cities keeps cities cooler (Fig. 3). Soils contain more carbon than the atmosphere and vegetation combined. Losing soils means that, like forests, they are becoming sources of carbon rather than sinks  – IPCC 5th Assessment Report


The Ocean:

Like the land, this varies from ocean to ocean. The Southern Ocean absorbs about 40%, where it dissolves in the surface water. Ocean circulation distributes and sinks it into deeper waters, where it builds up. However, the ability of oceans to do this varies. Moreover, greater concentration of CO2 in the ocean is decreasing the pH, leading to a more acidic environment that’s affecting oceanic life.


Burning rainforests:

Millions of hectares of tropical rainforest is still being cleared specifically to sell meat to overseas buyers including McDonald’s and Burger King, which buy vast quantities of beef from Brazil. Along with Kentucky Fried Chicken, McDonald’s and Burger King also serve chicken fed a diet of soya from Brazil.

References and further reading