Causes: Land use
Image: Aaron Greenwood
Fig. 1: Click ‘Change country/region’ in the upper left to select more detailed data. (Graph: 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 services—including clean water and a liveable climate—necessary for the planet to remain habitable.
Video 1: The effect of land use change across Aotearoa since the arrival of humans: Parliamentary Commissioner for the Environment.
More CO2 means plants grow faster, but…
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.
Isn’t agriculture cooling the atmosphere?
Due to the albedo effect and short term cooling from evapotranspiration, changes in land use have caused a slight decrease in the average temperature of the troposphere over some farmlands. This does not mean agriculture is ‘cooling’ the planet; the same effect of trees in cities keeps cities cooler (Fig. 3).
Agricultural lands do not store nearly as much water in the plants and crucially, their soils. And equally crucially, industrial-scale agriculture such as dairy farming in Canterbury, is leading to the rapid loss of soils. Soils contain more carbon than the atmosphere and vegetation combined. Losing soils is like losing forests. Agricultural soils are becoming sources of carbon emissions rather than carbon sinks – IPCC 5th Assessment Report.
The UN Food and Agriculture Organization analysis showed that 21% of global CO2 emissions, 53% of global methane emissions and 78% of global nitrous oxide emissions came from agri-food systems in 2019, with enteric fermentation (methane from livestock) contributing the largest portion.
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 are 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
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.
Our carbon footprint
Any pale coloured surface, for example ice, snow, and many agricultural crops including pasture grasses grown for dairy cows and grain crops such as wheat, generally have higher a albedo, that is, they reflect more solar radiation back into space than darker surfaces such as the ocean and forests. However, agricultural lands are losing far greater quantities of carbon from their soils, so any short-term localised cooling does not in any way offset the degree to which industrial scale agriculture is contributing to climate change.
