What causes climate change?
(Image: Aaron Greenwood)
- 50% of the world’s habitable land is used for agriculture (Fig. 1), 1% of land is used for urban development and infrastructure, and residents of just 100 cities account for 20% of humanity’s overall carbon footprint.
- Expansion of agriculture and forestry and chemically enhancing them to increase productivity have contributed to greenhouse gas (GHG) emissions including methane and nitrous oxide, loss of biodiversity and destruction of life-supporting ecosystem services.
- Globally, agriculture uses ~70% of global fresh-water and accounts for ~23% of GHG emissions.
- In 2016, 45.3% (12.1 million ha) of NZ’s land was used for agriculture and horticulture, producing 47.8% of GHG emissions
- Soils contain more carbon than the atmosphere and vegetation combined, but they are running out. No-tillage agricultural soils or vanishing 10 – 20 times faster, conventional-tillage agricultural soils are vanishing more than 100 times faster than new soils are forming.
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.
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 photosynthesis—known 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.
“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
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.
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
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.
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
- Our World in Data: Land Use
- Statistics New Zealand: Agricultural and horticultural land use
- 2020 IPCC: Climate Change and Land Use; summary for policymakers
- 2020 IPCC: Climate Change and Land Use; all chapters
- 2020: Sciblogs NZ :Why higher carbon dioxide levels aren’t good news, even if some plants grow faster
- 2020: Hubau et al; Asynchronous carbon sink saturation in African and Amazonian tropical forests, Nature 579, 80–87
- 2020: Shi et al; The age distribution of global soil carbon inferred from radiocarbon measurements,
- Nature Geoscience 13, 555-559
- 2020: Du et al; Global patterns of terrestrial nitrogen and phosphorus limitation, Nature Geoscience 13, 221–226
- 2019 World Economic Forum; Oceans absorb almost 1/3 of global CO2 emissions, but at what cost?
- 2019: Carbon Brief, In-depth Q&A: The IPCC’s special report on climate change and land
- 2019: National Geographic: Ocean acidification explained
- 2018: Dong et al; Effects of Elevated CO2 on Nutritional Quality of Vegetables: A Review, Frontiers in Plant Science Aug. 2018
- 2018: Moran et al; Carbon footprints of 13 000 cities, Environmental Research Letters 13/6 pp
- Website affiliated with the paper: City Carbon Footprints
- Scientific American article about this research: Here’s How Much Cities Contribute to the World’s Carbon Footprint
- 2017: DeVries et al; Recent increase in oceanic carbon uptake driven by weaker upper-ocean overturning, Nature 542, 216
- Carbon Brief article on the above: Scientists solve ocean ‘carbon sink’ puzzle
- 2016: Zhang; The Impacts of Land-Use and Land-Cover Change on Tropospheric Temperatures at Global and Regional Scales, American Meterological Sociey: Earth Interactions.
- 2014 IPCC: Chapter 8 Urban Areas in Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects.
- 2013 IPCC: Anthropogenic and natural radiative forcing. Climate Change 2013: The Physical Science Basis, T. F. Stocker et al., Eds., Cambridge University Press, 659–740.
- 2011: Pan et al; A Large and Persistent Carbon Sink in the World’s Forests, Science 333 6045, 988-993
- 2010: McCarthey et al; Climate change in cities due to global warming and urban effects, Geophyscial Research Letters 37/9