Human-produced carbon dioxide (CO2) emissions are one of the defining phenomena of our times. The main cause of global warming, these emissions also lead to other forms of climate change and ocean acidification. But the production of CO2 is not an equal business across all parts of the globe. Here, we take a closer look at the information available about carbon emissions including where they are produced, why they are produced, and how much is produced.
Human emissions vs natural emissions
CO2 is released naturally into the atmosphere all the time through sources such as the ocean carbon cycle, decomposition of organic matter, respiration and volcanic eruptions. In turn, natural sinks such as forests and the ocean remove CO2 from the atmosphere. But excess emissions produced by humans ever since the Industrial Revolution have led to increased concentrations of the gas in the atmosphere, more than existing sinks can absorb. As a result, the atmosphere has warmed and is continuing to do so.
Human emissions of CO2 are small compared with natural sources. Around 750 gigatons (one gigaton = one billion tons) of CO2 move through the carbon cycle each year naturally, while humans currently produce an additional 36 gigatons. Though small in comparison, this still has a large effect. Excess CO2 disrupts the natural cycle and cannot be absorbed in full by existing carbon sinks. When CO2 is released into the atmosphere from the burning of fossil fuels, approximately 50 per cent remains in the atmosphere, while 25 per cent is absorbed by plants and trees, and the other 25 per cent is absorbed into certain areas of the ocean.
Human activities are responsible for almost all of the increase in greenhouse gases in the atmosphere over the last 150 years.
Sources of carbon emissions
A wide range of industrial processes release CO2 into the atmosphere, the largest being generation of electricity, heat and transportation through the burning of fossil fuels such as coal, oil and gas, accounting for 87 per cent of all human-produced CO2 emissions.
The remainder comes from the clearing of forests and other land-use changes (nine per cent), as well as some industrial processes such as cement manufacturing (four per cent) and waste disposal (including incineration).
Carbon emissions and global warming
It is now well-understood that large quantities of CO2 in the atmosphere cause it to heat up. CO2 absorbs heat from the sun that would otherwise bounce back into space. Some of this energy is re-emitted back to Earth, causing additional heating of the planet.
Over the last few decades, global temperatures have risen sharply — to approximately 0.7℃ higher than the 1961-1990 baseline. When extended back to 1850, temperatures were a further 0.4℃ colder. Overall, this amounts to an average temperature rise of 1.1℃.
The 2015 Paris Climate Agreement set a target to hold the rise in global average temperature to well below two degrees Celsius and to try and limit the temperature increase even further to 1.5˚C. The world is not on track to meet this target. To halt warming to a 1.5˚C rise, carbon emissions would need to drop by around 15 per cent every year up to 2040.
Current predictions suggest that if the policies presently in place around the world are all followed, temperatures will rise by around 3.0°C above pre-industrial levels. If all unconditional pledges and targets that governments have made are followed, temperatures will rise to around 2.8°C above pre-industrial levels.
Global emissions increased from two billion tonnes of CO2 in 1900 to over 36 billion tonnes today. CO2 concentrations in the atmosphere are at their highest levels in over 800,000 years.
Levels of CO2 in the atmosphere fluctuate naturally, however from 803,719 BCE up until the Industrial Revolution, atmospheric concentrations of CO2 never exceeded 300 parts per million (ppm). We have seen a rapid rise in global CO2 concentrations over the past few centuries, and in recent decades in particular. Concentrations are now well over 400ppm.
Data from 2014 to 2017 suggested that global annual emissions of CO2 had approximately stabilised, but data from the Global Carbon Project reported a further annual increase of 2.7 per cent, and 0.6 per cent in 2018 and 2019, respectively.
Due to the coronavirus, 2020 will be a remarkable year. It is thought that the virus could trigger the largest ever annual fall in CO2 emissions, more than during any previous economic crisis or period of war. Despite this, atmospheric CO2 levels are still expected to increase again this year, even if emission cuts are greater still. Rising CO2 concentrations – and related global warming – will only stabilise once annual emissions reach net-zero.
Not all countries contribute to rising emissions
The growth of global emissions in 2019 was almost entirely due to China. The rest of the world actually reduced its emissions, thanks to falling coal use in the US and Europe, as well as much more modest increases in India and the rest of the world, compared to previous years.
China is the world’s largest emitter today: it emits nearly 10 billion tonnes of CO2 each year, more than one-quarter of global emissions.
North America is the second largest regional emitter at 18 per cent of global emissions. It’s followed closely by Europe with 17 per cent.
Africa and South America are both fairly small emitters: accounting for three to four per cent of global emissions each.
In terms of individual countries, the top five emitters are China, the US, India, Russia and Japan.
Nevertheless, it is important to remember that every country that produces carbon emissions is contributing to global warming and climate change, even if those emissions are not rising. CO2 concentrations will only stabilise once annual emissions reach net-zero.
Current emissions don’t tell the whole story. Taking into account historic emissions, the United States has emitted more CO2 than any other country to date: at around 400 billion tonnes since 1751, it is responsible for 25 per cent of historical emissions – twice as much China.
UK carbon emissions
The UK’s CO2 emissions peaked in the year 1973 and have declined by around 38 per cent since 1990, faster than any other major developed country.
UK emissions have declined from around 600 million tonnes of CO2 in 1990 to 379 million tonnes in 2018, placing it 17th in a global ranking of countries by total emissions in 2018.
According to Carbon Brief, the most significant factors include a cleaner electricity mix based on gas and renewables instead of coal, as well as falling demand for energy across homes, businesses and industry.
Other greenhouse gases
Other gases produced by human activities also have a greenhouse (warming) effect. The key offenders are methane (CH4), nitrous oxide (N2O) and to a smaller extent other gases, such as the group known as ‘F-gases’.
Both methane and N2O have a higher global warming potential than CO2 but are present in the atmosphere in much smaller quantities. Methane is, per unit, more than 20 times as potent as CO2 as a greenhouse gas. That means that over a 20-year period, the global-warming potential of one tonne of atmospheric methane is similar to that of around 85 tonnes of CO2, according to the Intergovernmental Panel on Climate Change. N2O is 300 times more potent than CO2.
Both methane and N2O remain in the atmosphere for much shorter periods that CO2, leading some scientists to suggest that targeting them could have a faster impact on global warming.
Global methane emissions have risen nearly 10 per cent over the past two decades, resulting in record-high atmospheric concentrations. In 2017, global yearly emissions of the gas reached a record 596 million tonnes, according to scientists with the Global Carbon Project, resulting in atmospheric concentrations of more than 2.5 times above pre-industrial levels. The increase is driven mainly by agriculture and the natural-gas industry.
The largest source of N2O is agriculture, stemming in particular from fertiliser-use and animal waste. N2O emissions have increased considerably during the past two decades, especially from 2009 onwards. A recent study suggested that N2O emissions increased globally to approximately 10 per cent of the global total between 2000-2005 and 2010-2015.