The greenhouse gas carbon dioxide blankets Earth’s atmosphere, preventing heat energy from escaping. That extra heat raises temperatures, warming the planet and disrupting nature’s usual balance.
This animation, by Carbon Brief, shows national responsibility for historical emissions. It ranks nations by their total cumulative CO2 emissions from fossil fuels and land use change since 1850, including those emitted under colonial rule.
1. The Carbon Cycle
Carbon is a vital element for all living organisms and is found in various forms throughout the natural world. The carbon cycle describes the continuous circulation of carbon between Earth’s reservoirs – the atmosphere, oceans and soil. Carbon atoms are constantly moving between these reservoirs through processes such as photosynthesis, respiration and decomposition of living organisms, and volcanic eruptions.
As greenhouse gases, carbon dioxide and other GHGs trap radiant energy from the sun within the atmosphere, warming the planet by increasing its average temperature. This is known as the greenhouse effect and it is essential to the existence of life on Earth, but it can become dangerous if concentrations of GHGs increase too much.
GHGs are emitted into the air when carbon-containing matter is combusted, such as when fossil fuels like coal, oil and gas are burned. This causes CO2 levels in the atmosphere to rise, which is then absorbed by the ocean and land, leading to a positive feedback loop that amplifies climate change.
Normally, natural processes keep atmospheric CO2 levels in balance. However, since the onset of industrial times, human activities have caused a significant rise in CO2 levels. This is because burning fossil fuels releases carbon that’s been locked away in geological formations for millions of years, transferring it from the slow to the fast (biogenic) carbon cycle. This influx has led to excessive levels of atmospheric CO2 that the biogenic carbon cycle can’t cope with, and which is driving climate change.
Figure 1 shows the long-term trend of atmospheric CO2 observed at Mauna Loa since 1958. While the trend is clearly upwards, there are also a series of wiggles within the data. These are caused by seasonal cycles in photosynthesis, which cause the levels of CO2 in the atmosphere to rise and fall each year.
Human activity is causing CO2 levels to rise at a rate faster than they have in the past 3.6 million years, and this has triggered the climate change we are currently experiencing. This figure shows the total cumulative emissions of CO2 from human activities since 1850, broken down by country and source (in billions of tonnes). The breakdown includes fossil fuels, cement, land-use change and forestry.
2. Fossil Fuels
Fossil fuels are a group of non-renewable materials that can be burned to produce energy. They include coal, crude oil, natural gas and other hydrocarbons that formed from the fossilized remains of plants and animals.
When fossil fuels are burnt they release carbon dioxide and other greenhouse gases into the atmosphere. These gases trap heat from the sun and cause the world to warm. They also disrupt the Earth’s natural climate processes. This is causing a number of problems, including ocean acidification, loss of biodiversity and severe weather events.
In the United States, coal, oil and gas provide 81 percent of our energy. They are responsible for nearly three-fourths of the CO2 emissions from human activities since the beginning of the industrial revolution. Fossil fuels are a non-renewable resource, and waiting millions of years for new deposits to form is not an option. This means that we need to make a transition to renewable energy sources and increase efficiency in our existing buildings and vehicles.
It’s important to understand who is responsible for emissions because of the global nature of our economy and the complex ways that countries trade with each other. Emissions accounts that take into account consumption give full responsibility to the people who use the products that are made with fossil fuels, and tend to reduce the total for major exporters. However, there are practical challenges to obtaining such accounts, which require detailed trade tables and have only been available for the years since 1990.
A key issue is that countries themselves are arbitrary boundaries, created by accidents of history, geography and politics. This makes it difficult to compare the historical responsibility of different nations.
One way of looking at this is to use a method called “normalisation”. This involves splitting total emissions into categories according to the population of each country. This allows the share of a country’s emissions to be compared across countries with very different populations.
Carbon Brief has normalised country shares using the historical timeseries published by CDIAC and aggregated with other useful information by Our World in Data (OWID). The chart below shows the 20 largest contributors to cumulative emissions (1850-2021) and also breaks down the total into those from fossil fuel use and land-use change and forestry (green). A further chart shows these breakdowns for each of the years since 1990.
3. Energy
The energy we use to do work — like walking, cycling, driving cars and trucks, running factories and ships, and baking bread in ovens — depends on the chemical energy of fossil fuels. But this energy isn’t free — it comes with a cost in the form of greenhouse gas emissions.
Historically, fossil fuels have been the primary source of human-caused CO2 emissions. But a growing proportion of global energy consumption now stems from renewables and nuclear power, which are less carbon-intensive than fossil fuels. The renewables and nuclear sectors also produce fewer other types of harmful greenhouse gases, such as methane, nitrous oxide and sulfur dioxide.
It’s important to understand that there is a direct link between the total amount of CO2 humans release and the planet’s warming – and that it doesn’t matter if those emissions happen now or in the past. This is because the climate system is a long-term player, with impacts that persist far beyond our own lives.
The carbon cycle has natural’sources’ and’sinks’ for CO2. Volcanoes belch CO2 into the atmosphere, but a broader system of plants, oceans, and soils absorb and store it. Over time, the carbon sinks replenish themselves, but the rapid pace of human activity has overwhelmed them.
For this reason, the global climate system now has a ‘carbon budget’ that determines how much further the world can warm before it goes too far. This budget is the total amount of CO2 humans have already released into the air. If emissions continue at current levels, the budget for a two-thirds chance of keeping temperatures below 1.5°C would be used up within 10 years – and the remaining two-thirds chance in 2050.
Countries can take steps to reduce their historical emissions by using renewable and nuclear energy sources, and reducing their transport and heating and cooling demand. They can also invest in clean energy technologies, such as carbon capture and storage, which involves capturing the CO2 from fossil fuels before they enter the atmosphere, and storing it deep underground at a suitable geologic formation.
It’s also important to recognise that a country’s ranking in the chart above – and its per-capita ranking – depends on the methodology chosen. While Carbon Brief’s approach to normalising a country’s ranking by population is fairly straightforward, other methods can lead to markedly different results and should be considered carefully.
4. Renewable Energy
The rise in atmospheric CO2 is largely due to human activities. The burning of fossil fuels such as coal and oil, and deforestation have added a significant amount of heat-trapping greenhouse gases to the atmosphere. These gases trap the Sun’s heat in the atmosphere and change the climate.
Scientists are working on ways to reduce these changes by developing renewable energy sources. One way is to turn carbon dioxide into a useful fuel substitute by using it as raw material in the production of biofuels like ethanol and biomass. Another method is to capture the CO2 produced when fossil fuels are burned and store it deep underground.
These methods have the potential to slow, halt and even reverse global warming. But the key to a successful outcome will be implementing them on a large scale. This includes replacing a substantial proportion of cars with zero-emission vehicles, and providing access to clean energy for all, including remote, island and coastal communities.
Renewables such as wind and solar photovoltaics (PV) are increasingly popular, and the technology continues to improve and get cheaper every year. They are a vital part of the solution to climate change, but they will need to be matched by increased deployment of nuclear power and other low-carbon technologies, along with better batteries to store energy.
Observed changes in the Earth’s climate system, such as rising global air and ocean temperatures, melting of snow and ice, and long-term sustained reductions in regional weather patterns, are caused by extra heat being added to the atmosphere by humans through the use of fossil fuels, land-use change, agriculture, and deforestation. These human-caused inputs increase the concentration of heat-trapping greenhouse gases in the atmosphere.
The US is responsible for the highest total cumulative emissions of CO2 since 1850, as shown in this animation from Carbon Brief, which also ranks countries by their contribution to the global total and separates them into emissions from fossil fuels and land-use change/forestry.
A recent study has suggested that adding genetically modified E coli bacteria to sugar cane fields could make the plant produce three-fourths as much ethanol per hectare, and thus emit less CO2. This would reduce the need to harvest sugar cane for ethanol and cut the rate of deforestation in tropical nations where it is harvested.
