Greenhouse gases create the ‘greenhouse effect’ which warms the earth’s climate. These gases – carbon dioxide (CO2), nitrous oxide, methane, and others – are important in sustaining a habitable temperature for the planet: if there were absolutely no greenhouse gases (GHGs), the average surface temperature of the Earth would be about -18 degrees celsius.1
Since the Industrial Revolution, however, energy-production, that largely relied on fossil fuels, has led to a rapid increase in greenhouse gas emissions, which leads to global warming. The changing climate has a range of potential ecological, physical and health impacts, including extreme weather events (such as floods, droughts, storms, and heatwaves); sea-level rise; altered crop growth; and disrupted water systems. The most extensive source of analysis on the potential impacts of climatic change can be found in the 5th Intergovernmental Panel on Climate Change (IPCC) report; this presents full coverage of all impacts in its chapter on Impacts, Adaptation, and Vulnerability.2
In light of this evidence, UN member parties have set a target, in the Paris Agreement, of limiting average warming to 2 degrees celsius above pre-industrial temperatures.
This entry provides a historical perspective of how CO2 emissions have evolved, how emissions are distributed, and the key factors that both drive these trends and hold the key to mitigating climate change.
To set the scene, let’s look at how the planet has warmed. In the chart we see the global average temperature relative to the average of the period between 1961 and 1990.
The red line represents the average annual temperature trend through time, with upper and lower confidence intervals shown in light grey.
We see that over the last few decades, global temperatures have risen sharply — to approximately 0.8 degrees celsius higher than our 1961-1990 baseline. When extended back to 1850, we see that temperatures then were a further 0.4 degrees colder than they were in our 1961-1990 baseline. Overall, if we look at the total temperature increase since pre-industrial times, this amounts to approximately 1.2 degrees celsius.
In this chart you can also view these changes by hemisphere (North and South), as well as the tropics (defined as 30 degrees above and below the equator). This shows us that the temperature increase in the North Hemisphere is higher, at closer to 1.4 degrees celsius since 1850, and less in the Southern Hemisphere (closer to 0.8 degrees celsius). Evidence suggests that this distribution is strongly related to ocean circulation patterns (notably the North Atlantic Oscillation) which has resulted in greater warming in the northern hemisphere.3
The visualisation below presents the long-run perspective on global CO2 emissions. Global emissions increased from 2 billion tonnes of carbon dioxide in 1900 to over 36 billion tonnes 115 years later.
What do our most recent trends in emissions and concentrations look like? Are we making any progress in reduction?
Whilst data from 2014 to 2017 suggested global annual emissions of CO2 had approximately stabilized, the most recent (preliminary) data from the Global Carbon Project reported a 2.7 percent increase in 2018.
Where in the world does the average person emit the most carbon dioxide (CO2) each year?
We can calculate the contribution of the average citizen of each country by dividing its total emissions by its population. This gives us CO2 emissions per capita. In the visualization below we see the differences in per capita emissions across the world.
Here we look at production-based emissions – that is, emissions produced within a country’s boundaries without accounting for how goods are traded across the world. In our post on consumption-based emissions we look at how these figures change when we account for trade. Production figures matter – these are the numbers that are taken into account for climate targets4 – and thanks to historical reconstructions they are available for the entire world since the mid 18th century.
There are very large inequalities in per capita emissions across the world.
The world’s largest per capita CO2 emitters are the major oil producing countries; this is particularly true for those with relatively low population size. Most are in the Middle East: In 2017 Qatar had the highest emissions at 49 tonnes (t) per person, followed by Trinidad and Tobago (30t); Kuwait (25t); United Arab Emirates (25t); Brunei (24t); Bahrain (23t) and Saudi Arabia (19t).
Many of the major oil producers have relatively a relatively low population size meaning their total annual emissions are low, however. More populous countries with some of the highest per capita emissions – and therefore high total emissions – are the United States, Australia, and Canada. Australia has an average per capita footprint of 17 tonnes, followed by the US at 16.2 tonnes, and Canada at 15.6 tonnes.
This is more than 3 times higher than the global average, which in 2017 was 4.8 tonnes per person.
Since there is such a strong relationship between income and per capita CO2 emissions, we’d expect this to be the case: that countries with high standards of living would have a high carbon footprint. But what becomes clear is that there can be large differences in per capita emissions, even between countries with similar standards of living. Many countries across Europe, for example, have much lower emissions than the US, Canada or Australia.
In fact, some European countries have emissions not far from the global average: In 2017 emissions in Portugal are 5.3 tonnes; 5.5t in France; and 5.8t per person in the UK. This is also much lower than some of their neighbours with similar standards of living, such as Germany, the Netherlands, or Belgium. The choice of energy sources plays a key role here: in the UK, Portugal and France, a much higher share of electricity is produced from nuclear and renewable sources – you can explore this electricity mix by country here. This means a much lower share of electricity is produced from fossil fuels: in 2015, only 6% of France’s electricity came from fossil fuels, compared to 55% in Germany.
Prosperity is a primary driver of CO2 emissions, but clearly policy and technological choices make a difference.
Many countries in the world still have very low per capita CO2 emissions. In many of the poorest countries in Sub-Saharan Africa – such as Chad, Niger and the Central African Republic – the average footprint is around 0.1 tonnes per year. That’s more than 160 times lower than the USA, Australia and Canada. In just 2.3 days the average American or Australian emits as much as the average Malian or Nigerien in a year.
Who emits the most CO2 each year? In the treemap visualization below we show annual CO2 emissions by country, and aggregated by region. Treemaps are used to compare entities (such as countries or regions) in relation to others, and relative to the total. Here each inner rectangle represents a country – which are then nested and colored by region. The size of each rectangle corresponds to its annual CO2 emissions in 2017. Combined, all rectangles represent the global total.
The emissions shown here relate to the country where CO2 is produced (i.e.production-based CO2) , not to where the goods and services that generate emissions are finally consumed. We look at the difference in each country’s production vs. consumption (trade-adjusted) emissions here.
Asia is by far the largest emitter, accounting for 53% of global emissions. As it is home to 60% of the world’s population this means that per capita emissions in Asia are slightly lower than the world average, however.
China is, by a significant margin, Asia’s and the world’s largest emitter: it emits nearly 10 billion tonnes each year, more than one-quarter of global emissions.
North America – dominated by the USA – is the second largest regional emitter at 18% of global emissions. It’s followed closely by Europe with 17%. Here we have grouped the 28 countries of the European Union together, since they typically negotiate and set targets as a collective body. You can see the data for individual EU countries in the interactive maps which follow.
Africa and South America are both fairly small emitters: accounting for 3-4% of global emissions each. Both have emissions almost equal in size to international aviation and shipping. Both aviation and shipping are not included in national or regional emissions. This is because of disagreement over how emissions which cross country borders should be allocated: do they belong to the country of departure, or country of origin? How are connecting flights accounted for? The tensions in reaching international aviation and shipping deals are discussed in detail at the Carbon Brief here.
The same data is also explorable by country and over time in the interactive map below.
By clicking on any country you can see how its annual emissions have changed, and compare it with other countries.
In the interactive chart below you can explore each country’s share of global emissions. Using the timeline at the bottom of the map, you can see how the global distribution has changed since 1751. By clicking on any country you can see its evolution and compare it with others.
If you’re interested in which countries emit more or less than their ‘fair share’ based on their share of global population, you can explore this here.
The distribution of emissions has changed significantly over time. The UK was – until 1888, when it was overtaken by the US – the world’s largest emitter. This was because the UK was the first country to industrialize, a transition which later contributed to in massive improvements in living standards for much of its population.
Whilst rising CO2 emissions have clear negative environmental consequences, it is also true that they have historically been a by-product of positive improvements in human living conditions. But, it’s also true that reducing CO2 emissions is important to protect the living conditions of future generations. This perspective – that we must consider both the environmental and human welfare implications of emissions – is important if we are to build a future that is both sustainable and provides high standards of living for everyone.
Rising emissions and living standards in North America and Oceania followed soon after developments in the UK.
Many of the world’s largest emitters today are in Asia. However, Asia’s rapid rise in emissions has only occurred in very recent decades. This too has been a by-product of massive improvements in living standards: since 1950 life expectancy in Asia has increased from 41 to 74 years; it has seen a dramatic fall in extreme poverty; and for the first time most of its population received formal education.
Whilst all countries must work collectively, action from the very top emitters will be essential. China, the USA and the 28 countries of the EU account for more than half of global emissions. Without commitment from these largest emitters, the world will not come close to meeting its global targets.
Since 1751 the world has emitted over 1.5 trillion tonnes of CO2.5 To reach our climate goal of limiting average temperature rise to 2°C, the world needs to urgently reduce emissions. One common argument is that those countries which have added most to the CO2 in our atmosphere – contributing most to the problem today – should take on the greatest responsibility in tackling it.
We can compare each country’s total contribution to global emissions by looking at cumulative CO2. We can calculate cumulative emissions by adding up each country’s annual CO2 emissions over time. We did this calculation for each country and region over the period from 1751 through to 2017.6
The distribution of cumulative emissions around the world is shown in the treemap below. Treemaps are used to compare entities (such as countries or regions) in relation to others, and relative to the total. Here countries are presented as rectangles and colored by region. The size of each rectangle corresponds to the sum of CO2 emissions from a country between 1751 and 2017. Combined, all rectangles represent the global total.
There are some key points we can learn from this perspective:
- 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% of historical emissions;
- this is twice more than China – the world’s second largest national contributor;
- the 28 countries of the European Union (EU-28) – which are grouped together here as they typically negotiate and set targets on a collaborative basis – is also a large historical contributor at 22%;
- many of the large annual emitters today – such as India and Brazil – are not large contributors in a historical context;
- Africa’s regional contribution – relative to its population size – has been very small. This is the result of very low per capita emissions – both historically and currently.
All of this data is also explorable by country and over time in the interactive map below. By clicking on any country you can see the country’s cumulative emissions over time, and compare it with other countries.
In the visualizations above we focused on each country or region’s total cumulative emissions (1) in absolute terms; and (2) at a single point in time: as of 2017.
In the chart below we see the change in the share of global cumulative emissions by region over time – from 1751 through to 2017.
Up until 1950, more than half of historical CO2 emissions were emitted by Europe. The vast majority of European emissions back then were emitted by the United Kingdom; as the data shows, until 1882 more than half of the world’s cumulative emissions came from the UK alone.
Over the century which followed, industrialization in the USA rapidly increased its contribution.
It’s only over the past 50 years that growth in South America, Asia and Africa have increased these regions’ share of total contribution.
In the final visualization you can explore the same cumulative CO2 emissions as you have seen above but now visualizes by country. Using the timeline at the bottom of the chart you can see how contribution across the world has evolved since 1751. By clicking on a country you can see an individual country’s cumulative contribution over time.
The map for 2017 shows the large inequalities of contribution across the world that the first treemap visualization has shown. The USA has emitted most to date: more than a quarter of all historical CO2: twice that of China which is the second largest contributor. In contrast, most countries across Africa have been responsible for less than 0.01% of all emissions over the last 266 years.
What becomes clear when we look at emissions across the world today is that the countries with the highest emissions over history are not always the biggest emitters today. The UK, for example, was responsible for only 1% of global emissions in 2017. Reductions here will have a relatively small impact on emissions at the global level – or at least fall far short of the scale of change we need. This creates tension with the argument that the largest contributors in the past should be those doing most to reduce emissions today. This is because a large fraction of CO2 remains in the atmosphere for hundreds of years once emitted.7
This inequality is one of the main reasons which makes international agreement on who should take action so challenging.
CO2 emissions are typically measured on the basis of ‘production’. This accounting method – which is sometimes referred to as ‘territorial’ emissions – is used when countries report their emissions, and set targets domestically and internationally.8
In addition to the commonly reported production-based emissions statisticians also calculate ‘consumption-based’ emissions. These emissions are adjusted for trade. To calculate consumption-based emissions we need to track which goods are traded across the world, and whenever a good was imported we need to include all CO2 emissions that were emitted in the production of that good, and vice versa to subtract all CO2 emissions that were emitted in the production of goods that were exported.
Consumption-based emissions reflect the consumption and lifestyle choices of a country’s citizens.
In the interactive map below we see the emissions of traded goods. To give a perspective on the importance of trade these emissions are put in relation to the country’s domestic, production-based emissions.9
- Countries shown in red are net importers of emissions – they import more CO2 embedded in goods than they export.
For example, the USA has a value of 7.7% meaning its net import of CO2 is equivalent to 7.7% of its domestic emissions. This means emissions calculated on the basis of ‘consumption’ are 7.7% higher than their emissions based on production.
- Countries shown in blue are net exporters of emissions – they export more CO2 embedded in goods than they import.
For example, China’s value of -14% means its net export of CO2 is equivalent to 14% of its domestic emissions. The consumption-based emissions of China are 14% lower than their production-based emissions.
We see quite a regional East-West split in net exporters and importers: most of Western Europe, the Americas, and many African countries are net importers of emissions whilst most of Eastern Europe and Asia are net exporters.
You can find these figures in absolute terms (tonnes of CO2) for each country in the Additional Information section.
How did the differences between a country’s production and consumption-based emissions change over time?
In the interactive charts below you can compare production- and consumption-based emissions for many countries and world regions since the first data is available in 1990.10 One chart shows total annual emissions, the other one shows the same on a per capita basis. Using the ‘change country’ toggle of the chart you can switch between them.
Individual maps of consumption-based annual and per capita emissions can also be found in the Additional Information which follows this post.
We see that the consumption-based emissions of the US are higher than production: In 2016 the two values were 5.7 billion versus 5.3 billion tonnes – a difference of 8%. This tells us that more CO2 is emitted in the production of the goods that Americans import than in those products Americans export.
The opposite is true for China: its consumption-based emissions are 14% lower than its production-based emissions. On a per capita basis, the respective measures are 6.9 and 6.2 tonnes per person in 2016. A difference, but smaller than what many expect.
Whilst China is a large CO2 emissions exporter, it is no longer a large emitter because it produces goods for the rest of the world. This was the case in the past, but today, even adjusted for trade, China now has a per capita footprint higher than the global average (which is 4.8 tonnes per capita in 2017). In the Additional Information you find an interactive map of how consumption-based emissions per capita vary across the world.
The comparisons below provide the answer to the question whether countries have only achieved emissions reductions by offshoring emissions intensive production to other countries. If only production-based emissions were falling whilst consumption-based emissions were rising, this would suggest it was ‘offshoring’ emissions elsewhere.
There are some countries where this is the case. Examples where production-based emissions have stagnated whilst consumption-based CO2 steadily increased include Ireland in the early 2000s; Norway in the late 1990s and early 2000s; and Switzerland since 1990.
On the other hand there are several very rich countries where both production- and consumption-based emissions have declined. This has been true, among others, for the UK (chart), France (chart), Germany (chart), and the USA (chart). These countries have achieved some genuine reductions without outsourcing the emissions to other countries. Emissions are still too high in all of these countries, but it shows that genuine reductions are possible.
In most countries emissions increased when countries become richer, but this is also not necessarily the case: by comparing the change in consumption-based emissions and economic growth we see that many countries have become much richer while achieving a reduction of emissions.
In the interactive map below you see each country’s net imports or exports of CO2 each year, as measured in tonnes of CO2.
Countries which are net importers are shown in red (and given as positive values), with net exporters shown in blue (given as negative values).
In the interactive map below we see how consumption-based CO2 emissions vary across the world.11
In the visualization below we show how consumption-based emissions corrected for population size – emissions per capita – varies across the world.
Carbon dioxide emissions associated with energy and industrial production can come from a range of fuel types. The contribution of each of these sources has changed significantly through time, and still shows large differences by region. In the chart below we see the absolute and relative contribution of CO2 emissions by source, differentiated between gas, liquid (i.e. oil), solid (coal and biomass), flaring, and cement production.
At a global level we see that early industrialisation was dominated by the use of solid fuel—this is best observed by switching to the ‘relative’ view in the chart. Coal-fired power at an industrial-scale was the first to emerge in Europe and North America during the 1700s. It wasn’t until the late 1800s that we begin to see a growth in emissions from oil and gas production. Another century passed before emissions from flaring and cement production began. In the present day, solid and liquid fuel dominate, although contributions from gas production are also notable. Cement and flaring at the global level remain comparably small.
You can also view these trends across global regions in the chart below, by clicking on ‘change region’. The trends vary significantly by region. Overall patterns across Europe and North America are similar: early industrialisation began through solid fuel consumption, however, through time this energy mix has diversified. Today, CO2 emissions are spread fairly equally between coal, oil and gas. In contrast, Latin America and the Caribbean’s emissions have historically been and remain a product of liquid fuel—even in the early stages of development coal consumption was small.12
Asia’s energy remains dominant in solid fuel consumption, and has notably higher cement contributions relative to other regions.13
Africa also has more notable emissions from cement and flaring; however, its key sources of emissions are a diverse mix between solid, liquid and gas.
Which is the largest contributor to carbon dioxide emissions — transport or electricity, residential or manufacturing? In the chart below we see the share of CO2 emissions from fuel combustion derived from each sector.
Globally around half of global emissions were the result of electricity and heat production in 2014. Transport and manufacturing industries both contributed approximately 20 percent; residential, commercial and public services around 9 percent and other sectors contributing 1 to 2 percent.
The large growth in global CO2 emissions has had a significant impact on the concentrations of CO2 in Earth’s atmosphere. If we look at atmospheric concentrations over the past 2000 years (see the Data Quality and Measurement section in this entry for explanation on how we estimate historical emissions), we see that levels were fairly stable at 270-285 parts per million (ppm) until the 18th century. Since the Industrial Revolution, global CO2 concentrations have been increasing rapidly.
However, CO2 is not the only GHG we’re concerned about—emissions of nitrous oxide (N2O) and methane (CH4) have also been increasing rapidly through agricultural, energy, and industrial sources. Like CO2, the atmospheric concentration of both of these gases has also been rising rapidly.
Has a global stabilization of CO2 emissions over the last few years had an impact on global atmospheric concentrations? While it appears progress is being made on global emissions, atmospheric concentrations continue to rise, as shown below. Atmospheric concentrations have now broken the 400ppm threshold—considered its highest level in the last three million years. To begin to stabilise—or even reduce—atmospheric CO2 concentrations, our emissions need to not only stabilise but also decrease significantly.
Why would a stabilization in CO2 emissions not directly translate into the same for atmospheric concentrations? This is because CO2 accumulates in the atmosphere based on what we call a ‘residence time’. Residence time is the time required for emitted CO2 to be removed from the atmosphere through natural processes in Earth’s carbon cycle. The length of this time can vary—some CO2 is removed in less than 5 years through fast cycling processes, meanwhile other processes, such as absorption through land vegetation, soils and cycling into the deep ocean can take hundreds to thousands of years. If we stopped emitting CO2 today, it would take several hundred years before the majority of human emissions were removed from the atmosphere.14
Carbon dioxide is not the only greenhouse gas of concern for global warming and climatic change. There are a range of greenhouse gases, which include methane, nitrous oxide, and a range of smaller concentration trace gases such as the so-called group of ‘F-gases’.
Greenhouse gases vary in their relative contributions to global warming; i.e. one tonne of methane does not have the same impact on warming as one tonne of carbon dioxide. We define these differences using a metric called ‘Global Warming Potential’ (GWP). GWP can be defined on a range of time-periods, however the most commonly used (and that adopted by the IPCC) is the 100-year timescale (GWP100).15
In the chart below we see the GWP100 value of key greenhouse gases relative to carbon dioxide. The GWP100 metric measures the relative warming impact one molecule or unit mass of a greenhouse gas relative to carbon dioxide over a 100-year timescale. For example, one tonne of methane would have 28 times the warming impact of tonne of carbon dioxide over a 100-year period. GWP100 values are used to combine greenhouse gases into a single metric of emissions called carbon dioxide equivalents (CO2e). CO2e is derived by multiplying the mass of emissions of a specific greenhouse gas by its equivalent GWP100 factor. The sum of all gases in their CO2e form provide a measure of total greenhouse gas emissions.
In the chart below, we see the contribution of different gases to total greenhouse gas emissions. These are measured based on their carbon-dioxide equivalent values (as explained in the section above). Overall we see that carbon dioxide accounts for around three-quarters of total greenhouse gas emissions. However, both methane and nitrous oxide are also important sources, accounting for around 17 and 7 percent of emissions, respectively.
Collectively, HFC, PFC and SF6 are known as the ‘F-gases’. Despite having a very strong warming impact per unit mass (i.e. a high global warming potential), these gases are emitted in very small quantities; they therefore make only a small contribution to total warming.
Global greenhouse gas emissions are broken down by sectoral sources in the sections which follow (showing carbon dioxide, methane and nitrous oxide individually, as well as collectively as total greenhouse gas terms). The data below is based on UN reported figures, sourced from the EDGAR database. Sources define sectoral emissions groupings in the following way16:
- Energy (energy, manufacturing and construction industries and fugitive emissions): emissions are inclusive of public heat and electricity production; other energy industries; fugitive emissions from solid fuels, oil and gas, manufacturing industries and construction.
- Transport: domestic aviation, road transportation, rail transportation, domestic navigation, other transportation.
- International bunkers: international aviation; international navigation/shipping.
- Residential, commercial, institutional and AFF: Residential and other sectors.
- Industry (industrial processes and product use): production of minerals, chemicals, metals, pulp/paper/food/drink, halocarbons, refrigeration and air conditioning; aerosols and solvents; semicondutor/electronics manufacture; electrical equipment.
- Waste: solid waste disposal; wastewater handling; waste incineration; other waste handling.
- Agriculture: methane and nitrous oxide emissions from enteric fermentation; manure management; rice cultivation; synthetic fertilizers; manure applied to soils; manure left on pasture; crop residues; burning crop residues, savanna and cultivation of organic soils.
- Land use: emissions from the net conversion of forest; cropland; grassland and burning biomass for agriculture or other uses.
- Other sources: fossil fuel fires; indirect nitrous oxide from non-agricultural NOx and ammonia; other anthropogenic sources.
Although many people typically attribute CO2 emissions to energy production, there are other important contributing activities, such as transportation and agriculture. The most recent Intergovernmental Panel on Climate Change (IPCC) reported that the agriculture, forestry, and land use (AFOLU) sector was responsible for about one-quarter of global greenhouse gas emissions.17,18
Total greenhouse gas emissions (measured in their carbon-dioxide equivalent values) by sector are shown in the chart below. The combined figures for agriculture, forestry and land use yield a similar result to that of the IPCC: collectively these emissions account for approximately one-quarter of global emissions.
Why have emissions from agriculture been increasing with time? There are two key contributors to increasing emissions. Firstly, a growing global population requires an overall higher food production. This increased requirement for food has led to both expansion of agricultural land and an intensification of farming practices.19
Agricultural land often expands into previously forested areas, and this process of deforestation releases CO2 stored in trees and soils. These emissions are included in the accounting related to agriculture, forestry and land use (ALOFU), and it is estimated that up to 80 percent of deforestation is the result of agricultural expansion.
Secondly, global economic growth has not only resulted in an increase in food demand (richer people tend to eat more), but also in changes in dietary composition; that is, changes in what we eat. Economic growth is typically related to an increase in meat consumption.20
Livestock are an important source of greenhouse gas emissions, with variations between animal products (lamb and beef are usually the most carbon-intensive and chicken the least).21
A growing global middle class has led to significant increases in global meat consumption in recent decades—trends in meat consumption can be found at our entry on Meat and Seafood Production & Consumption.
What does the future of our carbon dioxide and greenhouse gas emissions look like. In the chart below we show a range of potential future scenarios of global greenhouse gas emissions (measured in gigatonnes of carbon dioxide equivalents), based on data from Climate Action Tracker. Interactive data of these pathways can be found here. Here, five scenarios are shown:
- No climate policies: projected future emissions if no climate policies were implemented; this would result in an estimated 4.1-4.8°C warming by 2100 (relative to pre-industrial temperatures)
- Current climate policies: projected warming of 3.1-3.7°C by 2100 based on current implemented climate policies
- National pledges: if all countries achieve their current targets/pledges set within the Paris climate agreement, it’s estimated average warming by 2100 will be 2.6-3.2°C. This will go well beyond the overall target of the Paris Agreement to keep warming “well below 2°C”.
- 2°C consistent: there are a range of emissions pathways that would be compatible with limiting average warming to 2°C by 2100. This would require a significant increase in ambition of the current pledges within the Paris Agreement.
- 1.5°C consistent: there are a range of emissions pathways that would be compatible with limiting average warming to 1.5°C by 2100. However, all would require a very urgent and rapid reduction in global greenhouse gas emissions.
Historically, CO2 emissions have been primarily driven by increasing fuel consumption. This energy driver has been, and continues to be, a fundamental pillar of economic growth and poverty alleviation. As a result, we see in the visualization below that there is a strong correlation between per capita CO2 emissions and GDP per capita.
This correlation is also present over time: Countries begin in the bottom-left of the chart at low CO2 and low GDP, and move upwards and to the right. Historically, where fossil fuels are the dominant form of energy, we therefore see increased CO2 emissions as an unintended consequence of development and economic prosperity.
While we see this general relationship between CO2 and GDP, there are outliers in this correlation, and important differences exist in the rate with which per capita emissions have been growing.
These differences are exemplified in global inequalities in energy provision, CO2 emissions, and economic disparities. In the chart below we see the change in CO2 emissions (i.e. the growth rates) over the last few decades (1998-2013) across the global spectrum of emitters.
On the x-axis we have the spectrum of global emitters (where those at the far left have very low per capita emissions, and those at the far right have the world’s highest per capita emissions). On the y-axis we have the growth (in %) in CO2 emissions that each segment of emitters has undergone from 1998-2013. We see that the middle of the spectrum—typically those near the middle of the global income spectrum—have experienced a large growth in CO2 emissions over the last few decades (most between 25-40%). Insofar as emissions are a correlate of development, this is good news and reflects the fact that a global middle class is developing, but it does present important challenges in terms of global CO2 emissions.
It is therefore concerning that at the bottom of the spectrum (the group of people of whom many are part of the world’s poorer population) have seen a 12% decline in CO2 emissions over this same period. While a decline in emissions is necessary and possible for individuals with high per capita emissions, for the poorest, this potentially suggests stagnation or decline in living conditions.
Growth rate in CO2 emissions (from 1998-2013) across the spectrum of global emitters22
Not only cross-country inequalities in CO2 emissions are important—there are also noticeable within-country inequalities. In fact, as the global inequalities in CO2 emissions between countries begin to converge, within-country inequalities become more important. As the chart below shows, in 1998 two-thirds of inequality in CO2 emissions were due to between-country differences. Within-country differences then became more important, and by 2013, within and between-country differences were responsible for roughly the same share of total inequalities.
Levels of CO2 inequality between and within countries23
The link between economic growth and CO2 described above raises an important question: do we actually want the emissions of low-income countries to grow despite trying to reduce global emissions? In our historical and current energy system (which has been primarily built on fossil fuels), CO2 emissions have been an almost unavoidable consequence of the energy access necessary for development and poverty alleviation.
In the two charts below, we see per capita CO2 emissions, and energy use per capita (both on the y-axes), plotted against the share of the population living in extreme poverty (%) on the x-axis. In general, we see a very similar correlation in both CO2 and energy: higher emissions and energy access are correlated to lower levels of extreme poverty. Energy access is therefore an essential component in improved living standards and poverty alleviation.24
In an ideal world, this energy could be provided through 100% renewable energy: in such a world, CO2 emissions could be an avoidable consequence of development. However, currently we would expect that some of this energy access will have to come from fossil fuel consumption (although potentially with a higher mix of renewables than older industrial economies). Therefore, although the global challenge is to reduce emissions, some growth in per capita emissions from the world’s poorest countries remains a sign of progress in terms of changing living conditions and poverty alleviation.
If economic growth is historically linked to growing CO2 emissions, why do countries have differing levels of per capita CO2 emissions despite having similar GDP per capita levels? These differences are captured by the differences in the CO2 intensity of economies; CO2 intensity measures the amount of CO2 emitted per unit of GDP (kgCO2 per int-$). There are two key variables which can affect the CO2 intensity of an economy:
- Energy efficiency: the amount of energy needed for one unit of GDP output. This is often related to productivity and technology efficiency, but can also be related to the type of economic activity underpinning output. If a country’s economy transitions from manufacturing to service-based output, less energy is needed in production, therefore less energy is used per unit of GDP.
- Carbon efficiency: the amount of CO2 emitted per unit energy (grams of CO2 emitted per kilowatt-hour). This is largely related to a country’s energy mix. An economy powered by coal-fired energy will produce higher CO2 emissions per unit of energy versus an energy system with a high percentage of renewable energy. As economies increase their share of renewable capacity, efficiency improves and the amount of CO2 emitted per unit energy falls.
In the chart below, we see that the global CO2 intensity has been steadily falling since 1990.25
The carbon intensity of nearly all national economies has also fallen in recent decades. Today, we see the highest intensities in Asia, Eastern Europe, and South Africa. This is likely to be a compounded effect of coal-dominated energy systems and heavily industrialized economies. The shift in industrial production from high-income to transitioning economies, and its impact on CO2 emissions, is discussed in the next section.
As seen in the section above, the general trend in carbon intensity at the global and national level is a downward trend over time. But how do levels of CO2 intensity change across different levels of prosperity?
In the chart below we have plotted average carbon intensities by country (y-axis) against gross domestic product (GDP) per capita (x-axis, log scale). As a cross-section across countries in any given year, we see an overall shape akin to an inverted-U. On average, we see low carbon intensities at low incomes; carbon intensity rises as countries transition from low-to-middle incomes, especially in rapidly growing industrial economies; and as countries move towards higher incomes, carbon intensity falls again.
This trend is approximately true as a cross-section across countries. However, such trends differ for individual countries over time. If we view these trends over the timeline from 1990 onwards we see that there are large variations in the evolution of carbon intensities, even for countries with similar income levels.
With an understanding of the link between CO2 and global temperatures, as well as knowledge of the sources of emissions, an obvious question arises: How much could we reduce our emissions by, and how much would it cost? The possible cost-benefit of taking global and regional action on climate change is often a major influencing factor on the effectiveness of mitigation agreements and measures. How we work out the potential costs of global climate change mitigation has been covered in an explainer post here.
In more recent years, global concentrations of CO2 can be measured directly in the atmosphere using instrumentation sensor technology. The longest and most well-known records from direct CO2 measurement comes from the Mauna Loa Observatory (MLO) in Hawaii. The MLO has been measuring atmospheric composition since the 1950s, providing the clearest record of CO2 concentrations across the 20th and 21st century.
To reconstruct long-term CO2 concentrations, we have to rely on a number of geological and chemical analogues which record changes in atmospheric composition through time. The process of ice-coring allows for the longest extension of historical CO2 records, extending back 800,000 years. The most famous ice core used for historical reconstructions is the Vostok Ice Core in Antarctica. This core extends back 420,000 years and covers four glacial-interglacial periods.
Ice cores provide a preserved record of atmospheric compositions—with each layer representing a date further back in time. These can extend as deep at 3km. Ice cores preserve tiny bubbles of air which provide a snapshot of the atmospheric composition of a given period. Using chemical dating techniques (such as isotopic dating) researchers relate time periods to depths through an ice core. If Looking at the Vostok Ice Core, researchers can say that the section of core 500m deep was formed approximately 30,000 years ago. CO2 concentration sensors can then be used to measure the concentration in air bubbles at 500m depth—this was approximately 190 parts per million. Combining these two methods, researchers estimate that 30,000 years ago, the CO2 concentration was 190ppm. Repeating this process across a range of depths, the change through time in these concentrations can be reconstructed.
Historical fossil fuel CO2 emissions can be reconstructed back to 1751 based on energy statistics. These reconstructions detail the production quantities of various forms of fossil fuels (coal, brown coal, peat and crude oil), which when combined with trade data on imports and exports, allow for national-level reconstructions of fossil fuel production and resultant CO2 emissions. More recent energy statistics are sourced from the UN Statistical Office, which compiles data from official national statistical publications and annual questionnaires. Data on cement production and gas flaring can also be sourced from UN data, supplemented by data from the US Department of Interior Geological Survey (USGS) and US Department of Energy Information Administration. A full description of data acquisition and original sources can be found at the Carbon Dioxide Information Analysis Center (CDIAC).
As an example: how do we estimate Canada’s CO2 emissions in 1900? Let’s look at the steps involved in this estimation.
- Step 1: we gather industrial data on how much coal, brown coal, peat and crude oil Canada extracted in 1900. This tells us how much energy it could produce if it used all of this domestically.
- Step 2: we cannot assume that Canada only used fuels produced domestically—it might have imported some fuel, or exported it elsewhere. To find out how much Canada actually burned domestically, we therefore have to correct for this trade. If we take its domestic production (account for any fuel it stores as stocks), add any fuel it imported, and subtract any fuel it exported, we have an estimate of its net consumption in 1900. In other words, if we calculate: Coal extraction − Coal exported + Coal imported − Coal stored as stocks, we can estimate the amount of coal Canada burned in 1900.
- Step 3: converting energy produced to CO2 emissions. we know, based on the quality of coal, its carbon content and how much CO2 would be emitted for every kilogram burned (i.e. its emission factor). Multiplying the quantity of coal burned by its emission factor, we can estimate Canada’s CO2 emissions from coal in 1900.
- Step 4: doing this calculation for all fuel types, we can calculate Canada’s total emissions in 1900.
Providing good estimates of CO2 emissions requires reliable and extensive coverage on domestic and traded energy—the international framework and monitoring of this reporting has significantly improved through time. For this reason, our understanding of emissions in the late 20th and 21st centuries is more reliable than our long-term reconstructions. The Intergovernmental Panel on Climate Change (IPCC) provide clear guidelines on methodologies and best practice for measuring and monitoring CO2 estimates at the national level.27
There are two key ways uncertainties can be introduced: the reporting of energy consumption, and the assumption of emissions factors (i.e. the carbon content) used for fuel burning. Since energy consumption is strongly related to economic and trade figures (which are typically monitored closely), uncertainties are typically low for energy reporting. Uncertainty can be introduced in the assumptions nations make on the correct CO2 emission factor for certain fuel types.
Country size and the level of uncertainty in these calculations have a significant influence on the inaccuracy of our global emissions figures. In the most extreme example to date, Lui et al. (2015) revealed that China overestimated its annual emissions in 2013 by using global average emission factors, rather than specific figures for the carbon content of its domestic coal supply.28
As the world’s largest CO2 emitter, this inaccuracy had a significant impact on global emissions estimates, resulting in a 10% overestimation. More typically, uncertainty in global CO2 emissions ranges between 2-5%.29
Carbon Dioxide Information Analysis Center
- Data: CO2 emissions (also by fuel type), and data on trace gas emissions, aerosols, the carbon cycle, the Full Global Carbon Budget (1959-2013), land use and more.
- Geographical coverage: Global, regional, national, subnational (for some) and globally gridded (1°x1°; since 1751).
- Time span: Since 1751
- Available at: Online here.
- CDIAC is the climate-change data and information analysis center of the U.S. Department of Energy (DOE).
- The Historical Carbon Dioxide Record from the Vostok Ice Core is available here – it covers the period 417,160 – 2,342 years BP.
- The Atmospheric Carbon Dioxide Record from Mauna Loa is available here – it goes back to 1958.
- The Clio Infra Project is also using CDIAC data. The data is available for download here.
T.A. Boden, G. Marland, and R.J. Andres. 2017. Global, Regional, and National Fossil-Fuel CO2 Emissions
- Data: CO2 emissions by source
- Geographical coverage: Global- by region
- Time span: 1751-2013
- Available at: http://cdiac.ornl.gov/trends/emis/overview
National Oceanic and Atmospheric Administration (NOAA)
- Data: Global CO2 concentrations
- Geographical coverage: Global
- Time span: 1980-2016
- Available at: www.esrl.noaa.gov/gmd/ccgg/trends/
Met Office Hadley Centre for Climate Science and Services
- Data: Atmospheric and marine global temperatures and pressure data
- Geographical coverage: UK-based and global
- Time span: 1850-2017
- Available at: http://www.metoffice.gov.uk/hadobs/
- Data: National, regional and global level analysis on progress on greenhouse gas mitigation and targets
- Geographical coverage: Global, regional and national
- Time span: 1990-2100 (projections)
- Available at: http://climateactiontracker.org/global.html
World Resources Institute (WRI)
- Data: National and global level GHG emissions, global temperature trends and climate change impacts
- Geographical coverage: Global, regional and national
- Time span: 1860-2015
- Available at: http://www.wri.org/blog/2017/04/climate-science-explained-10-graphics
Intergovernmental Panel on Climate Change (IPCC)
IPCC reports are produced periodically, and provide the most complete and comprehensive aggregation of our knowledge and understanding of climatic change, including emissions, temperature correlation, mitigation and adaptation potential. This analysis provides a long-term historical outlook and covers data at both a national, regional and global level. IPCC publications and datasets are available at: https://www.ipcc.ch/