The Earth’s atmosphere regulates the temperature of the planet’s surface, among other functions. On the one hand, it controls the solar radiation received and, on the other hand, when the Earth warms, part of this heat is returned to the atmosphere in the form of infrared radiation. Some atmospheric gases (aka greenhouse gases), such as carbon dioxide and water vapour, absorb part of this energy and prevent it from escaping into outer space. This natural phenomenon is known as the greenhouse effect, and it is essential as it keeps the Earth’s surface temperature within values suitable for terrestrial life.
Nonetheless, there are some singularities caused by nature itself, while living organisms trigger others. Some of these processes generate gas emissions that intensify the greenhouse effect on our planet; there is a particular interest in those caused by man since they are somehow under our own activity as a species. Modern humans have generated significant changes in their environment over the years, most of them searching for progress in all aspects of life. The emission of gases with greenhouse effect could be grouped into agriculture, forestry, land use, energy generation and use, and industrial waste management.
But why do greenhouse gas emissions matter? And how are greenhouse gas emissions and concentrations changing? Population growth and life quality improvements have multiplied the presence of most greenhouse gases (GHG) in the atmosphere up to unprecedented levels. As human activities have been increasing over the years, global net greenhouse gas emissions related to these activities increased by 35% in 20 years (1990 to 2010) . Carbon dioxide (CO2) concentration has augmented by 147%, methane (CH4) by 259%, and nitrous oxide (N2O) by 123% since pre-industrial times . Consequently, global average temperatures have increased by more than 1℃ since then.
The four major types of GHG are grouped in the following categories:
Emitted basically as a result of burning fossil fuels (oil, natural gas, and coal), trees and wood products, and solid waste, CO2 is usually interpreted as a synonym of greenhouse gases. Land use conversions also play a role. Soil degradation and deforestation add carbon dioxide to the atmosphere, while forest regrowth does the opposite. Its lifetime cannot be represented with a single value because the gas is not disintegrated over time. Instead, it moves among the ocean–atmosphere–land system in a complex equilibrium.
CH4 enters the atmosphere during the production and transport of oil and natural gas and coal, also from livestock and several agricultural practices and from the anaerobic decomposition of organic waste in municipal solid waste landfills. Its average lifetime in the atmosphere is about 12.4 years, while its global warming potential is 28 to 36 times that of CO2.
This gas is emitted during industrial and agricultural activities, as well as during combustion of fossil fuels and solid waste. It has a much longer atmospheric lifetime, approximately 121 years.
This is a group of gases that contain fluorine, including hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride, among other substances. These gases do not occur naturally; instead, they are released from various industrial processes and commercial and household uses. Their average lifetime in the atmosphere ranges from a few weeks to thousands of years, depending on the compound.
A carbon dioxide equivalent (CO2eq), is a measure used to compare the emissions from various GHG through the lens of their global warming potential (GWP), by converting volumes of other gases to the equivalent amount of carbon dioxide with equivalent GWP. The CO2eq for a greenhouse gas is obtained by multiplying the corresponding GWP by the
Hence, emissions of 1 million metric tonnes of CH4 and nitrous oxide are equivalent to 25 and 298 million metric tonnes of CO2.
Beyond the Earth’s atmosphere, GHG emissions cause many other noteworthy changes in soils and oceans. Such alterations have adverse effects on individuals, society and the environment, including plants and animals. Many GHG remain in the atmosphere for tens, hundreds or even up to thousands of years after their release. Their warming effect on climate persist for a long time and therefore affect both present and future generations.
1.2 Country Profiles
China is the largest pollutant by far, accounting for 27% of global emissions, nearly 10 billion tonnes each year. As China, India, and other Asian countries are home to 60% of the world’s population, per capita emissions in the continent are slightly lower than the world average. The USA is the second-largest emitter at 15% of global emissions. It’s followed by the European Union (EU) with 9.8%. Here are the 28 countries of the EU since they normally negotiate and set targets as a collective body. India at 6.8%, Russia at 4,7%, and Japan at 3.3% are also significant contributors to global GHG emissions. African and South American countries are reasonably small emitters, each continent accounting for 3-4% of global emissions. Both have emissions just about the magnitude of international aviation and shipping.
As stated before, it is also useful to calculate the contribution of each country’s average citizen by dividing its entire carbon footprint by its population. This gives us CO2 emissions per capita and what stands out first is that there are enormous inequalities in per capita emissions across the world.
The major oil-producing countries are the world’s largest per capita CO2 emitters, particularly those with relatively low population sizes. Most are in the Persian Gulf: Qatar had the highest emissions at 49 tonnes (t) per person, followed by Trinidad and Tobago at 30 t; Kuwait at 25 t; the United Arab Emirates at 25 t; Brunei at 24 t; Bahrain at 23 t, and Saudi Arabia at 19 t; all this from 2017 data. More heavily populated countries with top emissions per capita—and thus high total emissions—are Australia (17 t), the US (16.2 t), and Canada (15.6 t). This is more than three times higher the global average, which was 4.8 tonnes per person in 2017.
Since there is such a marked correlation between per capita carbon emissions and income, one would expect that countries with high prosperity would have a high carbon footprint. But what becomes clear is that, even between countries with similar living standards, other variables (climate, culture, extension of national territory) can engender large differences in per capita emissions. High standard of living is a primary driver of CO2 emissions, but traditional, policy, and technological choices make a difference. Far from it, in many of the poorest countries in Sub-Saharan Africa countries, the average footprint is around 0.1 tonnes per year.
1.3 Sector Profiles
It is time to discuss the breakdown of emissions by sector and show why the energy sector is central to efforts against climate change.
The world emits around fifty billion tonnes of carbon dioxide equivalents (CO2eq) each year . To find out how we can most effectively mitigate emissions and what emissions can and can’t be removed with current technologies, we need first to comprehend where our emissions come from. The most recent breakdown of global emissions by sector, published by the World Resources Institute and Climate Watch and , disclosed that almost three-quarters of emissions come from energy use; one-fifth from agriculture and land use the remaining 8% from industry and waste. These categories are based on descriptions provided in the IPCC’s Fifth Assessment Report and the World Resources Institute’s methodologies.
Energy consumption is by far the most significant source of human-caused GHG emissions when accounting for transportation, electricity and heat, buildings, construction, fugitive emissions and other fuel combustion. Within the energy sector, heat and electricity generation is responsible for most emissions, followed by transportation and manufacturing and construction. From this, it also turns out that the main hotspots driving energy-related emissions are buildings and cars. End-use activities driving most energy emissions are road transportation, residential and commercial buildings. Their emissions include direct emissions from combustion of fuels and indirect emissions such as electric power use.
It is clear from the breakdown in Figure 3 that a variety of processes and sectors contribute to global emissions. That is to say; there is no single or straightforward fix to tackle climate change: focusing on electricity, or deforestation, or transport, or food alone is not enough. Even within the energy sector—which explains the three-quarters of global emissions—there is no simple solution. Even fully decarbonising electricity supplies, we would also need to electrify our heating and road transport.
Let’s first clarify two critical concepts, primary energy and secondary energy. Primary energy consists of fuels or energy that are naturally present on our planet and can be used directly, including oil, natural gas, coal, uranium, wood, wind, biomass, sun, tides, rivers (from which hydropower can be obtained) and the Earth heat (from which geothermal energy can be obtained). In contrast, secondary energy has already undergone a previous transformation from a primary energy source, such as gasoline, fuel oil, biofuels, electricity, hydrogen, and heat. For example: in 2019, around 63% of global electricity came from fossil fuels. Of the remaining 37% from low-carbon sources, renewable energy made up to 26% and nuclear reached 10%. The relative contribution of fossil fuels and low-carbon electricity has remained nearly unchanged for decades. In the early 2000s, fossil fuels even gained ground due to nuclear’s share declined whilst renewables grew.
Carbon emissions from energy can arise from various sources and fuel type: coal, oil, gas, and gas flaring. As global and national energy systems have transitioned over the decades, the contribution of different fuel sources to carbon emissions has changed geographically and temporally.
The average annual rise in emissions over 2018 and 2019 combined was greater than its 10-year average. The sharp increase in CO2 emissions in 2018 was driven in part by extreme weather effects. In particular, a surge in the number of scorching and cold days boosting energy demand.
In the chart of Figure 4, we see global fossil fuel consumption broken down by coal, oil and gas since 1800. Data disclosed in BP’s Statistical Review of World Energy  from 1965 onwards has been combined with earlier data sourced from Vaclav Smil’s work on energy transitions . Over the past half-century, fossil fuel consumption has risen dramatically, about eight times since 1950, and has approximately doubled since 1980. Fuels we rely on has also moved, from coal solely towards a combination with oil, and then gas. At the moment, coal consumption is falling in many parts of the world, whereas oil and gas are still proliferating.
Oil is the world’s greatest energy source today. Oil is only behind a small share of electricity production worldwide. Yet, it is on the rise, essentially driven by increasing demand for transport, up an average of 1.9% per year and making up just over one-third of global fossil fuel emissions. About half of oil is used in land transport, with demand rising in developing countries and some developed nations. In the US, there is almost one vehicle per person. Simultaneously, in many developing markets this ratio is far lower, e.g. one car for every six people in the People’s Republic of China and one for every forty people in India. Projections for increasing private vehicle ownership in developing markets of South East Asia and Latin America suggest oil demand will continue to grow for years to come. Airline travel is also growing, but it still represents only 8% of total oil use emissions. The start-up of large petrochemical projects drove product demand, which partially offset a slowdown in gasoline demand growth due to the impact of COVID-19 in 2020.
Oil output is a significant metric to track—it allows us to understand where it is being extracted, who the leading oil producers are, and how this relates to oil reserves. Despite strong US output growth, global oil production fell by 60,000 b/d due to a decline in OPEC production (-2 million b/d), with sharp declines in Iran, Venezuela, and Saudi Arabia. But we also mind where that oil is being consumed—that tells us what role it is playing in the transport and energy system of each country. Oil consumption growth in 2019 was led by China (680,000 barrels/day) and other emerging economies, even though demand fell in the OECD cluster (-290,000 b/d).
2.2 Natural Gas
Natural gas is experiencing consecutive years of strong growth driven by growing energy demand, substitution from coal and RE backup. Let’s review some of the key figures from 2019 to understand this phenomenon. Natural gas consumption increased by 2%, well below the exceptional 5.3% growth seen in 2018. Nevertheless, the weight of gas in primary energy rocketed to a record high of +24%. Increases in gas demand were driven by the US and China, while Russia and Japan saw the largest declines. In turn, gas production grew by 3.4%, with the US accounting for almost two-thirds of this increase. Australia and China were also key contributors to growth.
Previously hooked on pipelines for transport, natural gas markets are becoming more global as liquified natural gas (LNG) needs grow. It is worth mentioning that inter-regional gas trade expanded at a rate of almost 5%, more than double its recent average, driven by a record increase in LNG. LNG supply growth was driven by the US and Russia, with most incremental supplies heading to Europe: European LNG imports rose by more than two-thirds.
Coal is the prime contributor to fossil fuel emissions, making up 42% of the global total. Albeit the share of coal in primary energy demand and electricity generation slowly decreases, it remains the largest electricity source and the second-largest source of primary energy. Consumption of coal continued to grow in some emerging economies, particularly in China, Indonesia, and Vietnam; the latter appointing a record increase in part related to a sharp drop in hydroelectric power. rAs with consumption, the largest declines in production came from the US and Germany. Coal prices fell in 2019, with the Chinese and Northwest Europe marker prices declining by 34% and 14% respectively.
As renewable and low-emissions power sources become more competitive and more countries turn away from coal due to its impact on health and climate, there are signs that coal is clearly in decline. Recent data show that US generation from coal is projected to decline to levels unseen in the last 50 years. In Europe, coal-based emissions fell 10% in 2019. And in the UK, coal plummeted from 42% in 2012 to only 5% of electricity generation in 2018.
2.4 Energy Efficiency
Technical efficiency gains continue to bring reductions in energy-related emissions. Between 2015 and 2018, technological efficiency improvements reduced energy-related GHG emissions by 3.5 gigatonnes of carbon dioxide, roughly the equivalent of the energy-related emissions of all Japan over the same period. This helps bring the world closer to an emissions trajectory consistent with achieving global climate change goals.
Although efficiency accounts for the highest proportion of the cumulative emission reductions expected in the Sustainable Development Scenario, these targets are not currently on track to be achieved . Its vast potential for carbon emissions abatement clashes with slower rates of improvement in recent years. For instance, intensity improvement slowed down from nearly 3% in 2015 to 1.2% in 2018. Reversing this trend and improving energy efficiency is possible with cost-effective measures.
Energy efficiency has tremendous potential to avoid greenhouse gas emissions. But according to the International Energy Agency (IEA), three factors are driving the slowdown. In terms of demand, energy-intensive industries in China and the US deepened their production share and pushed up the use for all primary energy fuels. Weather is also playing a role: warmer summers drive up energy use for cooling. On the supply side, more robust electricity demand means coal power generation to increase to supply growth. More fossil fuel-based electricity increases primary intensity because energy is wasted when these fuels are converted from primary to final energy.
Longer-term structural factors are also playing a role. Although technologies and processes are becoming more efficient, structural factors are dampening these advances on energy demand and slowing global energy intensity improvements. This is the case for inefficient changes in transport modes and more building floor area per person. For example, in transport, energy use continues to grow: amongst other factors, consumers prefer larger cars, and typical vehicle occupancy rates have fallen.
Transport is considered a subcategory within energy-related emissions. It currently accounts for around one-fifth of global CO2 emissions, and it grew by 71% since 1990. More travel by cars is the principal reason transportation emissions are on the rise . They’re also projected to grow faster than any other sector, posing a significant challenge for pollution reduction efforts in compliance with the Paris Agreement and other global goals. If we look at the US as an example, emissions plateaued in 2005 and have now risen every year since 2012. It is not entirely surprising the transport sector surpassed the electric power industry in 2016 to become the single greatest US emitter of GHGs for the first time.
In terms of transport modes, 72% of global transport emissions come from road vehicles. Since the entire transport sector accounts for 21% of total emissions, road transport accounts for 15% of total emissions. These have also increased in other transport modes, such as international and domestic aviation, and coastal shipping. Railways are an exception; powered by a significant share of electricity, rail emissions have actually declined. In terms of geography, transportation GHG mostly come from upper-middle-income and high-income countries. South Asia and Sub-Saharan Africa contribute much less than other regions.
It must be outsourced that aviation—while it often gets the most attention in manifestations and queries for action against climate change—accounts for less than 12% of the transport sector emissions. It emits about one billion tonnes of CO2eq each year—around 2.5% of total global emissions. International shipping reaches a similar amount, at 10.6%.
3 AGRICULTURE, FORESTRY AND LAND USE
3.1 Livestock and manure
Global food production accounts for one-quarter of GHG emissions , with livestock and fisheries accounting for around one-third of food emissions. Livestock (animals bred for meat production, dairy, eggs, and even certain shellfish) contribute to emissions in various ways. Ruminant livestock, namely cattle, produce methane through their digestive processes, in a process known as “enteric fermentation”. Manure management, pasture management and fuel consumption of fishing vessels also fall into this category. This 31% of emissions relate only to “production” emissions on the holding: it does not include land use change or value chain emissions from crop production for animal feed: those figures are included separately in the other categories.
Land use justifies 24% of food emissions. The emissions resulting from land use for livestock make up to 16%, the remaining 8 % is for crops. Here comes the concept of deforestation, in which agricultural expansion results in the transformation of forests, grasslands and other carbon ‘sinks’ into farmland or pastures, resulting in carbon dioxide emissions. The term “land use” here is the sum of the change in land use, the burning of savannahs, and soils’ organic cultivation.
3.3 Supply chains
Supply and logistic networks account for 18% of food emissions. Approximately 25% of the world’s calories are thrown away; are spoiled or thrown throughout value chains; or are wasted by consumers, restaurants and retailers. To produce this food we need energy, water, land and fertilisers, all of which has an environmental cost. Poore and Nemecek’s 2018 study shows that 24% of food emissions come from food lost in supply chains or wasted by consumers . Nearly two-thirds of this, 15% of food emissions, come from supply chain losses resulting from low storage and handling techniques; lack of refrigeration; deterioration in processing and transport. The remaining 9% comes from food thrown away by retailers and consumers. This means that food waste is responsible for about 6% of total global GHG emissions. In fact, it is likely to be a little higher as their analysis does not include food losses during farming and harvesting.
Crop production raches 27% of food emissions. 21% of food emissions come from crop production for direct human consumption and 6% come from animal feed production. These are direct emissions resulting from agricultural production, including elements such as the release of nitrous oxide by applying fertilisers and manure, methane emissions from rice production, and carbon dioxide from agricultural machinery.
In the US, soil management is the largest source of agricultural greenhouse gas emissions, mostly in the form of N2O, which is naturally emitted by soils and fluctuates year to year with variations in weather, crop production. Most N2O emissions stem from the use of synthetic fertiliser.
4 CURRENT AND FUTURE TRENDS
4.1 What is next?
Global emissions have not yet peaked: they have grown 41% since 1990, and they are still climbing. In its Energy Technology Perspectives report , the IEA expects global transport (computed in passenger-kilometres) to double, automobile ownership rates to increase by 60%, and demand for passenger and freight aviation to triple by 2070. All these factors combined, they would result in a large increase in transport emissions. Major technological innovations can help offset part of this rise: as the world shifts towards low-carbon electricity, electric vehicles offer a sustainable option to reduce passenger vehicles emissions.
Transport remains hugely dependent on oil, and the sector accounted for about two-thirds of global oil consumption in 2015, with road transport alone accounting for half of oil consumption. Besides traditional fuels, electricity share in transport energy consumption has increased marginally from 0.7% in 2000 to 1% in 2015. With electrification and hydrogen technologies, some transportation sub-sectors could decarbonise within decades. Assuming optimistic scenarios, the phase-out of emissions from motorcycles could be attained by 2040; rail by 2050; trucks by 2060; and, in many regions, including the EU, North America, China and Japan, conventional vehicles as early as 2040.
Other transport sectors though, will be much more difficult to decarbonise. The global marine industry is focused on reducing its carbon footprint as never before. In fact, over the next decade, the only issue perceived as having a more significant impact is a potential global financial crisis, according to findings from the Global Maritime Forum . Maritime transport emits around 1 billion tons of Co2eq per year, accounting for almost 3% of global emissions. Apparent increasing demand for shipping could see this figure increase to 1.7 billion tons by 2050. In this context, the UN’s International Maritime Organization, set a target of at least halving the sector’s total annual GHG emissions by 2050, compared to 2008. Fully decarbonising shipping entails moving away from conventional heavy fuel oil to alternative fuels and engines that are not yet commercially viable.
Can we decarbonise agriculture and power systems? With agriculture being one of the largest and most important sectors in developing countries, and globally, the use of renewable energy can provide solutions for energy, climate, and food security. The United States Department of Agriculture (USDA) projects augmented agricultural emissions, mostly due to population growth and demand. By 2050, carbon emissions from the three largest sources—enteric fermentation, manure management and soil management—will rise between 3.2 and 8.6% over 2005 levels in its reference scenario .
Food processing, i.e. converting agricultural products into end-use products, transportation, packaging and retail, requires energy inputs and resources. Many assume that eating local products is key to a low-carbon diet, yet transport emissions are often a tiny percentage of total food emissions: only 6% globally. While supply chain emissions may seem high, at 18%, it is essential to reduce emissions by avoiding food waste. Food waste emissions are immense: a quarter of the emissions (3.3 billion tons of CO2 eq) from food production end up as waste, either from supply chain losses or by consumers. Durable packaging, refrigeration, and food processing can help prevent food waste. For example, the waste of processed fruits and vegetables is approximately 14% less than fresh alternatives and 8% less than seafood.
As the agricultural sector relies heavily on the population, it must compensate its demands with a large amount of energy consumption and GHG emissions. However, with Europe’s and other regions plans to become carbon neutral by 2050-2060, this provides the agriculture sector with new opportunities for clean innovation. Geothermal energy has been increasingly used for the last 25 years in aquaculture, greenhouses, soil heating and agro-industrial processes. Low or medium temperature geothermal heat is available worldwide, providing a clean, sustainable and renewable resource.
The power sector will also undergo a global transformation. Over the past decade, the costs of renewables have considerably dropped—solar power by as much as 80% and wind power by about 40%—making them economically competitive with conventional fuels, such as natural gas and coal, in the vast majority of international markets. As a result, renewables are growing fast: they accounted for most of the new power-generation capacity since 2018. Together with green hydrogen, renewables make up an essential element of any country’s plan to cut GHG emissions.
Current climate policies will reduce emissions, but not quickly enough to reach international targets. They will reduce, or at least slow down growth, in CO2 and other greenhouse gas emissions, with some impact on reducing future warming. But if our goal was to limit warming to well below 2°C, as set out in the Paris Agreement, it is clear that we are a long way off the track. Researchers drew up the global emission reduction scenarios needed to limit average global warming to 1.5°C and 2°C. Based on the IPCC Special Reports, these mitigation curves show that urgent and rapid emission reductions would be needed to achieve any objectives. And the longer it takes to reduce a peak in emissions; the more drastic these reductions should be. We may be moving slowly towards a world without climate policies, but we are still far from the progress rates we would need to achieve international goals.
As we see in the chart shown here, current implemented climate and energy policies would reduce warming relative to a world with no climate policies in place. The chart above maps out future GHG emissions scenarios under an array of assumptions: if no climate policies were implemented; if current policies persisted; if countries achieved their current future pledges for emissions reductions; and necessary pathways which are compatible with limiting warming to 1.5°C or 2°C of warming this century. In the net-zero scenario, nearly two-thirds of emission reductions come from technologies that are not yet commercially available. Moreover, reducing CO2 emissions in the transport sector over the next half-century will be a titanic effort.
It is not only about governments: a variety of stakeholders will need to work together to make the decarbonisation transition happen. Utilities and system planners must build more advanced ways to integrate expected energy flows and usage trends into their scenarios . They need to understand how a future energy scheme could work with the natural-gas network; the capability of behind-the-meter resources, such as distributed energy; the full potential of more complex resources, such as storage; and the ability to trade off different types of assets, such as transmission, hydrogen, among others. For regulators, sailing through these troubled waters means creating market signals and compensation structures that are effective and transparent. This is central given that power and transport systems will be increasingly complicated, with marginal assets dispatching at nearly zero marginal cost and the value of “firmness”—or reliable capacity—growing in consideration.
The combination of decelerated growth in energy demand and a shift in the fuel mix away from coal and toward renewables and natural gas leads to a significant slowdown in the evolution of carbon emissions. Yet, since three-quarters of global GHG comes from energy—the burning of coal, oil, and gas—we need to transition away from them to low-carbon sources promptly. It follows from the above that natural gas is the fastest-growing fossil fuel. Even with this fast growth, it contributes around half the emissions of coal. While natural gas is at times considered a “bridge fuel” between coal and renewables, the investments being made now in natural gas infrastructure will lock in its consumption and correlated emissions for decades to come, possibly delaying the shift to lower-carbon sources.
Meeting long-term climate objectives means changing how we produce and use energy, eat, produce goods, and move. It will require a greater emphasis on energy efficiency and innovation, speeding up the deployment of low-carbon technologies, accelerating the development of carbon capture, utilisation and storage (CCUS), electric mobility, and hydrogen. Cutting emissions from food production will be one of our main challenges in coming times. We will need changes to diets; food waste reduction; improvements in agricultural efficiency; and technologies that make low-carbon food alternatives scalable and affordable.