Despite significant efforts being made toward reducing carbon dioxide (CO2) emissions globally, a recent report by the United Nations Environment Program (UNEP 2024) revealed that based on current global policies, the maximum value that global warming can be limited to is 3.1°C over the course of the century, which is 1.6°C above the target of 1.5°C. This means that if the target of reducing emissions below 1.5°C is to be achieved, a yearly reduction of emissions by 7.5% until 2035 is needed.
Greenhouse gas (GHG) emissions continue to be responsible for this event. However, to effectively tackle this challenge, comprehensive questions need to be asked and answered.
This article evaluates these questions and discusses how we can improve the GHG value chain.
What Are GHGs?
GHGs are active gases that absorb and emit radiant energy within the thermal infrared range. Without these gases, Earth's average temperature would be near -18°C (Ma 1998). These gases are responsible for creating the “greenhouse effect,” i.e., the way the habitable temperature of the planet is maintained.
Solar energy which can either be direct or reflected from the Earth’s surface is the source of GHG radiation. The amount of radiated heat energy which is reflected towards the Earth’s surface is dependent on the temperature of the atmosphere and concentrations of various GHGs.
While we can have varying portions of such energy dissipated, their impact on the Earth’s surface is determined by a combination of factors.
Gas Concentration: This defines how much of the gas exists in the atmosphere. Concentrations are typically measured in parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt).
Gas Residence Time: This defines how long the gas remains in the atmosphere, typically measured in years.
Global Warming Potential: This defines how effective the gas is at trapping heat. It varies for each gas in contribution to global warming. It is a function of the residence time and efficiency of the gas. It can be defined on a range of time-periods, however, the most used (and that adopted by the Intergovernmental Panel on Climate Change
[IPCC]) is the 100-year timescale referred to as global warming potential (GWP) 100 (IPCC 2014).
However, the significant environmental and societal consequences of anthropogenically increasing their concentration remains an issue to be resolved.
What Are the Types of Greenhouse Gases?
GHGs may be natural compounds or synthetic gases. The main GHGs include:
Carbon dioxide (CO2)
This is known for its persistence in the atmosphere. In 2023, CO2 accounted for almost 64% of global human-caused emissions (UNEP 2024). Once emitted into the atmosphere, CO2 is not destroyed over time, but instead moves among different parts of the ocean, atmosphere, and land, with some remaining in the atmosphere for thousands of years. Over time, one could lose 60% after 100 years, 80% after 1,000 years, and 90% after 10,000 years.
Methane (CH4)
This is a waste product of anaerobic bacteria and the main component of natural gas. Based on mass, it is a much larger contributor to the greenhouse effect than CO2. It persists in the atmosphere for around 12 years after which it becomes CO2 and H2O but has much higher GWP—almost 30 times greater than that of carbon dioxide over a 100-year period.
Nitrous oxide (N2O)
This is a product of both plant decay and high-temperature combustion. It has a GWP that is around 270 times that of CO2 on a 100-year time scale, and it remains in the atmosphere, on average, a little over 100 years.
Water vapor (H2O)
This is the most abundant GHG. Its residence time at the global scale ranges from 8 to 10 days to a much shorter 4 to 5 days. It is the only GHG that is condensable, meaning it can change from gas to liquid, making it different from other GHGs. Also, it is the only GHG in which its concentration increases because the atmosphere is warming due to the increasing presence of other GHGs and causes the atmosphere to warm even more. Although it is not directly affected by human activities, it is subject to the resultant effect of human activities on other GHGs.
Fluorinated gases (F-gases)
Known for having a long residence time, fluorinated gases come from human-related activities and have no significant natural sources. There are four main categories of these gases: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur hexafluoride (SF6), and nitrogen trifluoride (NF3). Although emitted in smaller quantities than other GHGs, it traps substantially more heat. F-gases are powerful GHGs with an even higher warming potential than CO2. The GWP for these gases can range from 1,000s to 10,000s. Some are being phased out of use because of their high GWP.
Where Do These Greenhouse Gases Come From?
| GHG | Natural Sources | Anthropogenic Sources |
| CO2 | Released by plants and animals during respiration, through the decomposition of organic matter as well as volcanic events. | Burning fossil fuels for energy and transportation is a major source of CO₂ with deforestation and land-use changes further contributing to emissions. |
| CH4 | Wetlands and termites emit methane as part of natural biological processes. | Fossil fuel extraction and agriculture, especially livestock farming, are primary sources of human-caused methane emissions. |
| N2O | Bacteria in soil and ocean environments produce nitrous oxide, especially in nitrogen-rich conditions. | Fertilizer use in agriculture and certain industrial combustion processes are significant sources of nitrous oxide. |
| H2O | Results from the evaporation of water bodies, transpiration from plants, and sublimation of ice. | |
| F gases | HFCs, often used as substitutes for ozone-depleting CFCs and HCFCs in air conditioning and refrigeration, contribute to GHG emissions. |
How Do GHGs Contribute to Climate Change?
Since the Industrial Revolution, the atmospheric concentrations of GHGs have been rising rapidly. Reports show a significant increase in global CO2 concentrations (Forster et al. 2021). In 2023, CH4, N2O, and F-gas emissions collectively made up about 25% of total GHG emissions. Emissions from these gases continued to rise, with F-gases increasing at the fastest pace (4.2%), followed by CH4 (1.3%), and N2O (1.1%) as seen in Fig. 2.
Currently, anthropogenic CH4 emissions are the second-largest source of GHGs, primarily originating from ruminant livestock and manure management, rice farming, oil and gas venting, coal mining, and waste management—all of which saw growth in 2023. So, CO2 is indeed not the only GHG to be concerned about.
Nevertheless, the recent increasing growth in global CO2 emissions could explain the current global focus on its reduction. There have been increased concentrations of CO2 reported in Earth’s atmosphere. Over the past 2,000 years, atmospheric concentrations show that levels were stable at 270–285 ppm until the 18th century (Nong et al., 2021).
Despite the ongoing efforts in minimizing CO2 emissions, atmospheric concentrations continue to rise. The question then becomes: “Have the seemingly stabilized CO2 emissions over the past few years as shown in Fig. 3 below had an impact on global atmospheric concentrations? Why would a stabilization in CO2 emissions not directly translate into the same for atmospheric concentrations?"
The answer is simply because of CO2 residence time. Therefore, to begin to stabilize or even reduce atmospheric CO2 concentrations, emissions need to not only stabilize but also decrease significantly.
What Is the Distribution of GHG Emissions?
In the past, many studies considered CO2 as the only GHG emission, however, recent studies have highlighted the gaps (Nong et al., 2021), thereby making these studies account for non-CO2 GHGs. In the UNEP 2024 study, non-CO2 GHGs were converted to CO2e using global warming potentials with a 100-year time horizon from the IPCC WGI AR6 (Forster et al., 2021).
How Have GHGs Impacted the Earth?
GHG emissions have contributed to global warming, and have a range of potential ecological, physical, and health impacts, including
1. Extreme weather events (such as floods, droughts, storms, and heatwaves).
2. Sea-level rise.
3. Altered crop growth.
4. Disrupted water systems (ocean acidification).
5. Rising global temperatures.
6. Biodiversity loss.
Considering the collective effects of these gases, individual contributions of GHGs should be considered if we intend to achieve the overarching goal of the Paris Agreement—to limit the rise in the global average temperature to well below 2°C above pre-industrial levels and pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels.
For Further Reading
Emissions Gap Report 2024, United Nations Environment Program.
Net-Zero Emissions: From Why to How by B. Freake
Greenhouse Gases: Refining the Role of Carbon Dioxide by Q. Ma, NASA
AR6 Synthesis Report Climate Change 2023, The Intergovernmental Panel on Climate Change
Comprehensive Review: Effects of Climate Change and Greenhouse Gases Emission Relevance to Environmental Stress on Horticultural Crops and Management by I. Shah, M, Manzoor, W. Jinhui, et. al; Shanghai Jiao Tong University
Greenhouse Gas Emissions vs. CO2 Emissions: Comparative Analysis of a Global Carbon Tax by D. Nong, CISRO; P. Simhauser, Griffith University; N. Nguyen; Foreign Trade University
Indicators of Global Climate Change 2023: Annual Update of Key Indicators of the State of the Climate System and Human Influence by P. Forster, C. Smith, University of Leeds; T. Walsh, University of Oxford; et al.
Global Carbon Emissions in 2023 by Z. Liu, Z. Deng, University of Hong Kong; S. Davis, University of California, Irvine; et. al.
Adeshina Badejo is a petroleum engineering PhD student at Texas A&M University under the Texas A&M at Qatar Strategic Research Initiatives Program. He has a strong interest in reducing the environmental impact of the continued use of fossil fuels. His research focuses on flow assurance challenges of the CO2 value chain from the extraction point to the subsurface injectivity point with the integration of machine learning. Badejo has been actively involved with SPE since 2016 as a volunteer. He led the Heriot-Watt University PetroBowl team to the regional qualifiers in Zagreb, Croatia, and received the 2023 SPE Aberdeen Section Student Bursary Award. He also served as the 2018–2019 SPE Programs Chairperson during his undergraduate studies and co-initiated the inaugural edition of The Industry Discourse, a student-led energy conference. He holds a master’s degree in subsurface energy systems from Heriot-Watt University, Edinburgh, and a bachelor’s degree in petroleum and gas engineering from the University of Lagos, Nigeria.
Vera Dogo holds a master’s degree in reservoir evaluation and management from Heriot-Watt University, Edinburgh, and a bachelor’s degree in petroleum engineering from the Federal University of Petroleum Resources in Effurun, Nigeria. She is committed to fostering sustainability in the energy sector, focusing on optimizing fossil fuel practices while promoting a transition toward greener energy solutions. Her master’s degree research on reducing the carbon footprint in reservoir operations earned her second place at the 2024 SPE Student Paper Contest (Europe master’s division) in Zagreb, Croatia, where she also represented Heriot-Watt in the PetroBowl competition. Currently, she is part of the communications team for the SPE Integrated Reservoir Management Technical Section. Outside of work, she enjoys reading, cooking, and music.