All about Greenhouse Gases

What are greenhouse gasses and where do they come from? A beginner’s guide to greenhouse gasses.

Different Greenhouse Gasses

There are 10 primary greenhouse gasses and, of these, only water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) occur naturally. The other 6 are only present in the atmosphere due to industrial processes.

The largest greenhouse gas by volume is actually the one most people tend to overlook: water vapor. The concentration of water vapor in the atmosphere can vary significantly and depends on the temperature and other meteorological conditions and not directly upon human activities.  

The biggest anthropogenic, or human-induced, greenhouse gas is carbon dioxide, which accounts for about 76% of humanity’s contribution to the greenhouse effect.

Global Warming Potential (GWP) is a measure used to indicate a gas’ potential to trap the Earth’s heat. Carbon dioxide is used as the reference gas and has a GWP of 1. Greenhouse gas emissions are typically discussed in terms of mass of carbon dioxide equivalents (CO2e), which are calculated by multiplying the mass of emissions by the gas’s GWP.

What’s in the Air? 

The atmosphere contains many gasses, most of which are in small amounts, including some pollutants and greenhouse gasses. The most abundant gas in the atmosphere is nitrogen (roughly 78%), with oxygen second (almost 21%), and argon third (about 0.9%).  A mixture of trace gasses accounts for the other 0.1%, including the greenhouse gasses carbon dioxide, methane, nitrous oxide, and ozone. Concentrations of these greenhouse gasses are measured in parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt) by volume. In other words, a concentration of 1 ppb for a given gas means there is 1 molecule of that gas in every 1 billion molecules of air.

Tiny solid or liquid particles known as aerosols, which are produced both naturally and by human activities, are also present in variable amounts, along with human-produced industrial pollutants and natural and human-produced sulfur compounds.

Since 1750, atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased by 148%, 260%, and 123%, respectively, to levels that are unprecedented in the past 800,000 years. Here’s a table showing the concentration of these greenhouse gasses:

Figure 2. Visual representation of the composition of Earth's atmosphere by volume. Figure 2. Visual representation of the composition of Earth's atmosphere by volume.

Sources of Greenhouse Gas Emissions

Greenhouse gasses enter the atmosphere in different ways. Anthropogenic carbon dioxide is emitted primarily from fossil fuel combustion for electricity generation, transportation, and industrial and household uses. Iron, steel, cement, and production of petroleum-based resources are other significant sources of carbon dioxide emissions. 

Both methane and nitrous oxide are emitted from natural and anthropogenic sources. Approximately 40% of methane is emitted into the atmosphere by natural sources like wetlands and termites, and about 60% comes from anthropogenic sources such as ruminants, rice agriculture, fossil fuel exploitation, landfills, and biomass burning. Nitrous oxide is released into the atmosphere from both natural sources and anthropogenic sources, including oceans, soils, biomass burning, fertilizer use, and various industrial processes. Fertilizer alone accounts for 75% of anthropogenic nitrous oxide emissions

Hydrofluorocarbons are the fastest growing greenhouse gasses and are used in refrigeration, cooling, or as solvents in place of ozone-depleting chlorofluorocarbons.

How are the Levels of Carbon Dioxide Measured and Tracked

Atmospheric carbon from fossil fuel burning is currently the main human-caused factor in the escalating global warming. The level of carbon in our atmosphere is tracked using what is called the Keeling curve, which measures atmospheric carbon in parts per million (ppm). The Keeling curve forms part of the Scripps Carbon Dioxide program, which is run in parallel with other programs that measure changes in atmospheric oxygen and argon abundances. 

Each year, many measurements are taken at Mauna Loa, Hawaii, to determine the parts per million (ppm). Weekly, monthly, yearly, per decade, or extended Keeling curves are available and on record. At the beginning of the Industrial Revolution, around 1880, before fossil fuel burning, the atmospheric carbon ppm level was around 280 ppm. Here is the current Keeling curve graph for where we are today:

How Long Do Greenhouse Gasses Remain in the Atmosphere

The IPCC defines a gas’s lifetime as the amount of the gas in the atmosphere divided by the rate at which it is removed from the atmosphere. However, the calculation of a gas’ lifetime is not as simple because not all gasses are removed by just one, or mainly one, process. 

Ironically, carbon dioxide, being the most important greenhouse gas related to global warming, is the hardest to pin down. When carbon dioxide is released into the atmosphere, about 75% of it will dissolve into the ocean over a few decades. The other 25% is neutralized by a variety of longer-term geological processes, which can take thousands of years.

Here’s a table showing a selection of greenhouse gasses, their global warming potential (GWP), and their lifetimes:

Methane is naturally removed from the atmosphere by both chemical and biological processes, including reaction with atmospheric hydroxyl (OH) and chlorine, and by methane-consuming bacteria, called methanotrophs, in soil and water. Methane has an atmospheric lifetime of 12 years. Nitrous oxide is destroyed by photolytic reactions, chemical reactions involving photons or light and has a lifetime of around 114 years. 

In 2019, the global anthropogenic greenhouse gas emissions totaled  52.4 billion tons of carbon dioxide equivalents (CO2e) resulting in an increase of 0.57 billion tons CO2e. CO2e describes the global warming potential of all greenhouse gasses. In other words, the impact of different greenhouse gasses is expressed in terms of the amount of carbon dioxide that would result in the same amount of warming. 

Since 1990, annual anthropogenic greenhouse gas emissions have increased by 59% with an average increase in emissions of 0.4 billion tons CO2e per year from 1970-2000. What’s more, carbon emissions in 2021 grew to their highest ever level, as the world rebounded from the effects of the Covid-19 restrictions.

Global electricity generation from coal rose to an all-time high in 2021, which is a worrying trend, as coal is one of the biggest contributors to carbon emissions in the energy sector.  Luckily the massive push for low-carbon energy is also gaining momentum. In 2021, renewable and nuclear energy accounted for 39.5% of energy generation worldwide, and looks set to continue growing

Current Global Climate Change Policies

According to the Intergovernmental Panel on Climate Change (IPCC), to keep global warming below 2°C, emissions of carbon dioxide and other greenhouse gasses must be halved by 2050 compared to their 1990 levels.  Developed countries are expected to reduce more – between 80 and 95% by 2050. Advanced developing countries with large emissions like China, India, and Brazil, will also have to limit their emission growth. 

Different countries have different climate change policies in place. The European Union, for example, has adapted programs that are aimed at reducing emissions. These include the increased use of renewable energy, improved energy efficiency of buildings and industrial sectors, and the reduction of carbon dioxide emissions from new cars, among others. 

The Sustainable Energy Development Goals (SDGs) have also been adapted by the United Nations General Assembly (UNGA) in 2015, and provide a powerful framework for international cooperation toward a sustainable future. SDG 7 states 3 main targets: ensuring affordable, reliable and universal access to modern energy services; increasing the share of renewable energy in the global energy mix by 2030; and doubling the rate of global energy efficiency.

Future Scenarios and Targets

The Paris Agreement sets out the aim to reduce warming to well below 2°C. The graphs obtained from the website ‘Our World in Data’ show that current policies to reduce, or at least slow down growth in, carbon dioxide and other greenhouse gas emissions will have some impact on reducing future warming. This chart maps out future greenhouse gas emissions scenarios under a range of assumptions: if no climate policies were implemented; if current policies continued; if all countries achieved their current future pledges for emission reductions; and necessary pathways which are compatible with limiting warming to 1.5°C or 2°C of warming in this century. From the chart, we can see that current climate and energy policies would reduce warming relative to a world with no climate policies in place, but the still fall short of the target, which is to limit warming to well below 2°C. 

Hitting the Target: Close, but not Good Enough

If countries achieved their current ‘pledges,’ there would be an improvement even greater than the target of limiting warming to 2.1°C. In this regard, it is evident that the world is making progress. However, our aim is to limit warming to well below 2°C and we, therefore, will fall short of meeting these targets by 2100. 

Robbie Andrew, senior researcher at the Center for International Climate Research (CICERO), mapped out the necessary global emissions reduction scenarios that will be necessary to reduce global warming to 1.5°C and 2°C. Based on the combined findings in the IPCC’s special Report on 1.5 °C and Michael Raupach’s work, published in Nature Climate Change, the mitigation curves suggest that rapid urgent reductions in emissions are necessary to achieve either target. The longer we delay a peak in emissions, the more drastic these reductions would need to be. 

The image below shows the current pledge status of the different countries across the globe.

How Will We know if We are Making Progress in Reducing Global Warming?

Now that we know the importance of reducing our carbon footprint and combating climate change, how will we know if we’ve made progress or have reached the set targets? 

Well, besides tracking the average global temperature rise and reducing levels to below 2°C, as stipulated in the Paris Agreement, there are at least 2 other ways to measure whether we are making honest progress in reducing global warming. The first measure is to monitor the average annual increase in carbon ppm levels, which is currently at about 3 ppm per year. If we make progress, these levels will either start to drop, remaining at the current level, or, at the very least, rise at a slower rate. 

Another measure to track progress in global warming is the Keeling graph, which is a graph that shows the ongoing change in the concentration of carbon dioxide in the Earth’s atmosphere. Current carbon levels are around 417 ppm. If these levels are lowered to around 325-350 ppm, we’ve made significant progress.

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