Carbon dioxide (CO2) is the most significant greenhouse gas (GHG) contributing to anthropogenic (human-caused) climate change. Understanding why CO2 has become the leading driver of global warming involves examining the role of greenhouse gases, the concentration of CO2 in the atmosphere, and the unique characteristics of this gas compared to other greenhouse gases.

What are Greenhouse Gases (GHGs)?

Definition of GHGs:

• Greenhouse gases are gases that trap heat in the Earth’s atmosphere. They absorb infrared radiation (heat) emitted from the Earth’s surface and prevent it from escaping into space. This natural process is known as the greenhouse effect, which helps maintain temperatures suitable for life on Earth.

How GHGs Work:

• The Sun’s shortwave radiation reaches the Earth’s surface, warming it. The surface then emits this energy back as longwave radiation (infrared heat).
• CO2 and other GHGs like methane (CH4) trap this infrared radiation due to their molecular structure. Unlike simple molecules like nitrogen (N2) and oxygen (O2), which cannot absorb infrared radiation, CO2 and methane are more complex and can absorb a wider range of wavelengths, including heat.

Natural Role of GHGs:

• GHGs such as CO2, CH4, and water vapor occur naturally and are vital for the Earth’s climate system. Without these gases, the Earth would be too cold to support life.

Human Impact:

• The problem arises from the increase in the concentration of these gases, especially CO2, due to human activities like fossil fuel burning and deforestation. This results in more heat being trapped in the atmosphere, leading to global warming.

Why CO2 is the Primary Driver of Global Warming

Studies show that CO2 has contributed more than any other driver to recent global warming. Below are the main reasons why CO2 is the leading contributor:

CO2’s Role in Radiative Forcing

Radiative Forcing (RF):

• Radiative forcing refers to the change in the Earth’s energy balance due to the presence of greenhouse gases. The Intergovernmental Panel on Climate Change (IPCC) has studied the heating effects of different climate drivers, including GHGs, aerosols, and land use changes.
• Between 1750 and 2011, CO2 was found to have the highest positive radiative forcing, meaning it had the greatest warming effect on Earth compared to other GHGs and drivers.

CO2’s Prevalence in the Atmosphere

• While gases like methane (CH4) and hydrofluorocarbons (HFCs) are far more potent than CO2 in trapping heat (e.g., methane is 80 times more powerful than CO2), their atmospheric concentrations are much lower.
• CO2 is much more abundant in the atmosphere. Since the onset of the Industrial Revolution, human activities have increased atmospheric CO2 levels by 50%, making CO2 responsible for about 70% of global warming.

CO2’s Longevity in the Atmosphere

Persistence of CO2:

• Unlike other greenhouse gases such as methane, CO2 remains in the atmosphere for a much longer time.
• It takes about 10 years for methane to convert into CO2, and centuries for other gases like nitrous oxide (N2O) to break down.
• According to the Union of Concerned Scientists (UCS), 40% of CO2 remains in the atmosphere for 100 years, and 20% stays for over 1000 years, while 10% can stay for up to 10,000 years.
• This prolonged presence in the atmosphere ensures that CO2 continues to have a significant warming effect over long periods.

Comparing CO2 to Other Greenhouse Gases

While other gases such as methane (CH4) and HFCs are more potent, CO2’s concentration and longevity make it the primary driver of climate change.

Methane and Other GHGs

Methane’s Potency:

• Methane (CH4) is about 80 times more powerful than CO2 in trapping heat over a 20-year period. However, it is much less abundant and has a much shorter atmospheric lifetime of about 10 years.
• Once methane breaks down, it eventually turns into CO2, meaning its long-term impact on warming is similar to CO2, but it does not stay in the atmosphere as long.

Hydrofluorocarbons (HFCs):

• HFCs are synthetic gases that are even more potent than methane, and some are thousands of times more powerful than CO2. However, they are used in very small quantities, so their contribution to warming is limited compared to CO2.

Water Vapor’s Role

• Water vapor is the most abundant GHG in the atmosphere. However, it does not contribute to global warming in the same way as CO2 because it has a short cycle (about 10 days) and does not accumulate like CO2.
• However, as global temperatures rise due to increased CO2, more water evaporates into the atmosphere, creating a feedback loop that intensifies warming.

Recent Trends in CO2 Emissions

Global CO2 Emissions

• In 2024, global fossil-based CO2 emissions are expected to reach a record high of 37.4 billion tonnes, a 0.8% increase from 2023.
• India, among major economies, has seen the largest increase in CO2 emissions, projected to rise by 4.6%.

 Impact of CO2 on Global Temperature Rise

• The continued rise in CO2 emissions has significant implications for global warming. According to the Global Carbon Project, the ongoing rate of emissions could result in a 50% chance that global temperatures will consistently exceed 1.5°C above pre-industrial levels in about six years.
• This temperature increase exceeds the Paris Agreement’s target of limiting global warming to below 1.5°C.