Why CO2 is the primary driver of climate change

India’s carbon dioxide (CO2) emissions from burning fossil fuels are expected to increase by 4.6% in 2024, the highest among major economies, according to a new report by Global Carbon Project, an organisation that quantifies the world’s greenhouse gas (GHG) emissions.

Globally, fossil-based CO2 emissions are set to touch a record high of 37.4 billion tonnes this year, a rise of 0.8% from 2023, the report said. At this rate, there is “a 50% chance global warming will exceed 1.5 degree Celsius consistently in about six years”.

CO2 is one of the most important GHGs in the atmosphere and is the primary driver of anthropogenic climate change.

Here is a look at why CO2 has caused most of the global warming.

But first, what are greenhouse gases?

GHGs are those gases in the Earth’s atmosphere that trap heat. The Sun emits shortwave radiation or sunlight that passes through the atmosphere and is absorbed by the planet’s surface to warm it. However, some of this sunlight is reflected back by the surface as infrared radiation (heat) which has a longer wavelength.

GHGs such as CO2 and methane (CH4), which cannot absorb shortwave radiation, trap infrared radiation. That is because unlike oxygen or nitrogen molecules, CO2 and methane are made up of three or more atoms, which gives them a larger variety of ways to stretch and bend and twist. This means that they can absorb a wider range of wavelengths, including infrared radiation, Jason Smerdon, a climate scientist at Columbia University, told State of the Planet, a news site of the Columbia Climate School.

Simply put, GHGs act like a blanket that envelopes Earth and insulates it from the cold of space. This process of maintaining a warmer temperature is called the greenhouse effect. GHGs like CO2, CH4, and water vapour occur naturally and are a boon for the planet as in their absence there would not be the greenhouse effect without which there would not be liquid water and any form of life.

The issue is not the presence of GHGs but their concentration levels, meaning their amount in the atmosphere. For about a thousand years before the Industrial Revolution, the GHG concentration remained relatively constant. Since then, there has been a constant rise in the amount of some of the GHGs, especially CO2, due to human activities like burning fossil fuels. As a result, more heat is getting trapped in the atmosphere, leading to a rise in global temperatures.

Why has CO2 caused most of the global warming?

Studies have shown that CO2 has contributed more than any driver to climate change. In fact, CO2 is responsible for about 70% of global warming, according to an analysis by Facts on Climate Change, a Czech Republic-based independent think tank.

In 2013, the Intergovernmental Panel on Climate Change (IPCC) — the United Nations body that advances scientific knowledge about climate change — compared the “radiative forcing” (RF) or heating effect of three different climate drivers: GHGs, aerosols, and land use change. It was found that between 1750 and 2011, CO2 had the highest positive RF, meaning it had the greatest warming effect on the planet.

The analysis also showed that other GHGs such as CH4 or Hydrofluorocarbons (an entirely human-made GHG) which are much more potent — while CH4 is around 80 times more powerful than CO2, HFCs can be thousands of times more powerful — had less heating effect than CO2.

There are two reasons for this. One, CO2 is much more abundant in the atmosphere compared to CH4 and HFCs. Since the onset of industrial times in the 18th century, human activities have raised atmospheric CO2 by 50%, which means the amount of CO2 is now 150% of its value in 1750 (see chart), according to a report in the National Aeronautics and Space Administration (NASA).

Two, CO2 remains in the atmosphere longer than the other major GHGs emitted due to human activities. It takes about a decade for CH4 emissions to leave the atmosphere (it converts into CO2) and about a century for nitrous oxide (N2O).

“After a pulse of CO2 is emitted into the atmosphere, 40% will remain in the atmosphere for 100 years and 20% will reside for 1000 years, while the final 10% will take 10,000 years to turn over,” a report by the Union of Concerned Scientists (UCS), a US-based non-profit organisation, said.

Notably, water vapour is the most abundant GHG in the atmosphere. However, it has a short cycle (10 days on average) and does not build up in the atmosphere in the same way as CO2 does. Therefore, water vapour does not have much heating effect compared to CO2. But as global temperatures continue to soar, more water evaporates into the atmosphere, causing more warming.