Greenhouse gases (GHGs) emitted by human activity (also referred to as ‘anthropogenic emissions’) have increased substantially since the industrial revolution, particularly in recent decades.  

Although the Earth’s climate has been evolving for millions of years, research shows it’s extremely likely that these emissions have led to global warming in a short space of time. This in turn is causing climate change.  

This Insight gives an overview of the basic concepts underlying GHG emissions, global warming and climate change, with references to further information and underlying research. The Insight UK and global emissions and temperature trends provides information on the data discussed here.  

What is a greenhouse gas? 

The Earth’s atmosphere is made up mainly of nitrogen, oxygen and argon (78.1%, 20.9% and 0.934% in dry air, respectively), as well as a mixture of other gases at much lower concentration.  

When energy from sunlight reaches the Earth, much of it is absorbed by land or the oceans, heating the surface. Over time, the heated surface releases this energy. Some of it is absorbed by greenhouse gases, preventing it from leaving the atmosphere. 

GHGs are released during many day-to-day activities, such as driving petrol cars or heating homes, as well as industrial production. GHGs trap heat in the atmosphere. Carbon dioxide (CO2) is the most dominant GHG as it is emitted in greatest quantity by human activity – primarily through fossil fuel burning. CO2 tends to remain in the atmosphere for hundreds of years.  

Other GHGs emitted by human activity have a stronger greenhouse effect than CO2. However, these either naturally break down in the atmosphere more quickly (such as methane, which takes a few decades) or are emitted in small quantities (such as nitrous oxide, and some of the man-made chemicals known as F-gases). While water vapour is the most abundant GHG, it only lasts for a few days before returning to the surface as precipitation. 

Human emissions tip the natural balance 

Greenhouse gases have always been an integral part of the atmosphere. Although GHGs currently only make up a small fraction (less than 1%, excluding water vapour) they play an important role in retaining heat from the sun and ensuring we have a liveable planet.  

Many natural processes absorb or emit CO2. For example, photosynthesis absorbs CO2 and decomposing organic matter releases it. In total, roughly the same amount of CO2 is emitted and removed by natural processes. This ‘carbon cycle’, in which emissions are balanced by removals, has kept the atmospheric concentration of CO2 stable for many thousands of years. 

However, emissions of CO2 resulting from human activity (and other GHGs, particularly methane), have risen sharply since the industrial revolution. This tips the carbon cycle out of balance and increases the atmospheric concentration. This is despite the fact that human emissions of CO2 are approximately 20 times smaller than natural emissions. 

What is global warming? 

The atmospheric concentration of CO2 is now at its highest in several million years, and research shows that the heat trapped by this and other GHGs is increasing the average global temperature. This increasing temperature trend is known as global warming.  

Other factors such as changes in the Earth’s orbit and volcanic eruptions also affect the global climate, but research has shown it is extremely likely these factors alone cannot account for the warming that has taken place, without also considering human effects. 

There has been 1.0°C of global warming since pre-industrial times. This is a clear warming trend shown in measurements averaged over decades and geographic areas. Since records began in 1884, all ten of the UK’s ten warmest years have occurred since 2002. Globally, the past five years have been the warmest of the last 140 years

What is climate change? 

Climate change refers to the changes in global weather patterns driven by global warming.  

Increasing global temperature has widespread effects on natural systems over land and oceans. While weather varies locally from day to day, when looking at long-term trends over large geographical areas, there are statistically noticeable shifts in weather patterns.  

Climate change has been observed in patterns of temperature, humidity and rainfall, as well as in the frequency or intensity of extreme weather events. These are complex effects that are influenced by many natural and human factors, making it difficult to precisely predict how they will develop in future.  

Warmer air holds more moisture, so rainfall is increasing on average across the planet, but there is a lot of regional variation. Generally, over the course of the 21st century wet areas are projected to become wetter, and dry areas to become drier. This also means that the intensity of droughts and heavy rainfall is likely to increase, along with the impacts on food security and other human systems that this entails. 

Temperature limits and global emissions 

Under the Paris Agreement, nearly all governments worldwide agreed to limit global warming to between 1.5 and 2.0°C by the end of the century. The Intergovernmental Panel on Climate Change estimates that to limit global warming to 1.5°C, global emissions need to be roughly halved by around 2030 (compared to 2018) and reach net zero around 2050.  

Net zero refers to a situation in which emissions reduce to almost zero, and any remaining emissions are removed from the atmosphere. The UK Government has a goal of net zero GHG emissions by 2050. 

Further reading 

About the author: Jack Miller is Energy Adviser at the Parliamentary Office of Science & Technology (POST)