They are a group of gases that due to their molecular structure, absorb heat when present in the atmosphere.
The primary greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2), and ozone (O3).
Greenhouse gases, when present in the atmosphere, trap heat energy that is reflected by the earth's surface. This is not the case for other gases such as oxygen or nitrogen.
This trapped energy then causes extra warming on the earth’s surface. This effect is known as the “greenhouse effect”.
Each greenhouse gas is unique, traps different amounts of energy, and lasts for a different amount of time. Hence each gas contributes a different amount to global warming.
In order to estimate the overall impact of different greenhouse gases, each greenhouse gas is converted into the equivalent amount of carbon dioxide emissions.
This enables the discussion on greenhouse gases to be condensed down to a single number, measured in carbon dioxide equivalent or CO2e.
Before the industrial revolution, for every million molecules of air in the atmosphere, there were 283 molecules (known as parts per million or ppm) of CO2e. This is now up to more than 500 ppm in the present day.
In 2019, global CO2e emissions were 59 Gigatonnes of carbon dioxide equivalent. 
This is approximately equivalent to adding 3.4 ppm of CO2 each year. 
The main drivers of greenhouse gas emissions are the burning of fossil fuels, land-use change, and the release of methane, nitrous oxide, and fluorinated gases into the atmosphere.
The main effect of greenhouse gases so far has been to increase average global temperatures by 1 degree Celsius since 1880.
Greenhouse gases will continue to increase average temperatures as well as contribute to more extreme weather, increased natural disasters, sea level rises, and the melting of polar ice.
Scientists have conducted extensive modeling to predict what the effect of increased greenhouse gases in the atmosphere will be.
The current scientific consensus is that 1.5C is the maximum temperature that should be allowed without even more severe consequences.
Limiting temperature rises to 1.5C will require a dramatic and immediate decrease in the amount of greenhouse emissions being emitted each year.
Net-zero is the net amount of emissions to the atmosphere is zero. This can be achieved by balancing emissions of greenhouse gases with their removal (often through carbon offsetting) or by eliminating emissions altogether.
Many different proposals have been put forward to reduce emissions. John Doerr, in his book Speed & Scale, condensed the plan to the following actions:
- Electrify transportation
- Decarbonise the grid
- Fix Food
- Protect Nature
- Clean up industry
- Remove Carbon