Gas molecules absorbing thermal infrared radiation and present in large quantities to change the climate system are known as greenhouse gases. These gases play a crucial role in the Earth's climate system by trapping heat from the sun and preventing it from escaping back into space. This phenomenon is commonly referred to as the greenhouse effect.

Here is a detailed explanation of why greenhouse gases are the correct answer:

1. Greenhouse Effect:
- The greenhouse effect is a natural process that helps regulate the Earth's temperature.
- Sunlight passes through the Earth's atmosphere and heats up the surface.
- The Earth then radiates some of this heat back into space in the form of thermal infrared radiation.

2. Greenhouse Gases:
- Greenhouse gases are specific gases in the atmosphere that can absorb and re-emit thermal infrared radiation.
- These gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor (H2O).
- They are called greenhouse gases because they function like the glass in a greenhouse, allowing sunlight to enter but trapping the heat inside.

3. Absorption of Thermal Infrared Radiation:
- When greenhouse gases absorb thermal infrared radiation, they become energized.
- This energy is then re-emitted in all directions, including back towards the Earth's surface.
- As a result, the Earth's surface receives additional heat energy, leading to an increase in temperature.

4. Climate Change:
- Human activities, such as burning fossil fuels and deforestation, have significantly increased the concentrations of greenhouse gases in the atmosphere.
- This enhanced greenhouse effect is causing the Earth's temperature to rise, resulting in climate change.
- Climate change leads to various impacts, including rising sea levels, extreme weather events, and disruptions to ecosystems.

In conclusion, greenhouse gases are molecules in the Earth's atmosphere that absorb and re-emit thermal infrared radiation, thereby trapping heat and contributing to the greenhouse effect. Their presence in large quantities due to human activities is causing climate change and its associated consequences.