Climate change means long‑term changes in Earth’s climate, especially global warming. The main reasons are:

### 1. Greenhouse gases from human activities  
These are the dominant cause of current climate change.

Key gases:
- **Carbon dioxide (CO₂)** – from:
  - Burning fossil fuels (coal, oil, gas) for electricity, heat, transport, industry  
  - Deforestation (trees that used to store carbon are cut and often burned)  
  - Cement production
- **Methane (CH₄)** – from:
  - Livestock (cows, sheep) digestion and manure  
  - Rice paddies  
  - Landfills  
  - Leaks from oil, gas, and coal operations
- **Nitrous oxide (N₂O)** – from:
  - Nitrogen fertilizers  
  - Some industrial processes  
  - Burning of biomass and fossil fuels
- **Fluorinated gases** – synthetic chemicals from refrigeration, air‑conditioning, and industry; small in amount but extremely powerful at trapping heat.

These gases build up in the atmosphere and **enhance the natural greenhouse effect**, trapping more heat and raising global temperatures.

### 2. Land use changes  
- **Deforestation** reduces the number of trees that absorb CO₂.  
- **Agriculture** (especially intensive farming) increases methane and nitrous oxide.  
- **Urbanization** (cities, roads, concrete) changes how land absorbs and reflects heat and water, contributing to local warming and altering regional climate patterns.

### 3. Aerosols and air pollution  
- Some air pollutants (sulfates) **cool** the climate by reflecting sunlight.  
- Others (black carbon/soot) **warm** the climate by absorbing sunlight and darkening snow and ice.  
Human activity has changed the balance of these particles, slightly masking some warming while also causing regional changes to rainfall and cloud formation.

### 4. Natural factors (minor for recent warming)  
These affect climate but do **not** explain the rapid warming since the mid‑20th century:

- **Volcanic eruptions** – can temporarily cool the planet by putting reflective particles in the stratosphere.  
- **Solar variability** – the Sun’s output changes slightly over time, but measurements show no long‑term increase that matches recent warming.  
- **Natural cycles** (e.g., El Niño/La Niña, ocean oscillations) – shift heat between ocean and atmosphere, causing year‑to‑year and decadal variations, but not the long‑term upward trend.

### 5. Why scientists say humans are the main cause  
- The **pattern** of warming (faster over land, more at high latitudes, warming lower atmosphere but cooling upper atmosphere) matches what models predict from greenhouse gas increases.  
- Climate models that include **only natural factors** cannot reproduce the observed warming; models that include **human emissions** can.  
- Isotopic “fingerprints” in atmospheric CO₂ show it mainly comes from burning fossil fuels and deforestation.

In summary:  
Climate has always changed, but the **rapid warming over the last ~150 years is overwhelmingly driven by human emissions of greenhouse gases and land‑use changes**, with natural factors playing a much smaller role in the recent trend.
