Presently, the main cause of global warming is

Options

A. Increasing `CO_2` concentration
B. Depletion of ozone layer
C. Increased energy production by sun
D. The earth getting closer to sun

Explanation

### Step-by-Step Solution:

1. **Understanding Global Warming**:
- Global warming refers to the long-term increase in Earth's average surface temperature due to human activities, primarily the emission of greenhouse gases.

2. **Identifying Causes of Global Warming**:
- There are several factors that contribute to global warming, including:
- Increasing carbon dioxide (CO2) concentration
- Depletion of the ozone layer
- Increased energy production by the sun
- The Earth getting closer to the sun

3. **Evaluating Each Cause**:
- **Increasing Carbon Dioxide Concentration**:
- CO2 is a greenhouse gas that traps heat in the atmosphere. Its concentration has been rising due to activities like burning fossil fuels, deforestation, and industrial processes.
- **Depletion of Ozone Layer**:
- While the ozone layer protects the Earth from harmful UV radiation, its depletion does not directly cause global warming.
- **Increased Energy Production by the Sun**:
- Solar energy output does vary, but current scientific consensus indicates that this is not a primary driver of recent global warming.
- **Earth Getting Closer to the Sun**:
- The Earth’s orbit does change slightly over long periods, but this is not a significant factor in current global warming trends.

4. **Conclusion**:
- The main cause of global warming is the increasing concentration of carbon dioxide in the atmosphere. This is primarily due to human activities that release CO2, leading to the greenhouse effect and an increase in global temperatures.

### Final Answer:
The main cause of global warming is the increasing carbon dioxide concentration in the atmosphere.

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