The statistic you mentioned—that global warming causes more than 300,000 deaths annually—is often associated with reports from organizations like the Climate Vulnerability Monitor or similar studies that assess the impacts of climate change on human health and mortality.

However, it's important to note that this number can vary depending on the methodology and time frame considered. Other sources might provide different figures. The World Health Organization (WHO) has estimated that between 2030 and 2050, climate change is expected to cause approximately 250,000 additional deaths per year, from malnutrition, malaria, diarrhea, and heat stress alone.

Factors contributing to these deaths include:
1. **Extreme Weather Events**: Heatwaves, storms, and floods.
2. **Changes in Disease Patterns**: Increased spread of vector-borne diseases.
3. **Agricultural Impact**: Reduced crop yields leading to food shortages and malnutrition.
4. **Water Scarcity**: Leading to poor sanitation and hygiene.

These estimates underscore the urgent need for global action to mitigate climate change and adapt to its unavoidable impacts.