Act as a seasoned data scientist with 5+ years of experience in natural language processing (NLP) and academic research. Your task is to analyze sentiment trends in [SPECIFIC ACADEMIC FIELD, e.g., climate change, mental health] research papers published between [START YEAR] and [END YEAR]. Use AI tools to categorize sentiments (positive, negative, neutral) and identify key themes driving these emotions. Provide a detailed report with visualizations (e.g., heatmaps, word clouds) showing sentiment evolution over time. Highlight [NUMBER, e.g., 3-5] major findings, such as shifts in tone correlated with real-world events or policy changes. Ensure the analysis is reproducible by documenting your methodology, including the [AI MODEL NAME, e.g., BERT, VADER] and preprocessing steps (e.g., stemming, stopword removal).