Correct Option
The correct option is 1998.
Explanation
Global mean surface temperatures are influenced by long-term anthropogenic warming trends and short-term natural climate variability, most notably the El Niño-Southern Oscillation (ENSO).
Analysis:
- 1998: This year is statistically recorded as the warmest year of the 20th century. The extreme global temperatures were driven by the "Super El Niño" event of 1997-1998, which released significant heat from the Pacific Ocean into the atmosphere, amplifying the background trend of global warming.
- Other Options: While the 1990s was the warmest decade of the 20th century, years such as 1990, 1995, and 1999 did not reach the peak temperature anomalies observed in 1998.
Key Takeaway: The year 1998 holds the record for the highest global average temperature in the 20th century, primarily due to a historically strong El Niño event.