Which of the following is a serious direct impact of climate change?

The increase in average sea level is a serious direct impact of climate change primarily due to the thermal expansion of seawater and the melting of glaciers and polar ice caps. As global temperatures rise, glaciers melt, contributing more water to the oceans, while warmer water occupies more space and results in sea level rise. This rise poses significant risks to coastal communities, ecosystems, and infrastructure due to the potential for flooding and erosion.

In contrast, while the destruction of coral reefs is a significant environmental concern linked to climate change, it is often considered an indirect impact, as it arises from multiple factors including ocean warming, acidification, and pollution. The possibility of higher crop yields in temperate zones may occur in some scenarios but is not a universally beneficial outcome; it is heavily dependent on various factors, including water availability and extreme weather events. Lastly, a reduction in global temperature variation does not reflect a serious impact; instead, increased variation due to climate change can lead to more extreme weather conditions. Thus, the increase in average sea level stands out as a clear, direct consequence of climate change.