Final answer:
The melting of polar ice sheets due to global warming leads to rising sea levels due to both added meltwater and the expansion of warmer seawater, posing threats to coastal regions.
Step-by-step explanation:
The melting of polar ice due to increasing average temperatures of the Earth's atmosphere has several significant effects. One expected effect is the phenomenon of rising sea levels, which occurs not only due to the addition of meltwater from polar ice sheets and glaciers into the oceans but also because water expands as it warms. This contributes to coastal erosion, loss of coastal ecosystems, and poses risks to human communities living in low-lying coastal areas and small island nations. The other options listed (decreased global temperature, increased polar ice formation, and reduced ocean acidity) are not consistent with the effects of increasing temperatures and polar ice melt.