By some estimates, the annual average extent of Northern Hemisphere sea ice has ________ since the 1950s.

The annual average extent of Northern Hemisphere sea ice has halved since the 1950s due to various factors related to climate change, such as rising global temperatures and changing oceanic currents. Scientific observations and satellite data indicate a significant decline in sea ice extent, especially during the summer months when ice melts more rapidly. This reduction is critical not only for the native ecosystems that rely on the ice for habitat and hunting grounds but also for global climate systems, as sea ice plays a role in regulating Earth's temperature by reflecting sunlight. The ongoing loss of sea ice is a stark indicator of the impacts of climate change and highlights the urgent need for action to address environmental challenges.