请生成一篇500字的英语文章，主题是全球变暖对极地生态的影响。
As the Earth's temperature continues to rise, the effects of global warming are becoming increasingly apparent in polar ecosystems. The poles are particularly sensitive to changes in temperature due to their delicate balance of ice, water, and wildlife. This essay will explore the impact of global warming on polar ecosystems.

One of the most significant effects of global warming on the poles is the melting of sea ice. In the Arctic, sea ice has declined by approximately 13.1% per decade since the late 1970s. As a result, species that rely on the ice, such as polar bears and walruses, are struggling to find food and habitat. Polar bears, for example, hunt on the ice for seals, but with less ice available, their hunting grounds are shrinking. Walruses use the ice to rest and nurse their young, but without it, they are forced to come ashore where they are more vulnerable to predators.

The melting of ice is also affecting the oceans' circulation systems, which could have far-reaching consequences for global weather patterns. As melted freshwater enters the ocean, it reduces the salinity, which affects the ocean currents that help regulate the Earth's climate. This could lead to more extreme weather events, such as hurricanes and droughts.

Another impact of global warming on polar ecosystems is the acidification of the ocean. As the ocean absorbs carbon dioxide from the atmosphere, the pH level decreases, making it more acidic. This is particularly concerning for animals with calcium carbonate shells, such as mollusks and corals, who are struggling to form and maintain their protective shells. This, in turn, affects the entire food chain, as these animals are an important food source for many aquatic species.

Finally, global warming is causing the thawing of permafrost, which affects land-based ecosystems in the Arctic. Permafrost is a layer of frozen soil that acts as a natural freezer, preserving organic materials and regulating the release of carbon into the atmosphere. But as the permafrost melts, the organic material decomposes, releasing large amounts of carbon into the atmosphere, contributing to further global warming.

In conclusion, global warming is having a significant impact on polar ecosystems, affecting species that rely on ice, altering ocean currents and affecting the food chain, and releasing large amounts of carbon into the atmosphere. Urgent action is needed to address global warming and prevent further damage to these delicate ecosystems.