The article is relatively long, so the author's division of the content into sections presents the information in manageable parts. Each section has a heading that specifies the content of the section; the headings also serve to better organize the key ideas. Without these section headings, some readers might lose track of how the article progresses.
The use of sections also helps to structure the article into a logical progression. The article starts with observed facts and expert comments on the facts to establish the background—that the Arctic ice is melting. It then proceeds to the immediate effects (the rising sea level). Finally, it discusses the long-term effects that might occur.
The division into sections with headings increases the chances that readers will understand the multiple aspects of the article. Also, starting with facts that most readers will not doubt and then shifting to potential future effects increases the chances that readers will accept the underlying message of the article—that the melting of the Arctic ice is harmful.
