A paragraph may have any number of sentences, but must develop one main idea. It can be thought of as a composition in miniature. It is usually developed in much the same way as a composition: by examples, comparisons and contrasts, explanations (or definitions), or arguments (or reasons) that make the meaning of the topic sentence clear.

When you are writing, you must be clear. You must be sure that you are clear within a sentence and that your antecedents and pronouns logically link to one another. You must have logical links between sentences so that they have a flow. Your sentences need to be linked together by one common topic so that your paragraph will be unified. Once you can do all of this, you need to make sure that your paragraphs flow together from one into another so that your entire paper is cohesive. Unfortunately, there is no short cut to learning this process. It is only through practice and attention to detail that you can make the logical and solid papers that you desire.

The topic sentence is: Climate change and renewable energy sources.
In the second paragraph, a change of focus (in point of view) on renewable energy sources has been used.

Paragraph 1: Climate change impacts pose a global threat since a report carried out by the IPCC (Intergovernmental Panel on Climate Change) warns that global warming may be irreversible. The importance of renewable energy solutions in mitigating climate change has been proven: the actual dependence on fossil fuel energy sources highly contributes to global greenhouse gas emissions, the leading cause of global warming.

Paragraph 2: However, such alternative energy sources, because of their intermittency, (in the sense that, for instance, the wind doesn't always blow nor does the sun always shine) can’t be that efficient. Therefore, world leaders should join efforts to set up enhanced environmental policies so as to tackle global warming consequences.