阅读下列短文，根据文内容填空，在未给提示词的空白处仅填写1个适当的单词，在给出提的空白处用括号内所给词的正确形式填空。

Then what is causing the increase in the global average surface temperature? Climate scientists often mention a key climate process【小题1】(call) the “greenhouse effect”, which has two common meanings: the “natural” greenhouse effect and the “man- made” greenhouse effect. The “natural” greenhouse effect 【小题2】(refer)to the fact that heat from the sun enters the atmosphere and warms Earth’s surface as short-wave radiation. The heat is released back into space at longer wave lengths. Greenhouse gases in the atmosphere, such as methane carbon dioxide, trap some of the heat, 【小题3】 (keep) Earth’s climate warm and habitable.

【小题4】 this process, Earth could not sustain life. However, the “man-made” greenhouse effect has now become a big problem.