Choose a, b, c, d, or e for the correct answer.

    In recent years, global warming has been the subject of a great deal of political controversy. As scientific knowledge has grown, this debate is moving away from whether humans are causing warming and toward questions of how best to respond.

    Signs that the Earth is warming are recorded all over the globe. The easiest way to see increasing temperatures is through the thermometer records kept over the past century and a half. Around the world, the Earth's average temperature has risen more than 1 degree Fahrenheit (0.8 degrees Celsius) over the last century, and about twice that in parts of the Arctic.

    This doesn't mean that temperatures haven't fluctuated among regions of the globe or between seasons and times of day. However, if you average out the temperature all over the world over the course of a year, you see that temperatures have been creeping upward.

Source: http://environment. nationalgeographic. com/environment/ globa/-warminglgw-rea
 

In line 8, the word "this" refers to ____