Based on the context and the answer, generate a question. 

Context: The last time I checked my carbon budget , I came in at a fifth of the national average . I rarely fly , even when booked to address faraway audiences about my personal obsession , climate change - a subject I ' ve covered in three books . Whenever the word " nuclear " comes up at my talks , a shudder runs through the room . Because everyone knows that real environmentalists loathe nuclear power .

Answer:

he spends carefully because he knows about carbon 's effects on the environment



why did he come in at a fifth of the national average for his carbon budget ?