你的句子是倒装句。原因是“only+条件状语”被放在了句首。如果不倒装应该是：
We can have the hope to reduce their “speed, severity and harm” only if we discuss the consequences of our biophysical limits, the December warning letter says.
又如：
Only if the red light comes on is there any danger to employees. 只要红灯一亮，就表示有危及职工的险情。
Only if a teacher has given permission is a student allowed to leave the room. 学生只有得到老师的许可才能离开教室。
Only by shouting was he able to make himself heard. 他只有叫喊才能让别人听到他。
Only when the war was over could he go on studying. 只是在战争结束后他才得以继续学习。