Although these molecules allow radiation at visible wavelengths, where most of the energy of sunlight is concentrated,to pass through,they absorb some of the longer-wavelength,infrared emissions radiated from the Earth's surface,radiation that would　otherwise be transmitted back into space. (4)

虽然这些分子允许可见波长(visible wavelength)的辐射——阳光的绝大部分能量就汇集于此——不受阻挡地穿透，但它们却会吸收某些较长波长(longer- wavelength)，亦即从地球表面辐射出的红外发射(infrared emission)，这种辐射若不是二氧化碳的缘故就会被重新输送回太空。

难句类型：

复杂修饰、抽象词

解释：

前面的分句中有一个不算很长的插入语，但是因为它插入的位置正好在固定搭配allow something to do somthing中间，将allow和to分得很远，所以读起来让人感觉很不舒服。

后面的分句中的最后一行radiation that would otherwise be transmitted back into space是其前面的infrared emissions的同位语。其中的otherwise是副词作状语，表示如果后面的分句所说的they absorb some of the longer-waverlength, infrared emissions不发生时的后果。

意群训练：

Although these molecules allow radiation at visible wavelengths, where most of the energy of sunlight is concentrated, to pass through, they absorb some of the longer-wavelength， infrared emissions radiated from the Earth's surface,radiation that would　otherwise be transmitted back into space.