But science does provide us with the best available guide to the future, and it is critical that our nation and the world base important policies on the best judgments that science can provide concerning the future consequences of present actions.
解析:
并列句。第一個分句主干:science does provide us。其中with the best available guide to the future為狀語修飾provide,the best available和to the future分別為guide的前置定語和后置定語,由此本句賓語被復雜化; 第二個分句主干:it is critical that...,其中,it為形式主語,that從句為真正的主語,從句主語為名詞短語our nation and the world, base...on...為固定搭配,第二個that從句為judgments的定語從句,分詞結構concerning...為judgments的后置定語。