This book presents a reasonable conclusion but reaches it through a weak argument. In a nutshell the conclusion is that technological fixes for climate change are a bad idea. Nothing wrong with that conclusion, after all it would be difficult to find anyone who would reasonably argue that it's not better to avoid a problem rather than fix it after things have gone wrong. What's wrong with this book is that Hulme's argument is not an argument but an assertion masquerading as an argument. He is far too prone to asserting something in one chapter only to then refer to it in a later chapter as something he has shown or even proven. He may have asserted it, it may have been a reasonable assumption and there may be good evidence to agree with that assumption but he has not argued that it is true and certainly not proven his case.
His conclusions may be valid and it is certainly better to avoid the problem rather than to try and fix it afterwards but his argument is weak.