This is the logical fallacy known as "recourse to authority". It says that you should believe something, not because the writer has presented any actual evidence to back it up, but simply because some expert says it is so.
If the expert believes that something is true because he has evidence to back up his assertion, then he should present the evidence. In that case you will be convinced by the evidence (assuming that the evidence is, in fact, convincing), and not by the fact that an expert said so.
If the expert does not have any evidence, then his opinion is no more well-informed than any random person's off the street.
Either way, you should be convinced by the evidence, not the fact that an expert said so.
In recent years I've heard some try to justify this sort of flawed argument by renaming the fallacy, "recourse to INAPPROPRIATE authority", and claiming that if the authority has the right credentials, that you should believe them. A little thought will show that this is a hollow distinction. OF COURSE anyone who says that you should believe something because an expert said so will claim that the expert's credentials make him authoritative. Who would say, "Bill Smith says that X is true. Of course Bill Smith knows absolutely nothing about the subject, but you should just believe him anyway." They're always going to say that Bill Smith is an expert on the subject. People who make this argument are trying to divert you into an argument about Bill Smith's credentials instead of the subject at hand. But even if Bill Smith has a master's degree in a relevant field and has worked in this field for 30 years, if he has no evidence to back up his claims and is just tossing out unsubstantiated opinion, what difference does his degree and experience make?
I'm not going to get into the debate about global warming per se here, that's a whole 'nother subject. But to embrace the analogy:
If someone told me that 97% of scientists say this plane is going to crash, I would likely ask who this person is who is telling me and who the 97% of scientists are.
How did he get this statistic? Did he really poll an unbiased collection of relevant scientists? It's quite possible that the statistic is from a biased poll, or that he just made it up. On controversial, politically charged subjects, that's not uncommon.
What is his definition of a "scientist"? Everyone with a PhD in science? But what would someone with a degree in biology or astronomy know about the reliability of an airplane? Their opinion is unlikely to be any more informed than any random person off the street. They're probably just repeating what they've heard from others.
Often when you hear statements that some large majority of experts agree that X, it turns out that they are defining X to only include people who agree with them. Like, "We asked the scientists who work for Foobar Airlines, and 97% of them agreed that planes from Whatsit Airlines are dangerous and will probably crash." Or, "We asked scientists who really know about the dangers of air travel -- that is, the scientists who are members of the Man Will Never Fly Society -- and 97% agreed, etc."
Experts are human beings, too. They can be biased by their political or social beliefs. A scientist who makes his living studying X is unlikely to say that X is totally bogus. And scientists who have no specific knowledge of the subject in question can fall into the "everybody knows that ..." trap just as easily as the general public.
For example, a number of years ago the president at the time called a conference of economists to evaluate his economic plan. And I saw many stories in the news about how all the economists who attended the conference agreed that the president's plan was basically a good idea, the only question was whether it went far enough. Well, duh. They said that like that proved something. But surely he didn't invite economists who he knew would say he was going in the wrong direction.