Which piece of evidence comes from an unreliable source?

To determine which piece of evidence comes from an unreliable source, we need to evaluate the credibility of each individual quoted in the text regarding scientific claims about global warming.

The text introduces three main figures:

• Bill McKibben: Described as "a leading environmental scientist." His statement, "When you are in a hole, stop diggin'!", implies that global warming is real and action is needed. As a leading environmental scientist, he is presented as a reliable source on this topic.
• David Vitter: Described as "a senator from Louisiana." He "calls scientists evidence 'ridiculous pseudo-science garbage'." A senator is a political figure, not a scientific expert. His dismissal of scientific evidence as "ridiculous pseudo-science garbage" makes him an unreliable source for scientific claims.
• Gerrit van der Lingen: Described as "a Dutch paleoclimatologist, or a scientist who studies ancient weather patterns." He stated "the world has not been warming any more since 1998." The text also notes he "echoed Vitter's view." While he is a scientist, the fact that he "echoed Vitter's view" (who is clearly presented as dismissive of scientific evidence) and makes a specific claim that is often used by climate change skeptics, suggests his statement might be biased or controversial within the scientific community. However, he is still identified as a scientist.

Now let's analyze the options:

• Option 1: "Global warming is 'not a belief system. We know that beyond a shadow of a doubt.'" This statement is not attributed to any specific person in the provided text. Therefore, we cannot evaluate its source based on the given information.

• Option 2: "Global warming is a hole we are in and 'when you are in a hole, stop diggin!'" This is a direct quote from Bill McKibben, who is identified as "a leading environmental scientist." A leading environmental scientist is generally considered a reliable source for information on global warming.

• Option 3: "Global warming is not true because 'the world has not been warming any more since 1998.'" This is a direct quote from Gerrit van der Lingen, who is identified as "a Dutch paleoclimatologist, or a scientist who studies ancient weather patterns." While he is a scientist, the text notes he "echoed Vitter's view," which aligns him with a skeptical perspective. However, he is still a scientist making a scientific claim.

• Option 4: "Global warming is not real, but rather 'ridiculous pseudo-science garbage.'" This is a direct quote from David Vitter, who is identified as "a senator from Louisiana." A senator is a political official, not a scientific expert. His statement dismisses scientific evidence as "ridiculous pseudo-science garbage," which indicates he is an unreliable source for scientific information.

Comparing the options, David Vitter, a senator, is the most clearly unreliable source for scientific claims about global warming because he is not a scientist and explicitly dismisses scientific evidence as "ridiculous pseudo-science garbage."

The final answer is $\boxed{Global warming is not real, but rather 'ridiculous pseudo-science garbage.'}$