Can someone explain to me
Sure!
Here we go.
Northwest Passage
Two points here: Firstly, they thought sure as anything they would find a passage between North and South America as well, and what do you know? Just because it is thought something may exist doesn't mean that it does.
The NW Passage does indeed exist, and has been impassable without icebreakers until last year. The first time it was traversed that we know of was in the early 1900s, and that was due to greater knowledge of the area and the newer and easier-to-deal-with ice along the route they chose. The Vikings are the first that we know of to use the Passage, and they didn't cross all the way to the Pacific. They only made it as far as the eastern Coast of Canada, just off the western coast of Greenland. Nothing about the Northwest Passage indicates that it has ever been open to free travel and has, instead, always been at least partially blocked with ice. This example offers no rationale to discredit current global warming trends.
Greenland
The southern portion of Greenland is not covered in ice and during the summer makes one feel that the place is named very appropriately. It is also theorized that the name is actually a transcription error from the original Norse, which may have been "Gruntland" which translates to "Ground Land" as it is depicted on some early maps. In addition, the island was probably greener during the Medieval Warm Period, which coincides with Vikings' settling in the area. This example offers no rationale to discredit current global warming trends.
Piri Res
Not to understate how cool the early map is...but it's not very accurate (example: there are two sets of Virgin Islands and it includes the mythical island of Antilla) mainly because it was compiled from about 20 other maps and not through eye-witness account. But, we'll assume that it was in fact an eye-witness account of Antartica depicted on an earlier map (which is quite the assumption - nearly all maps had a theoretical southern landmass to balance out the northern portion of the planet, but no one actually knew of Antartica's existence until 1820), it is hardly accurate enough to take the coastline at face value and claim that it wasn't under a mile of ice at the time because the coastline doesn't match what we have now. The missing portion of the South American continent fits the mysterious landmass better, and the map states that "spring comes early" to the islands off that coast, which is not at all true of Antartic islands, but is true of South American ones. The map also states that the area is hot and contains a variety of snakes. Now, even if Antartica hadn't been covered in ice at that time, "hot" is simply not possible. Thus, Occam's Razor takes hold - it is far more likely that they didn't have a proper grasp of latitude and longitude and the bottom of the map is the southeast portion of South America than it is that they somehow managed to see below a mile of ice to the actual coastline of a continent no one had seen and simply forgot to mention finding it for 300 years. This example offers no rationale to discredit current global warming trends.
Little Ice Age
I can explain it if you'd like, but it doesn't make any difference, as it is not at all linked to man-made global warming.
No one contends that the Earth goes through periods of warm and cold on its own. That would ludicrous. The argument is that this period of warming is suspiciously rapid and happens to coincide a little too closely with the industrial revolution, the subsequent proliferation of automobiles, and the rising levels of greenhouse gases in the atmosphere and does not fit into the pattern that the planet has exhibited for millions of years prior to our, shall we say, manipulation of our environment to be tossed off as not at all the responsibility of humans. Some of the warming is naturally caused. It would be hard to find a scientist that didn't think so. But that does not forgive human kind from our hand in quickening the process. We are not responsible for all of it. We are responsible for some of it. And we should be attempting to stop making it worse if we wish to stay on this planet. In the words of a great man (George Carlin): "The Earth will be fine. It's the humans that are f*cked."
You don't have to agree with it. But your lack of agreement doesn't mean that the concepts don't make sense.