(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Newfoundland was so warm in the Medieval Warm Period that when the Vikings landed they called it Vineland and brought boatloads of grapes back to Europe. Answer: Once again: you can't draw conclusions about global climate from an anecdote about a single region, or even a few regions. You need detailed analysis of proxy climate indicators from around the world. These proxy reconstructions have shown that the Medieval Warm Period (around the time the Vikings are said to have discovered North America) was not as pronounced or as warm as today's warmth. From NOAA's paleoclimate website comes these quotes:
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: The Hockey Stick graph -- the foundation of global warming theory -- has been shown to be scientifically invalid, perhaps even a fraud. Answer: The first order of business here is to correct the mischaracterization of this single paleoclimate study as the "foundation" of global warming theory. What's going on today is understood via study of today's data and today's best scientific theories. Reconstructions of past temperatures are about, well, the past. Study of the past can be informative for scientists, but it is not explanatory of the present nor is it predictive of the future. The scientific foundation of global warming theory contains much more than a few tree-rings and the temperature during the Medieval Warm Period. RealClimate has an interesting article about what it would mean for today's climate theories if the MWP had indeed been warmer than today. Now, about that pesky bit of sporting equipment ...
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Global warming has been going on for the last 20,000 years. Answer: It is true that 20,000 years ago the temperature was some 8 to 10° C colder than it is today. But to draw a line from that point to today and say, "look, 20K years of global warming!" is dubious and arbitrary at best. If you have look at this graph of temperature, starting at a point when we were finishing the climb out of deep glaciation, you can clearly see that rapid warming ceased around 10,000 years ago (rapid relative to natural fluctuations, but not compared to the warming today, which is an order of magnitude faster). After a final little lift 8,000 years ago, temperature trended downward for the entire period of the Holocene. So the post-industrial revolution warming is the reversal of a many-thousand-year trend.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: When the Vikings settled it, Greenland was a lovely, hospitable island, not the frozen wasteland it is today. It was not until the Little Ice Age that it got so cold they abandoned it. Answer: First, Greenland is part of a single region. It can not be necessarily taken to represent a global climate shift. See the post on the Medieval Warm Period for a global perspective on this time period. Briefly, the available proxy evidence indicates that global warmth during this period was not particularly pronounced, though some regions may have experienced greater warming than others. Second, a quick reality check shows that Greenland's ice cap is hundreds of thousands of years old and covers over 80% of the island. The vast majority of land not under the ice sheet is rock and permafrost in the far north. How different could it have been just 1,000 years ago? Below is a brief account of the Viking settlement, based on Jared Diamond's "Collapse".
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: It was just as warm in the Medieval Warm Period (MWP) as it is today. In fact, Greenland was green and they were growing grapes in England! Answer: There is no good evidence that the MWP was a globally warm period comparable to today. Regionally, there may have been places that exhibited notable warmth -- Europe, for example -- but all global proxy reconstructions agree it is warmer now, and the temperature is rising faster now, than at any time in the last one or even two thousand years.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: It was warmer during the Holocene Climatic Optimum than it is today -- without any human influence. Answer: Though some temperatures during that period were in the same range as today, they were confined to the northern hemisphere and the summer months.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: In 1988, Hansen predicted dire warming over the next decade -- and he was off by 300%. Why in the world should we listen to the same doom and gloom from him today? Answer: While in some instances it is ignorant repetition of misinformation, at its source this story is a plain lie.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: The alarmists were predicting the onset of an ice age in the '70s. Now it's too much warming! Why should we believe them this time? Answer: It is true that there were some predictions of an "imminent ice age" in the 1970s, but a cursory comparison of those warnings and today's reveals a huge difference.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Climate science can't even fully explain why the climate did what it did in the past. How can they claim to know what is going on today? Answer: There are two requirements for understanding what happened at a particular point of climate change in geological history. One is an internally consistent theory based on physical principles; the other is sufficient data to determine the physical properties involved.