XKCD has a web cartoon making the rounds that nicely summarizes 22,000 years of climate and human history. One important point: As the last ice age ended and temperatures heated up, humans did better and civilizations advanced. Vox invites readers to consider its implications and send it along to those folks who serenely opine, "The climate always changes." From Vox:
Randall Munroe, the author of the webcomic XKCD, has a habit of making wonderfully lucid infographics on otherwise difficult scientific topics. Everyone should check out today's edition on global warming. It's a stunning graphic showing Earth's recent climate history. Take some time with it. Stroll through the events like the domestication of dogs and the construction of Stonehenge. And then ponder the upshot here….
But Munroe's comic below hits at the "why worry." What's most relevant to us humans, living in the present day, is that the climate has been remarkably stable for the past 12,000 years. That period encompasses all of human civilization — from the pyramids to the Industrial Revolution to Facebook and beyond. We've benefited greatly from that stability. It's allowed us to build farms and coastal cities and thrive without worrying about overly wild fluctuations in the climate.
And now we're losing that stable climate.
During the last ice age global temperatures averaged about 4 to 5 degrees Celsius lower than the Holocene average. As one scrolls down the trendlines in the graphic, one notes that about 9,000 to 7,000 years ago global average temperatures were higher than currently. In the cartoon, Munroe says that temperatures "start to level out slightly above the 1961-1990 average" around 8,000 BCE.
Of course, determining what temperatures were thousands of years ago is a fraught exercise, but it is generally thought that during the Holocene Optimum global average temperatures were 1 to 2 degrees Celsius higher than they are now. (Yes, I know it's a link to Wikipedia, but I checked a bunch of different studies and it turns out that Wikipedia pretty much cited and linked to the most relevant of them, so click the links if you've got doubts.)
At the end of the XKCD cartoon, it shows current average temperature (which is around where it was 9,000 years ago), and then appends the steeply rising projections of various climate models. Since most doubters are contesting the model projections—not the actual temperature trends—I expect that sending the graphic along to them will do little to change their minds. It will, however, nicely feed into the confirmation biases of those who are fully on-board with those projections. I do note that the cartoon mentions that the Northwest Passage has recently opened. About 9,000 years ago, it was at least as warm in that region as it is now.
In any case, go check out the XKCD cartoon and learn some interesting history.
Just a reminder: I do think that man-made global warming could likely become a significant problem for humanity by the end of the century.