My friend and debate partner emphasized the debate side of our friendship and promptly replied (the break in time since then has been me looking for a moment to carefully consider what he said). He wrote:
11,000 years ago corresponds to the "last ice age" (cave men, wooly mammoths, etc. -- so I've been told all my life, give or take a millennium). Is that the "peak in the cycle" you mention, i.e. does "peak" mean "coldest"? If we have a 22,000 year cycle (says this model) and an ice age 11,000 years ago, we should now be halfway around this cycle, near the opposite of the ice age, no? That fits with rising temperatures during that entire half-cycle. The model suggests that the earth will begin to cool toward the "next ice age" (per this model) "pretty soon" and continue to cool for about 11,000 years. Thus, the article appears to argue against global warming caused by human activity -- the opposite conclusion from the one I hear you reaching.So I went checking. First (and only) stop was Wikipedia (yeah, it may be suspect, but it is right there). I'll start with some clarification of terminology. We are supposedly still in an ice age because ice still covers Antarctica and Greenland. However, we are not in a glacial period, in which glaciers cover a big portion of the northern continents. Specifically, we're in an interglacial period in the Quaternary Ice Age, and that started about 2.6 million years ago. The Analog article I referenced in the earlier post didn't distinguish between ice age and glacial period.
It took a while to confirm my friend's understanding of the most recent glacial period because most of the articles on ice age and glaciation have charts with timelines in the hundreds of millions of years. 11,000 years is a tiny blip. In the article on the Quaternary Period I found this:
The current interglacial began between 10,000 and 15,000 years ago, which caused the ice sheets from the last glacial period to begin to disappear. Remnants of these last glaciers, now occupying about 10% of the world's land surface, still exist in Greenland and Antarctica.The last glacial period article is here and says the same thing.
So, yes, the last glacial period ended about 11,000 years ago.
Back to the Analog article. And, yes, this was the fact article from that issue. I'm quite aware of the scientific veracity of fiction, even science fiction. I wrote:
We hit a peak in that cycle about 11,000 years ago, which means we should be in an ice age right about … now. But it didn't happen. The drop was steady and according to schedule until about 5,000 years ago when temperatures began to rise again.Though I had written about temperature changes, the original article wrote about changes in atmospheric methane. That change shouldn't matter to my conclusion.
I'll go back to the article itself. It is The Ice Age That Wasn't by Richard A. Lovett, in the April 2007 edition of Analog Science Fiction and Fact.
Greenhouse gases are ones that trap atmospheric heat. Carbon dioxide (CO2) is the most plentiful, but molecule for molecule, methane (CH4) is a good deal more powerful.Earlier, the article said the source of the changes in sunlight in the Northern Hemisphere are due to wobbles of the earth on its axis and the eccentricities of the earth's orbit that happen in cycles of 41,000 and 22,000 years.
These [ice] cores reveal that during eras when the Northern Hemisphere receives weaker summer sunlight (i.e., ice ages), methane is lower, In eras when solar energy is higher in the arctic, methane increases.As the solar energy fluctuates, so does the level of methane, and so does its action as a greenhouse gas, and so does the world's temperature.
This pattern means that the atmosphere's methane level should have reached a peak 11,000 years ago and been dropping ever since. And that's exactly what happened until 5,000 years ago. Then something went awry, and it began to rise.To me (one who wasn't all that aware of glaciation history), a peak in methane levels implies a peak in temperatures, and forms a convincing case that 11,000 years ago was not a glacial period.
And that appears to contradict my friend's understanding and the several articles in Wikipedia.
The Analog article doesn't offer a way out of that contradiction, so I'll leave it at that. I checked the letters column in issues of Analog over the next year (they're already in my book closet) but there was no mention of this article.
Going on to another part of my friend's response. I had written:
There are three dips in the steady rise in temperature. All three correspond to devastating plagues -- bubonic plague at the time of the collapse of the Roman Empire, the Black Death of the 14th Century, and the European diseases that swept through the Americas in the 16th Century. Reduce the number of humans and our effect on climate is reduced.My friend responded:
Per your next-to-last paragraph, less human activity means less warming. But less human activity would presumably be balanced at least in part by more animal activity, as animal populations should rise greatly with reduced human predation. And the very small human population (compared to the current level) that prevailed until a few hundred years ago (pre-modern-medicine) should have had much less impact on the earth than we are having now. So I'm questioning the interpretation of these three "dips".I had written about this quite a while ago. The dips in global temperature after times of plague when large parts of the population died were because fewer people meant less farming. That meant fields were allowed to return to forest, and that sucked carbon dioxide out of the atmosphere. Less carbon dioxide, less greenhouse effect, and lower temperatures. The "Little Ice Age" ended when there were enough people to clear forests for farms, releasing all that sequestered carbon.