In a time when everyone worries about global warming, I tend to worry more about global cooling. We’re currently living in a brief interglacial period. The last one, about 100,000 years ago, only lasted a few thousand years before lapsing back into a new and prolonged ice age. Our current one has already lasted 12,000 years or more. Why aren’t we in a new ice age? Shouldn’t we be worrying more about the next ice age than any warming taking place right now?
Today I came across a YouTube video featuring Dr. Daniel Britt, a Professor of Astronomy and Planetary Sciences at the Department of Physics, University of Central Florida, that addressed exactly this question.
The cycle of ice ages over the past million years seems to closely track Milankovitch cycles of Earth’s varying tilt, orbital eccentricity and inclination, which result in quite large variations in the amount of sunlight received at high latitudes. The graph below, from 30:30 minutes into the video, shows the variation of solar radiation received at 65°N from 50,000 years ago, to 50,000 years hence.
It shows that right now 65°N is getting something like half the summer sunlight that it was getting 10,00 years ago. So why hasn’t the Earth been cooling?
“So what should be happening is that we should be dropping into the next ice age. Climate should be deteriorating. Things should be getting colder, given the solar input.”
Exactly my point.
“The problem is that it hasn’t been. Solar input is dropping, but the ice is melting. So what’s going on? And what’s going is us, and greenhouse gases – CO2 and methane. What happened is that our ancestors were smart kids. Up until about 8,000 years ago, the atmosphere stuck with historic trends. But about 8,000 years ago our ancestors invented agriculture, and started clearing forests. When you clear a forest what you do is slash and burn. You put lots of carbon back in the atmosphere. You control the forests with fire. Very carbon-intensive forms of agriculture. Methane the same way. About 5,000 years ago they invented rice cultivation and domesticated livestock. Nothing is better producer of methane than a herd of beef cattle… Rice is a swamp grass. And so what you need are swamps in order to grow rice. When you don’t have swamps, you go create them…[and swamps produce methane]. Human input into the atmosphere started a long time ago. It’s not just us in the industrial age. Basically in the pre-industrial age you started out with about 260 parts per million CO2, and human activity stopped the decline of CO2 and methane in the atmosphere and cranked it up to about 280 ppm. That’s 80 gigatons of carbon, which is about 10% of the biomass.”
So, it’s really only because of anthropogenic global warming (AGW) generated by our ancestors over the past 8,000 years that we haven’t slipped into a new ice age.
At this point in the lecture, I half-expected him to call for a round of applause for our industrious ancestors, because without them we’d already be in an ice age, with temperatures several degrees lower than now.
But instead he moved on to recent industrial additions of CO2 to the atmosphere, which are far greater than those over the previous 8,000 years, and returned to the standard global warming narrative of rising temperatures, and rising sea levels.
But the conclusion I drew from it was that, but for the existing global warming, we’d now be experiencing falling temperatures and falling sea levels and crop failures and worse. We’d be in an ice age. And a little bit of global warming is a Good Thing. And we actually need more global warming than we were getting from pre-industrial agriculture, particularly since we no longer use slash-and-burn agriculture. And if we ever achieve a Zero Carbon solar and windmill-driven economy, CO2 levels in the atmosphere will fall, and we’ll be plunged into the ice age we’ve been forestalling for the past 5000 years.
All this supposes that rising CO2 really is raising temperatures – something Britt regarded as unquestionably happening. But given that human populations over the past 8,000 years have been very low, is it actually likely that humans could have produced that much global warming? Where’s the evidence? We simply don’t know what human agricultural patterns were like 4,000 years ago. So I suspect that this ancestral AGW is really just a guess. It’s one way of explaining why we aren’t in an ice age right now. But it may not be the only way.
But at least it seriously addressed a question that’s been puzzling me for a decade or more.