Of Ice Ages and Men

Note: I reworked and (hopefully) improved this essay in my "Ice Ages Revisited: A Story of Climate" (2018). Among other things, I thought it would be good to remove some of the snark that appears below. That might be a better place to start. But because I also simplified some of the scientific details in writing that later piece, you might want to come back here after reading it.

_________________________________________________

You sometimes hear it said that it's absurd to think human beings could alter the climate. Global warming, if it exists at all, can't be caused by humanity, because the climate is far too vast and inexorable to yield to human trifling. Senator James Inhofe said the notion that people could alter what God is "doing in the climate" was "arrogance" and "outrageous," but he was making a theological argument, and a weak one at that. "God's still up there," the senator said. So? Somewhere along the way he forgot the Christian understanding that God does not block the human exercise of free will, even when it causes great harm. Such is the nature of sin, and sin is consequential. Senator Inhofe should leave theology to theologians.

Theology aside, whether or not the climate is too colossal to be moved is a question for science, and one that science can answer. Unfortunately, science is frequently bypassed, and the question relegated to "common sense," which is always a ready crutch for the poorly-informed. But answer it we shall, by way of thinking about ice ages.

The connection between ice ages as an apparently irresistible natural phenomenon, on the one hand, and the possibility of human impact on the climate, on the other, came to me while discussing global warming with a climate change skeptic. My skeptic friend (I really shouldn't call him a skeptic; he's actually a climate change denier, or contrarian) was convinced that one of the reasons the scientific consensus on anthropogenic climate change is so overwhelming is that scientific inquiry has been strongly skewed to the study of man-made climate effects, to the detriment of understanding natural causes of climatic variation—variation which, after all, has occurred throughout the history of the earth, with ice ages being but one prominent example. That's his position when he actually acknowledges the scientific consensus; much of the time he claims that no such consensus exists. Do not expect most climate change deniers to have a consistent story.

Of course, the assertion that science hasn't sufficiently studied or considered natural causes of climate change is absurd, if for no other reason than it's impossible to understand and characterize anthropogenic effects without first understanding how the climate operates as a completely naturogenic system. Climate study is at its core an understanding of all the factors that influence the climate, and the positive or negative increment each factor contributes. Indeed, "natural" effects weigh importantly in the calculations that explain the net outcomes we observe and predict today. Many climate scientists focus their efforts on understanding the drivers of climatic variation observed over geologic time scales (a discipline called paleoclimatology), reaching back tens or hundreds of millions of years before the appearance of humanity on the earth. Importantly, understanding how the earth has operated on geologic time scales is essential to explaining why the present situation is so unusual and dangerous. The notion that scientific inquiry is somehow biased toward anthropogenic causes is just one of the simplistic conspiracy theories that the poorly-informed skeptic instinctively indulges.

My poorly-informed skeptic did a lot of rhetorical hand waving to assert that the climate has always had dramatic variation from completely natural causes, and dismissively asked whether science can explain sun spots, ice ages, and "water vapor" as examples or drivers of variation. That last one was puzzling; I didn't understand why he included it, and neither did he. He must have heard something once about water vapor being important to his position.

Water vapor is in fact a powerful greenhouse gas, which tends to increase in the atmosphere in a warming climate due to increased evaporation, particularly from large bodies of water. (This can cause large precipitation events, including snow, and these are predicted by climate science.) Conversely, water vapor in the form of clouds tends to increase the reflectivity (technically, "albedo") of the earth's atmosphere, which might result in a net cooling effect from clouds. (Clouds are actually rather complicated and hard to model.) Because the impact of water vapor on climate has, potentially, both positive and negative temperature components (and, indeed, participates in both positive and negative feedbacks), science necessarily works to understand their interaction. The uncertainty over clouds notwithstanding, it is clear there is an overall net warming and a net positive feedback with respect to water vapor. ("Positive feedback" means increased warming begets increased water vapor which causes even more warming and thus even more water vapor, and so on.) The point is that anthropogenic warming caused by burning fossil fuels increases the concentration of water vapor in the atmosphere and thus causes even more warming.

Back to my skeptic. Sometimes you ask a rhetorical question but don't actually want an answer, such as when my skeptic offered ice ages as (presumably) one of those poorly explained but hugely consequential natural phenomena that upend the science of climate change and demonstrate how forlorn and hapless those silly climate scientists are. Little did he realize that, far from being an imponderable force of nature opaque to science, ice ages are well understood; their study is in fact important to understanding natural climatic variation, and the feedbacks that potentiate it. My skeptic offered ice ages as a throwaway example of how humanity's climate influence must be negligible and science's understanding deficient, but ended up inadvertently inviting an explanation of ice ages that shows that neither is the case.

James Hansen, one of the world's preeminent climate scientists, spends a lot of time explaining ice ages in his book Storms Of My Grandchildren. Hansen, incidentally, got his start at NASA studying the climate of Venus (which assuredly has had no human influence) in the 1970s, and later became director of NASA's Goddard Institute for Space Studies, a position he held for approximately three decades. The information that follows comes from Hansen's book.

Ice ages are certainly impressive, and the earth has been oscillating in and out of them for millions of years. During the most recent one, which peaked a mere twenty thousand years ago, ice sheets as much as two miles thick covered much of Canada and some of the United States—which, I assume you realize, weren't at the time officially recognized as nation-states. Immense glaciers have been periodically advancing and retreating for a very long time. If that isn't natural climate variation, I don't know what is. In fact, the variation is so impressive that my skeptic friend is gobsmacked by it, and why not? He considers it prima facie evidence that natural effects swamp human ones. Compared to ice ages, how can humanity's influence on the climate make any material difference?

To see how, we must first answer the question of what causes ice ages. There are two phenomena, both quite weak, that cause variation in where, when, and how much solar insolation (INcident SOLar radiATION) strikes the earth. Both phenomena are caused by gravitational pulls on the earth by other planets—especially the large ones—that slightly alter the earth's tilt on its axis, and also the shape of its orbit. The first effect results in a small change in the earth's tilt over time, with a variation between about 22.1 and 24.5 degrees. The earth periodically and very slowly tilts to its maximum and then straightens back up, in a cycle that takes about 41,000 years to complete. Increased tilt exposes both polar regions to increased solar insolation; it also causes the amount of summer insolation to increase, and winter insolation to decrease, resulting in increased ice melt during the warmer summers. Conversely, with decreased tilt (which favors ice accumulation) the polar winters are warmer, which results in increased snowfall, because the warmer atmosphere contains more moisture. Snow, which is compacted under great weight to form ice, is the raw material of glaciers. Additionally, the cooler summers caused by decreased tilt provides a bit of protection from melting for that additional winter snow, allowing some of it to survive across seasons and glaciers to grow.

So the first effect is from axis tilt. The second effect results from very small changes in the the earth's orbit, and earth's proximity to the sun, over time. Because the earth's orbit is slightly elliptical, there are times when the earth is closer to the sun and times when it is farther away. When closer to the sun, the earth receives more solar insolation, which promotes warming. The day of the year when the earth is closest to the sun varies through the entire calendar over a period of about 20,000 years. Furthermore, the eccentricity (the amount of deviation from circular) of the earth's orbit varies from nearly zero (almost circular) to nearly six percent (a bit more elliptical), and does so with a non-uniform periodicity. The variation of the second effect (solar proximity and eccentricity) can either reinforce or weaken the first effect (axis tilt), depending on how much the two effects are in or out of phase with each other.

Both effects are currently pushing the northern hemisphere toward a buildup of ice sheets—or they would be if only natural causes were operating. Despite this "natural" bias toward more ice and, eventually, a new ice age, anthropogenic global warming is presently causing the amount of Arctic ice cover to decrease because of rapid melting, and that melting is accelerating. So current warming factors are currently opposing natural effects that, in the geologic past, would have initiated an ice age. In fact, as will soon become clear, humanity need not worry about enduring a new ice age ever again.

As an aside, neither of these changes in tilt or orbit has much effect on the total amount of sunlight striking the earth; their ability to induce (or oppose) ice ages comes from altering the geographical and seasonal distribution of solar insolation.

We will get back to ice ages shortly, but first a quick diversion that allows us to understand the strength of the factors that cause them. Scientists measure the amount of climate forcing from various physical phenomena, in units of watts per square meter averaged across the earth's surface. Hansen defines "forcing" as a disturbance (change) of the planet's energy balance, which causes the earth to reach a new equilibrium that is warmer or cooler than some previous equilibrium. That is, it's a change in energy balance from some previous state. Note that the effect an increment of forcing actually has on the climate depends on the climate's sensitivity to forcing, which is something that climate scientists deal with a lot. If the climate is very sensitive, small forcings can cause large changes. If it isn't very sensitive, even large forcings result in small changes. We will not speak further of sensitivity here, but you can read Hansen's book for more on this.

To get a sense of the magnitude of climate forcings, consider that the earth receives approximately 240 watts of sunlight per square meter of surface, averaged over a twenty-four hour period (day and night). Forcings can be either positive or negative, depending on whether they cause the earth to warm or to cool. A one percent increase in the amount of sunlight absorbed would thus represent a (positive) forcing of 2.4 watts (as always, per square meter).

Hansen gives values, relative to conditions that prevailed at the beginning of the Industrial Revolution, for the most important natural and human-caused forcings. These forcing values come from a presentation Hansen gave to the Climate Task Force headed by Vice President Dick Cheney in 2000, so the comparison period is approximately 1750 to 2000.

Natural forcings (w/m²)

+0.25 Solar irradiance
+0.10 Volcanoes

Human-caused forcings (w/m²)

+1.50 Carbon dioxide
+1.25 Other greenhouse gasses
+0.30 Ozone
+0.60 Black carbon aerosols
-1.00 Reflective aerosols
-1.00 Aerosol cloud changes
-0.20 Land cover changes

We make explicit the sign of each forcing to show whether it causes warming (positive) or cooling (negative) of the planet. A net forcing, positive or negative, can be calculated by simple summation.

Solar irradiance refers to the brightness of the sun, which can fluctuate up and down by a small amount over relatively short time scales. (As it ages, over many millions of years, the sun will gradually become ever brighter, but that's of no consequence for the current discussion.) Scientists think that irradiance was slightly greater than average over the comparison interval, so irradiance represents a positive natural climate forcing. Volcanoes normally have a net cooling effect (by expelling aerosols into the atmosphere, which block sunlight), but scientists think volcanic activity was a bit below average over the comparison interval, so we show a slight positive forcing.

As you can see, the natural climate forcings sum to 0.35, and the human-caused forcings to 1.45, for a net forcing of 1.8 watts per square meter. That is, the earth's energy balance is currently 1.8 watts per square meter in excess of what would maintain the earth's temperature at the current equilibrium level. Another way of saying this is that the earth is receiving more energy than it can currently (at its present temperature) radiate out into space. The "solution" is simple physics: the earth will warm until incoming and outgoing radiation are back in balance, at a new, higher, equilibrium temperature. (The warmer the earth becomes, the more heat it radiates out into space, which is how incoming and outgoing radiation reach a new equilibrium at a higher temperature.) It's thought that up to the middle of the twentieth century natural forcings were at least as large as man-made forcings, so natural causes can plausibly explain a good bit of the earth's warming up to that time. After the middle of the twentieth century, however, human forcings begin to dominate natural ones, resulting in a dramatic acceleration of warming in the latter decades of the twentieth century.

We can now return to ice ages. The (negative) climate forcing from the axis and orbit effects described above is tiny: just a small fraction of 1 watt. It is so small that it isn't even shown in the table above. But, because of slow feedbacks that occur over long time scales, that small forcing is enough to cause an ice age!

The feedbacks work like this: The initial increase in regional snow and ice caused by the changed insolation patterns (warmer winters, cooler summers) increase the earth's reflectivity (albedo), which results in slightly cooler global temperatures, which allows slightly greater snow and ice coverage, which cause greater increases in reflectivity, and so forth. The initial conditions that caused expanded ice coverage near the poles allows that ice to persist and spread to lower latitudes. As that happens, greenhouse gas feedbacks kick in. As the global temperature gradually cools, greenhouse gas concentrations (the most important of which is CO₂) also decrease, resulting in further cooling. (Note that temperature changes do indeed precede greenhouse gas changes, but this is not an argument that science has things backward with present-day warming.) The feedbacks ramp up, and next thing you know (after thousands of years, actually) you've got ice sheets over a mile thick covering Canada. The process eventually unwinds itself as the insolation patterns change in ways that once again favor warmer interglacial periods (the periods between ice ages). The entire cycle takes many tens of thousands of years.

Who knew that it took so little natural forcing, maintained over a very long period, to cause an ice age? Science has known it for a long time. Now you do too. Even very small forcings, including man-made forcings, can have very large climate effects. Is it "arrogance," then, to claim that humans can alter the climate? Clearly not. Are natural climactic processes so immense as to be immune from human control? No. We have seen that the current magnitude of total man-made climate forcings is many times greater than the magnitude of (negative) forcing necessary to initiate an ice age when only natural conditions are in effect. According to Hansen, "forces instigating ice ages ... are so small and slow that a single chlorofluorocarbon factory would be more than sufficient to overcome any natural tendency toward an ice age. Ice sheets will not descend over North America and Europe again as long as we are around to stop them."

What goes down can also go up, and we are in the process of taking a ride to a much warmer climate that we will not enjoy. The remarkably stable climate (and sea levels) humankind has enjoyed during the current interglacial period, which is called the Holocene, is coming to an end thanks to human activities. That stable climate has allowed human civilization to take hold and flourish. The frightening speed with which we are now heating up the planet, melting polar ice, and emitting greenhouse gasses is unprecedented in the history of the earth. Human-caused changes are occurring far faster than any natural process ever has (short of an asteroid hitting the earth, such as the one that exterminated the dinosaurs). But that is a topic for another day. Today we will let James Hansen have the final word:

Both global surface albedo and greenhouse gas amount are now under human control. The slow-feedback processes that cause glacial-to-interglacial oscillations are still operating, of course, but they respond, as they always have, to global temperature. The global cooling trend needed to cause the slow feedbacks that would take Earth into its next ice age no longer exists. Thus any thought that natural processes can somehow move Earth toward the next ice age is utter nonsense. Humans, by rapidly burning fossil fuels, have caused global warming that overwhelms the natural tendency toward the next ice age.

Update:  Climate scientist Michael E. Mann graciously tweeted about this post. In private correspondence James Hansen said "it's good -- the difficulty is in getting people to read it -- and carefully enough to understand it." Dr. Hansen did not fact check this piece, and is not responsible for anything I've gotten wrong here. Same for Dr. Mann. See also my post on the scientific consensus on climate change.

Copyright (C) 2014 James Michael Brennan, All Rights Reserved

The latest from Does It Hurt To Think? is here.