Frozen Earth: The Once and Future Story of Ice Ages (36 page)

A hallmark of ice age climate change, at least when viewed from the perspective of its impact on human societies, is abruptness.
With little or no warning, there have been drastic shifts in temperature, storminess, and precipitation, both regionally and globally.
Frequently, these shifts, although very rapid, leave the climate system in a new mode that persists for a relatively long time.
The Mediaeval Warm Period and the Little Ice Age, as well as the occasional abrupt fluctuations that occurred within them, are good examples from the past millennium.
As already discussed, there are even more striking events, such as the Younger Dryas interval, on a longer timescale.
There is little doubt that similar changes will occur in the future, and understanding the underlying causes of such events is important if there is to be any hope of predicting them or mitigating their impact on society.
Abrupt climate change is currently a hot topic in the environmental sciences, and a large cadre of scientists from diverse disciplines are working on the problem.
In a relatively short time, much has already been learned, and although definitive answers—always elusive in science in any case—are not available, some general conclusions are.

Most of the attempts to understand why rapid climate shifts happen involve several concepts that are quite familiar.
The first is the idea of a threshold, a state that, if crossed, more or less automatically shifts the climate into a different mode.
To cross a threshold, however, requires something else—usually an external “forcing” or trigger, some process or phenomenon that will change the system, either slowly or rapidly, until it reaches and crosses the threshold and flips into the new mode.
Such changes also usually require some type of positive feedback, a multiplying effect that ensures that the change will be global or universal.
None of these ideas is particularly new—recall James Croll’s belief that variations in the Earth’s orbit would act as an external forcing, cooling the Earth until snow persisted on the ground throughout the year.
Year-round snow was the threshold, and it would also provide positive feedback: it would amplify the cooling by reflecting more solar energy back into space, initiating rapid expansion of ice sheets and a new glacial interval.
By building such ideas into complex computer simulations of the global climate—an enormous task that requires great computing power—and by using accurately determined ocean and atmospheric conditions, it has been possible to examine the effects of different types of external forcing on the entire system—a kind of “what if ” approach.

One conclusion of the simulation studies, already known in a general way from earlier work, is that the ocean current system is very important for distributing heat.
In particular, changes in the way ocean circulation occurs in the North Atlantic Ocean have been implicated in some of the large and abrupt temperature changes observed in the Greenland ice-core data over the past few tens of thousands of years.
As discussed briefly in chapter 6, the warm surface water of the Gulf Stream moves northward in the Atlantic, evaporating and cooling as it goes.
Both evaporation, which increases the salt content of the water left behind, and cooling, which contracts it, cause the density of the water to increase.
By the time it reaches high latitudes, it has become so dense that it sinks, displacing the underlying lighter water.
There are a few other places, such as the Antarctic, where very cold surface water also becomes dense enough to sink, but the process is most important in the North Atlantic—so important that it is a driving force in the circulation of the entire ocean.
The cold sinking water spreads southward across the equator in a deep layer, south to the Antarctic and around into the Indian and Pacific Oceans.
In places, it upwells again to the surface, and surface currents make up the return flow, eventually again joining the Gulf Stream to complete the circuit.
If something were to shut down the sinking of North Atlantic seawater, the whole ocean circulation system
would slow down and either stop completely or reorganize.
The Gulf Stream would no longer carry warm tropical water into the North Atlantic.
Greenland and Europe would lose the warming benefit of this current, and their climates would abruptly become much colder.

That sounds very nice and simple in theory.
Could it actually happen?
Many researchers now believe that just such a scenario was responsible for the Younger Dryas cold period discussed in the previous chapter, and probably also for many other cold snaps that can be identified in the Greenland ice cores.
Hints can be found in sediment cores from the North Atlantic that during these intervals, the flow of Gulf Stream water slowed, and the amount of new dense bottom water being produced declined.
In addition, ice cores from the Antarctic show a slight warming at high southern latitudes, an effect that has been linked with a weak or nonexistent Gulf Stream.
Under conditions similar to those at present, the Gulf Stream cools the Antarctic slightly by drawing warm water out of the Southern Hemisphere and transporting it northward; if the Gulf Stream slowed or stopped, a small amount of warming would be expected.

That still leaves the question of cause.
The search for reasons for the on-again off-again nature of the Gulf Stream has focused on processes that could change seawater density, because, as explained above, density plays an important role in ocean circulation.
For a given batch of ocean water, density depends on temperature and dissolved salt content (and for this reason the circulation is referred to as thermohaline circulation).
If some process were to decrease the density of North Atlantic surface water, it would eventually cross a threshold value and float rather than sink, shutting down the thermohaline circulation.
This could happen by addition of low-density fresh water from rapidly melting glaciers, as was discussed in chapter 6.
Just such a scenario has been proposed for abrupt temperature decreases recorded in Greenland ice cores near 12,800 (beginning of the Younger Dryas) and 8,200 years ago, both of which correspond to sudden changes in glacial Lake Agassiz’s drainage that added large volumes of fresh water to the North
Atlantic.
It is also possible—paradoxically—that present-day global warming will lead to cooler temperatures in northern Europe through a similar effect.
Both accelerated addition of fresh water due to melting of the Greenland ice sheet and the general warming of seawater because of globally higher temperatures will decrease the density of surface water in the North Atlantic.
There is some evidence that the amount of sinking cold water in the North Atlantic has decreased slightly in recent years, but the measurements have not been carried out over a long enough period to determine whether this is a long-term trend or just a minor deviation from the average.

Although a strong case can be made that large-scale changes in North Atlantic ocean circulation were responsible for at least some of the rapid temperature changes recorded in Greenland ice cores, there is no evidence that the less severe climate variations of the past millennium, disruptive as they were for European civilization, had a similar origin.
As mentioned earlier in this chapter, one idea is that the sun’s activity may have been the important forcing factor.
Possibly that could have tipped the North Atlantic Oscillation into a mode that dominantly brought cold weather to the region.
Even volcanic activity has been implicated, not as a cause of Little Ice Age cold, but as a process that occasionally and temporarily exacerbated the already-cool climate.
That volcanic activity can have a measurable effect on temperatures worldwide is no longer in dispute—the volcanic dust and sulfurous gases blasted into the stratosphere during the 1991 eruption of Mt.
Pinatubo in the Philippines so reduced solar energy reaching the Earth’s surface that global temperatures were lowered by about half a degree Celsius for over a year.
That doesn’t sound like much, but it is a sizeable fraction of the average temperature reduction during the Little Ice Age.
The seventeenth century saw at least five large, explosive eruptions, beginning with the most massive, in the Peruvian Andes, early in 1600.
Ash from this eruption is easily identifiable in both Greenland and Antarctic ice cores.
Records from Europe and North America count the following summer, in 1601, as the coldest for hundreds of years.
In 1815, as the Little Ice Age was drawing
to a close, there was an even larger eruption on the island of Sumbawa in Indonesia.
Again, the following summer was frigid.
The year 1816 became known as the “year without a summer.”
Snow fell in New England in June, and crops failed in Europe.

If there is a lesson to be learned from our knowledge of the past millennium’s climate history, it is that surprises abound even for this very short snippet of geological time, which, when viewed from the long-term perspective of the entire Pleistocene Ice Age, enjoyed a relatively warm and stable interglacial climate.
Modern societies for the most part are better equipped to deal with such surprises than were those of even a hundred years ago, but are not entirely immune.
Just-in-time logistics systems and highly concentrated and specialized agriculture are as likely to be disrupted by abrupt climate change as some earlier technologies.
Energy grids even now have difficulty coping with high demand during heat waves, when millions of air conditioners are operating at full capacity.
Just as troubling is our inability to predict, even in a general way, what may happen to the climate system as a result of human influences.
A great, unintended experiment in “climate forcing” is under way as we add more and more greenhouse gases to the atmosphere.
Whether or not we shall reach one of those thresholds that seem to separate different climate modes, and what will happen if we do, is still unknown.

CHAPTER TWELVE

Ice Ages and the Future

It is worth reiterating here something that was pointed out in the first chapter of this book but may have drifted into the background since: the Earth is still in an ice age.
We are in a warm period, one of the many interglacial intervals that have occurred throughout the Pleistocene Ice Age, but even so, there are significant amounts of permanent ice in polar regions.
It is easy to forget that this may be just a short respite before another glacial interval begins.
Or perhaps the respite may be longer than we anticipate.
Man’s activities may intervene and confound our ability to use the natural climate variations of the past as a tool to predict the future.

In spite of the uncertainty about the future, we do know that the past few million years of the Pleistocene Ice Age have been fairly unrepresentative of the long history of our planet.
Over that four and a half billion years, only a few major ice ages have been identified, separated by hundreds of millions of years of warmer climates when no major ice sheets blanketed the continents.
We have touched on some of the possible causes for these unusual episodes in previous chapters: variations in the shape of the Earth’s orbit around the sun, and the tilt and wobble of its axis of rotation; the positions of the continents as they move about on the Earth’s surface due to plate tectonics; and the concentration of
greenhouse gases, especially carbon dioxide, in the atmosphere.
It is widely agreed that
within
the Pleistocene Ice Age, it is the astronomical parameters that have regulated the repeated cycles of glacier growth and decline.
And by analogy, the course of ancient ice ages may have been similarly controlled by these parameters.
But it is not at all clear that any conjunction of these astronomical conditions would on its own be sufficient to tip the Earth into a cold period and initiate an ice age.
Much more likely, most scientists now believe, is that a combination of factors—including at the least the astronomical parameters, greenhouse gases, and plate tectonics—must play a role.
Under the right conditions, a relatively small change in one of these—perhaps a fluctuation in the Earth’s orbit—may act as a trigger to initiate a cold interval.
An additional important ingredient seems to be positive feedback, some process that amplifies the effect of the other factors.

Recent concern about global warming, and especially the impact of mankind’s addition of greenhouse gases to the natural atmospheric inventory, has led to increased awareness of the role these gases may play in starting or ending ice ages.
Data from ice cores have further stimulated interest, because they show that on almost every timescale, the concentration of greenhouse gases has tracked changes in temperature.
Rapid addition of these substances to the atmosphere may have triggered some of the abrupt temperature rises recorded in the ice cores, and it has also become clear that for the future greenhouse gases will be a major factor in determining when the Earth will slip into the next glacial period of the Pleistocene Ice Age.
It is just possible that the greenhouse gas content of the atmosphere is
the
critical necessary condition that controls when an ice age will begin or end.

What, exactly, are the important greenhouse gases, and how do they affect the Earth’s temperature?
There are quite a few, but the most important in terms of their ability to trap solar energy in the atmosphere are water vapor (H
₂
O), carbon dioxide (CO
₂
), and methane (CH
₄
).
All three are molecules that absorb the heat energy radiated up into the atmosphere from the Earth’s warm surface.
The surface is
heated by incoming solar energy—the energy you feel as you lie on a beach in the sun—which is short-wavelength radiation that is not absorbed by the greenhouse gases.
Because of their structures, these gases only absorb energy of specific longer wavelengths, which coincidentally match those of the energy radiated away from the heated surface.
The greenhouse molecules not only absorb this energy, they also re-radiate much of it back into the atmosphere.
Although the analogy is not quite perfect, they act something like window glass that lets the sun’s energy in but blocks the heat in a warmed-up room from making the return journey into the outside air.