Frozen Earth: The Once and Future Story of Ice Ages (2 page)

One of these feedback processes was a reduction in reflectivity, or albedo, as the area covered by ice decreased.
With less snow and ice, the Earth retained more of the sun’s energy instead of reflecting it back into space, raising temperatures further.
This is happening today in the Arctic, which is warming more rapidly than other parts of the Earth as the extent of sea ice diminishes.
But even more important during the last deglaciation were changes in ocean circulation.

How does ocean circulation affect climate?
To understand this it’s necessary to remember that because of their huge volume, the oceans hold a tremendous amount of heat.
Circulating ocean waters carry this heat from one place on the globe to another.
Also—and especially important in terms of the warming and cooling cycles of ice ages—the
oceans contain a very large amount of carbon.
Some of this is present as dissolved carbon dioxide, and much of the rest can easily be transformed into carbon dioxide.
In total, the oceans contain about fifty times as much carbon as the atmosphere.
The rapid increase in carbon dioxide during the most recent deglaciation apparently happened when changes in ocean circulation released some of that carbon into the atmosphere.

Ocean circulation is strongly influenced by the geographical distribution of the continents.
In the present-day configuration it is largely driven by warm tropical water flowing northward in the Atlantic Ocean, cooling and becoming saltier due to evaporation as it goes.
Both these processes make the surface water denser, and in the North Atlantic it sinks, drawing even more tropical water northward to replace it, thus maintaining the circulation pattern (the dense, cold water descends to the deep ocean and flows south toward the Antarctic and eventually into the Indian and Pacific Oceans).
Shakun and his colleagues suggest that at the beginning of the most recent deglaciation, the slight warming of northern polar regions caused by increasing insolation slowed or even stopped this pattern of circulation.
How did this happen?
Fresh water (which is considerably less dense than salty seawater) from the melting glaciers flowed into the North Atlantic, decreasing the density of the surface water to the point where it could no longer sink.
This shut down the northward transfer of warm water from the tropics, leading to warming of the Southern Hemisphere and modest cooling, or at a minimum slower warming of, northern polar regions.
Climatologists refer to such ocean-driven temperature alternations between hemispheres as the bipolar seesaw.
As the southern oceans warmed, Antarctic sea ice cover decreased, and changes in southern ocean circulation released carbon dioxide into the atmosphere, enhancing warming globally.

If you’re not already familiar with some of these processes, following the scenario just described may set your head spinning.
It involves a complex series of interrelated events driven by multiple climate-forcing mechanisms, amplified by feedbacks such as changes in albedo or ocean
circulation patterns.
But then, all natural systems are complex, and the bottom line from the work of Shakun and his colleagues is that the greenhouse gas carbon dioxide was the primary forcing mechanism for global warming during the most recent deglaciation.
Currently, this research is the most extensive and thorough examination of what caused temperatures to rise globally from the LGM to the present.
It is always possible that future work will change some of the details, but for the moment this is one of our best guides for understanding how climate may react to future changes.

An interesting aspect of this work is its conclusion that the ultimate trigger for deglaciation was increasing insolation at high northern latitudes, even though—once the ice age glaciers had begun to melt—carbon dioxide was the primary forcing mechanism for the bulk of the warming.
One of the earliest workers who attempted to explain glacial cycles, James Croll, recognized the importance of changes in northern insolation more than 150 years ago; later (early in the twentieth century) Mulutin Milankovitch expanded on this idea (see chapters 5 and 7).
What these perceptive scientists didn’t understand, though, was that the key role of northern summer insolation in Pleistocene Ice Age cycles was at least partly due to the present-day configuration of the continents.

Why is this?
Think about the current situation: the South Pole lies within the Antarctic continent, the bulk of which is south of 70° latitude.
When global temperatures are low, snow and ice can build up quickly to form a continent-scale ice sheet.
But exceptional cold is required to maintain year-round sea ice beyond the continent, so such ice does not extend significantly farther north today.
In contrast, the North Pole falls in the Arctic Ocean, a small ocean surrounded by continents on which glaciers build up and retreat in response to relatively small temperature changes caused by variations in Northern Hemisphere insolation.
Feedback mechanisms then amplify these changes and affect temperatures globally.
About twenty-two thousand years ago, during the LGM, such processes allowed glaciers to reach as far south as 40°
north latitude in North America.
Ice ages in the Earth’s distant past (see chapter 8) occurred at times when the arrangement of continents was radically different from today’s.
Undoubtedly insolation changes were important for these too, but likely in quite different ways.

The work of Shakun and his colleagues examined only the most recent deglaciation, spanning approximately the past twenty thousand years.
But the Pleistocene Ice Age is characterized by multiple cycles of warming and cooling, of ice retreats and advances, stretching back two and a half million years or more.
Detailed, high-resolution records through all of these cycles are rare.
For example, Greenland ice cores, a primary source of information about past Northern Hemisphere climate changes, extend to only 130,000 years ago, covering little more than one complete cycle.
However, during the winter of 2008–9, a group of scientists and engineers operating under the aegis of the International Continental Scientific Drilling Program retrieved sediment cores that record the local climate in northeastern Siberia through nearly all of the Pleistocene Ice Age cycles.
The cores were drilled from a lake (with a tongue-twisting name: Lake El’gygytgyn) that occupies a 3.6-million-year-old meteorite crater about one hundred kilometers (sixty-seven miles) north of the Arctic Circle.
The availability of a continuous record of local Arctic environmental change through the Pleistocene Ice Age is tremendously important because it permits climatologists to compare the real climate variability, as recorded in the sediment cores, with that predicted by climate simulations run with different forcing factors.
This is especially valuable for the Arctic because both climate models and observations (including temperature records from the past few decades) indicate that northern polar regions are considerably more sensitive to global warming than other parts of the Earth.

The scientists who examined the Lake El’gygytgyn sediment cores recently summarized their work in the journal
Science
(“2.8 Million Years of Arctic Climate Change from Lake El’gygytgyn, NE Russia,” by Martin Melles and colleagues,
Science
337, 20 July 2012).
What did they learn?
Two observations stand out.
The first is that in northern
Siberia, many “super interglacials,” short intervals when local summer temperatures reached levels considerably higher than those of today, punctuated the long Pleistocene Ice Age.
The second is that these periods of high temperatures in Siberia correspond closely in time with episodes of ice sheet meltback in the Antarctic that are known from ocean sediment cores.

Melles and his colleagues looked in detail at several especially warm super interglacials, with summer temperatures 4°C to 5°C (7°F to 9°F) higher than those of today, and investigated possible forcing factors that could have produced such temperatures.
What they discovered is surprising.
Climate simulations that included the effects of both local summer insolation and greenhouse gas forcing (the latter probably more important) could not reproduce the observed high temperatures and instead predicted temperatures that were no higher than those of non-super interglacials.
And because the super interglacials at Lake El’gygytgyn correspond to periods of sharp deglaciation in Antarctica, it is clear that these high-temperature intervals were not simply the result of localized extreme warmth.
The super interglacials were global.

Why didn’t the climate models reproduce the high super interglacial temperatures experienced at the Siberian lake?
Clearly, still-unrecognized processes or forcing factors must have been involved.
Melles and his colleagues speculate that ocean circulation—that great mover of heat around the globe—might be part of the answer, but they can’t be sure exactly how.
These results are another reminder of just how complex the climate system is, and how difficult it is to construct simulations or models to predict accurately how temperatures, rainfall, and the like will change in the future.
More often than not questions answered spawn new questions, and climatologists—indeed, all scientists—always seem to face more work to get to the bottom of things.

Studies such as those described in the past few pages are remarkable achievements; they have detailed how surface temperatures, precipitation, vegetation, ocean circulation, and other aspects of the environment changed during the Pleistocene Ice Age.
Even though questions
remain, they have gone a long way toward elucidating the mechanisms behind glacial-interglacial cycles.
But what about the ultimate question: what initiated the Pleistocene Ice Age in the first place?

In chapter 12, I describe one possible answer, an idea that was suggested not long before the initial publication of this book in 2004: that chemical weathering of the evolving Himalayan Mountains “drew down” carbon dioxide in the atmosphere, reduced the greenhouse effect, and cooled the planet.
This may seem a bit confusing because the Pleistocene Ice Age began only about two and a half million years ago, when large-scale glaciers began to form in northern polar regions, yet the Himalayas are much older (they began to form about fifty million years ago when plate tectonic forces caused India to crash into Asia).
However, temperature proxies in deep-sea sediment cores show that global temperatures declined steadily from approximately the time of the India-Asia collision (when they were much higher than they are today) until the start of the Pleistocene Ice Age.
By about thirty-five million years ago, global temperatures were low enough for ice to begin to cover the Antarctic (which had previously been unglaciated), and climate feedbacks related to this ice cover further cooled the Earth until, eventually, Northern Hemisphere glaciation began.
So the question of what initiated the Pleistocene Ice Age rests on what caused the long-term cooling that began around fifty million years ago.

It is well known that carbon dioxide from ordinary air, when dissolved in rainwater, is the primary agent of rock weathering and that extensive weathering depletes its abundance in the atmosphere.
That young, rising mountain ranges are sites of intense chemical weathering is also well known.
The coincidence in timing between the rise of the Himalayas and a global temperature decrease suggests that weathering of this young mountain range could have been responsible for the lower temperatures, through its effect on atmospheric carbon dioxide.
But recently a new candidate has joined carbon dioxide drawdown as a possible cause of the global cooling: sulfur.
What, you may ask, does sulfur have to do with climate?
Potentially quite a lot.
Sulfur is plentiful; in
the form of sulfate (SO
^2-
₄
), it is the fourth-most-abundant ion in seawater.
Because of this, the oceans are a major source of sulfur-bearing aerosols in the atmosphere—suspended microscopic droplets that reflect incoming solar radiation.
When their concentration increases, they reflect more solar radiation and the Earth cools.
This effect was illustrated clearly in 1991, when a large eruption of Mt.
Pinatubo in the Philippines injected sulfur-bearing aerosols into the atmosphere, lowering global average temperatures by about 1°F for more than a year.

In a recent paper in the journal
Science
(“Rapid Variability of Seawater Chemistry over the Past 130 Million Years,”
Science
337, 20 July 2012), Ulrich Wortmann and Adina Paytan note that the record of past seawater sulfur content shows large and quite rapid changes, and they conclude that deposition and dissolution of vast quantities of the sulfurrich mineral gypsum almost certainly caused at least some of this variability.
Gypsum is abundant in so-called evaporite deposits, which are assemblages of minerals that form in hot, arid regions when salty seawater trapped in restricted basins evaporates.
Large-scale evaporite deposits have formed many times during our planet’s long history, as evidenced by the numerous salt mines found around the globe (in addition to being important sources of sulfur, evaporites provide us with table salt and potassium for fertilizer).
But evaporite minerals are not very stable at the Earth’s surface; when exposed to ordinary precipitation, they dissolve readily.

Wortmann and Paytan’s analysis indicates that the sulfur content of the oceans started to increase rapidly (geologically speaking) approximately fifty million years ago—near the time when uplift associated with Himalayan mountain building began.
The authors conclude that this uplift exposed large-scale evaporite deposits to erosion.
(Undissolved remnants of these deposits still exist, stretching from Oman to Afghanistan, Pakistan, and India.) Large amounts of gypsum in the uplifted deposits dissolved, substantially raising seawater sulfur content and thereby increasing the concentration of sulfur-bearing aerosols in the atmosphere, which ultimately resulted in global cooling.
Plate
tectonics—in this case the collision of India and Eurasia—thus played a major role in the cooling that led to the Pleistocene Ice Age, through both the drawdown of carbon dioxide and the supply of sulfur to the oceans.
These observations illustrate how deeply interconnected even seemingly disparate Earth processes are.