Orbital Changes Control Monsoon Cycles

One of the bleakest places on Earth today is hyperarid Sudan, south of Egypt. Dry winds blow sheets and dunes of sand across the landscape, and almost nothing lives there. But satellite photos and images from heat-sensing devices show subsurface traces of streams and rivers that once flowed eastward to join the Nile River in its northward course from well-watered source areas in the highlands of East Africa. Once, this desert area was green, with broad grasslands and tree-lined waterways inhabited by crocodiles, hippopotamus, ostriches, and rhinoceros. Their bones are found in dried-up stream sediments now covered by a thin layer of sand.

Indeed, the entire southern fringe of the Sahara was once green, with the grasslands of the modern-day Sahel region much farther north than now, and large lakes dotting the area. Modern-day Lake Chad in southern Libya is the largest lake in the region today, but not long ago it was ten times larger, based on evidence from freshly deposited lake sediments and notches cut by lake waves into surrounding rocks. Nomadic tribes in the North African desert tell stories of a wetter time in their ancestral past.

Until 25 years ago, geologists were still looking for an explanation for this vast greening of the southern Sahara. Most early attempts had linked this wet interval to the presence of ice sheets in the North. The prevailing hypothesis held that the ice sheets had interfered with the normal circulation of the atmosphere and redirected moist winds southward into a region they do not reach today. The midlat-itude storm track had supposedly been positioned much farther south, bringing rains to the arid core of the Sahara Desert and to the grasslands farther south. This explanation had seemed to work (and indeed still does) for the existence of lakes in arid basins of the southwestern United States, and it was used to explain similar climatic changes found in North Africa.

In this case, however, a very reasonable explanation turned out to be wrong. By the 1960s and 1970s, with the discovery and application of radiocarbon dating, the earliest reliable dates of the lake sediments in North Africa were coming in, and the lakebeds did not date to the time when the ice sheets had been largest, near 20,000 years ago. Instead, their ages were closer to 10,000 years ago, by which time the ice had almost completely disappeared from the northern continents. The unavoidable conclusion was that large ice sheets had not caused the filling of the lakes.

Some scientists then proposed the exact opposite view, arguing that the lake levels were higher during warm interglacial climates because the disappearance of the ice sheets and their chilling effect on climate had allowed the atmosphere to hold more moisture and thereby fill the lakes with rainwater. But this suggestion was doomed from the outset. It offered no explanation of why most of the North African lakes had dried out in the last 5,000 years. Because no ice sheets have appeared during that interval, northern ice sheets cannot be the reason for the recent drying trend.

By this point, all explanations of African lake levels tied to ice sheets had effectively reached a dead end. The cause of the greening of the southern Sahara Desert had to lie elsewhere. And as it turned out, the explanation was right there in the tropics—the overhead Sun. In 1981 meteorologist John Kutzbach came up with one of those simple explanations that make other scientists kick themselves for not having thought of it first. He based his hypothesis on the modern-day wet summer monsoon, a phenomenon that is the main cause of rainfall over much of North Africa. To most of us, the word "monsoon" invokes images of torrential rains falling in India and Southeast Asia, an area with the strongest monsoons in the world. But North Africa has its own monsoon, and it operates across the broad Sahel region south of the Sahara Desert. The North African monsoon delivers summer rains that allow grass and some trees to grow in the savanna south of the desert. In winter the rains cease and the land dries out, but the grasses are naturally adapted to seasonal rains and a long dry season. North of about 17°N lies the hyperarid Sahara Desert, beyond the reach of both summer monsoon rains from the south and winter storms from the north.

Kutzbach's simple but elegant insight was that the former existence of extensive lakes and grasslands in the Sahel and southern Sahara can be explained by simply strengthening the modern-day summer monsoon pattern and delivering heavier and more widespread rainfall. To explain this strengthening, Kutzbach called on the same physical process that drives the modern-day monsoon: heating of continents by the strong overhead Sun (fig. 5.1). Heating of land surfaces warms the overlying air, and heated air rises (just as in a hot-air balloon) because it expands and becomes less dense. As the heated air rises, it leaves behind at the surface a region of lower pressure caused by the upward loss of air. To replace this deficit, air moves in from nearby regions. If an ocean is nearby, the air that moves in contains water vapor evaporated from the sea surface. The arrival of this moisture-bearing air from the ocean sets the stage for the wet summer monsoon.

As the moist air flows in from the ocean, it is heated and joins in the upward flow above the hot land mass. But as the rising air penetrates tens of kilometers into the atmosphere where lower temperatures prevail, it cools. Cooled air cannot hold much moisture, and the water vapor condenses into tiny droplets that form

Hot land surface

5.1. Tropical monsoons are driven by changes in summer solar radiation at the 22,000-year orbital cycle. Intervals of strong radiation produce heavy monsoon rains that saturate tropical wetlands and produce large emissions of methane (CH4).

Hot land surface

5.1. Tropical monsoons are driven by changes in summer solar radiation at the 22,000-year orbital cycle. Intervals of strong radiation produce heavy monsoon rains that saturate tropical wetlands and produce large emissions of methane (CH4).

clouds and then raindrops that fall back to earth. You can see this process at work every summer at middle latitudes in the towering cumulonimbus clouds that form high above heated landmasses, usually as scattered clouds that bring afternoon showers. In the tropics, this monsoon circulation is far more widespread and persistent. Many regions receive drenching rains every afternoon and evening because of the midday solar heating.

So the basic operation of the summer monsoon is very simple: strong radiation from the Sun, strong heating of the land, an inrush of ocean air, and monsoon rains. In winter, everything works in exactly the opposite way. The Sun is weak, the land cools off, the overlying air becomes dense and sinks, and the sinking air is dry. This circulation is also called a monsoon—the cold dry winter monsoon. Winter is the dry season in much of the tropics for this very reason.

Kutzbach's hypothesis built directly on this knowledge of modern-day monsoons. Because the stronger radiation of summer (compared to winter) is the cause of the modern-day summer monsoon, he reasoned that past intervals with stronger-than-modern solar radiation in summer should have driven a stronger summer monsoon and filled those North African lakes. And intervals with summer solar radiation values higher than those today have occurred repeatedly in the past.

Solar radiation at tropical latitudes over orbital time scales is controlled by the 22,000-year cycle of orbital precession (chapter 3). At the present time, summer radiation is at a minimum value for that cycle, which implies that our modern-day summer monsoons must also be close to their minimum long-term strength. Kutzbach noted that solar radiation levels were considerably higher than those today at regular intervals of 22,000 years, with the last such time occurring just 10,000 years ago. According to his hypothesis, lakes in North Africa should have reached their largest size near this time, in agreement with the large amount of evidence amassed from radiocarbon dating.

Kutzbach tried out an initial test of his hypothesis using a numerical model of Earth's atmospheric circulation. These models are basically the same kind used in weather forecasting: they incorporate physical principles known to control the circulation of the atmosphere. Of course, making a comparison to weather forecasting always leads to the same wise-guy challenge: "How can you scientists expect to 'forecast' the weather thousands of years ago when you can't even get the weather for this coming weekend half-right?" The response to this (quite reasonable) wise-guy prod is that climate scientists like Kutzbach don't use these models to predict day-to-day weather but instead explore the longer-term average state of the climate system.

The annual climatic cycle provides a useful analogy. No scientist would attempt to use models to make a weather forecast for a specific July day months in the future, yet these same models can reliably predict the average conditions during a typical July day. The reason they can do so is that an average day in July is hot for a simple physical reason: the Sun's rays are direct and the days are long. The models do a fine job of simulating the average heating that occurs in response to the strength of the midsummer Sun. If they didn't, they would be totally useless. The same models can also simulate in a general way much of the day-to-day and week-to-week weather variability that occurs during a typical July, but again they cannot make a specific forecast for a particular day.

For the same reason, these models can be used for meaningful tests of the monsoon hypothesis. Kutzbach knew from calculations based on Earth's orbit around the Sun that solar radiation in the northern tropics 10,000 years ago was 8% higher than it is today, and he entered that value as an "initial condition" in running the model to see what kinds of climatic changes it would have caused. In effect, he wanted to find out what an "average July day" would have been like 10,000 years ago (rather than a specific weather forecast for a particular day, which would be impossible). The result from this first model experiment confirmed his hypothesis—stronger solar radiation drove a stronger monsoon that delivered more rain across the North African Sahel in the model simulation.

Kutzbach then turned to geographer Alayne Street-Perrot, who had begun a compilation of radiocarbon-dated lake levels across North Africa. Together they compared the model simulations of precipitation for several time intervals through the last 20,000 years against the radiocarbon-dated evidence from the lakes. The match was excellent. The lakes were low 20,000 years ago during a time when the model simulated a weak monsoon because solar radiation in summer was lower. The lakes were highest nearly 10,000 years ago when stronger radiation drove powerful monsoons. And the lake levels slowly dropped during the last 10,000 years as summer radiation levels waned. Beyond any doubt, the lake levels were following the tempo of the overhead summer Sun, not that of the distant ice sheets.

In the last two decades, the predictions of Kutzbach's hypothesis have been borne out so many times in so many regions that it has come to merit the higher stature of a theory. The basic tempo of dry today, wet 10,000 years ago, and dry again 20,000 years ago holds not just in North Africa but also across a great arc from southern Arabia, India, and Southeast Asia to southern China. Both the field observations and the model simulation shown in figure 5.2 agree that this entire region was wetter 10,000 years ago than it is today because the summer monsoon was stronger than it is now in response to greater amounts of summer solar radiation.

In my opinion, John Kutzbach's theory of the orbital control of monsoons ranks in importance right alongside Milankovitch's theory of the orbital control of ice sheets. Half of Earth's surface lies between 30°N and 30°S, and monsoon changes are the primary orbital-scale control of climate across this vast region. Most of the world's population lives at these latitudes today and has done so since humans evolved. We have lived with these monsoon fluctuations for several million years.

Perhaps the most impressive confirmation of Kutzbach's hypothesis has come from a remote source—ancient air bubbles trapped in the Antarctic ice sheet. Working at station Vostok at the very top of the ice sheet for years in the "warmth" of Antarctic summers (—20°F!), Russian engineers gradually drilled a 3,300-meter sequence of cores down through the entire mass of ice, except a bottom layer left untouched because of concern that an underlying lake might be contaminated by the chemicals used for drilling. Water exists in this improbable location because the pressure of the overlying ice causes melting of the deepest layers of the ice sheet.

Air bubbles trapped in the long column of ice retrieved at Vostok hold a history of changes in the amount of methane in the atmosphere (fig. 5.3). Scientists developed the initial time scale for this ice record based on models that estimate how snow is first compressed and crystallized into ice and then flows slowly down into the ice interior and out toward the ice margins. The methane record produced by this initial time scale showed a strong resemblance to changes in solar radiation at the 22,000-year precession cycle. Based on this evidence, scientists

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