Lecture 34

Introduction

So, here we are settled nicely inside an Icehouse age, and probably to remain there for many millions of years. Nevertheless, as clearly shown in Figure 1 (which we saw before), we are currently sitting very close to the glacial/interglacial boundary condition and tend to oscillate back and forth across it - probably in response to the Milankovitch factors. Roughly 21,000 years ago the Earth was at a glacial maximum; the climate was cold, dry, and windy. Large mammals now extinct, such as mammoths, saber-toothed tigers and giant sloths, were hunted by our ancestors.

In this final lecture we will look at a couple of interesting climatic events since that glacial maximum, examine the current anthropogenetic effect on climate, and then speculate upon the future.

The Younger Dryas Event

As global climate warmed from the 21,000 glacial maximum, one dramatic cooling event, called the Younger Dryas Event, occurred and is well documented in pollen records (from an Arctic plant called Dryas) from Europe. As the North American ice sheet melted back to the north, starting at about 15,000 years ago warm water flowed northward along the European coast. For 2000 years, the climate warmed; ice sheets were melting, plants and animals were reoccupying the deglaciated lands, sea ice in the North Atlantic had all but disappeared. Clearly, the glacial period was over. Suddenly, at about 13,000 years ago and lasting until about 11,700 years ago, there was a major reversal (Figure 2). Within 10 years, the climate began to cool precipitously. Ocean temperatures fell, sea ice reappeared, and glaciers grew. Our records are very good, thanks to the pollen distribution of Dryas octopetala, which is limited to polar climates. Just as quickly as it started, the event came to a close about 1000 years later; marking the close, temperatures rose about 7^oC in 40 years!

The effects of the Younger Dryas Event are most pronounced around the North Atlantic, thus that is where we should look for the cause. What happened is that enormous fresh water (glacial melt water) lakes formed in the depressions as the glacier retreated at the end of the main glacial period. At one point, the retreating glaciers uncovered a natural drainage- way between these lakes and the north Atlantic. Melt-water suddenly flowed rapidly into the ocean where it formed a freshwater 'lid' over the denser salty marine water. The cold surface melt-water reduced evaporation, shut down the natural ocean circulation system, and pushed cold air across Europe. As soon as the flow of melt-water was reduced as glaciers grew again, the ocean currents once again resumed their normal pattern, and the climate change to a warmer one. Why didn't the process just happen all over again as warming resumed? I can only guess that in the 1000 year interval, the reduced load of glacial ice interrupted the drainage-way or the volume of melt water was much reduced.

The Little Ice Age

As the last millennium began, scattered evidence from Europe and the high latitudes of North America suggests a time of relatively warm climate near 1000 to 1300 called the Medieval Climatic Optimum; interestingly, this coincides with a very 'liberal' time of learning through much of Europe (I guess if you don't have to worry much about crops and freezing to death, you have time to work on cultural things)! During this time, Nordic people settled southwestern Greenland and even managed to grow wheat there. Sea ice, common today around the Greenland coast, was scarce. The population of Iceland reached about 80,000, and the North Atlantic was generally warmer and more storm-free than now. By about 1400 climate had worsened considerably, to the point that the North Atlantic was subjected to frequent storms, sea ice advanced dramatically, and the climate of Europe became cold and wet. All shipping in the North Atlantic gradually ceased and the Norse/Viking settlements on Iceland, Greenland and North America were abandoned. During the very late 1500s and the earliest 1600s some recovery took place, but conditions worsened again rapidly.

Worldwide temperatures were not actually vastly colder than at present: just 1^o to 1.5^oC lower. However, the practical impact of this small drop in temperature was profound. Alpine glaciers in the Alps and elsewhere advanced dramatically. The high mountains of Ethiopia were blanketed in snow (a first for humans!). At times one could walk from Staten Island to Manhattan on ice. The river Thames in London (the other one!) froze more than two dozen times. The early American colonists apparently had to endure winters far more bitter than those of today. Reduced evaporation from the colder oceans led to devastating droughts in some parts of the world, while the shortened growing season and wet summers in other parts contributed to food shortages. The 1810-1819 decade was the coldest in Europe since the beginning of the Little Ice Age, and that was the last straw for many, who immigrated to North America where the effects of the change did not seem so severe. The Little Ice Age came to an end with a general warming trend in the early to mid 1800s.

So, what factors influenced the beginning, duration and end of the Little Ice Age? Take a look at Figure 3; this is a record of climate change as interpreted from Greenland ice cores. The last interglacial period seems to have been roughly 30 thousand years in duration, and the intervening glacial period roughly 100 thousand years. One probability is that Milankovitch factors controlled these major boundaries. That's the interpretation shown in the top part of Figure 4 (i.e. there's been fairly constant cooling since the peak warmth of the last interglacial, and there's more cooling to go!). I think we would accept that this interpretation is a bit too simple; there is no accommodation whatever for climatic influences other than variation in solar system geometry. The second interpretation is shown in the lower half of Figure 4; here, short- term factors such as atmospheric dust accumulations (from large volcanic eruptions), changes in sediment load from rivers, temporary changes in deep ocean circulations, etc (collectively known as "millennial oscillations") are superimposed upon the long-term orbital geometry pattern. OK, can we point to a particular millennial oscillation factor that sufficiently influenced climate as to produce the Little Ice Age? The short answer is "No"! As far as significant volcanic activity was concerned, there was the huge basalt flow in Iceland at Laki that occurred in 1783 (Remember, this is the one that filled the air with a 'blue fog' from the high fluorine content it held. It also was said to be responsible for the death of 25% of the people of Iceland because of starvation). Then, along came the largest volcanic eruption in recorded history: Tamboro in Indonesia in 1815, following on the heels of a Pinatubo-type eruption (Mayon) in the Philippines the year before. But these events came in the latter stages of the Little Ice Age; they certainly didn't start it! It seems to me that, at this stage, we have to accept that there was a millennial oscillation event, but we cannot yet define its cause. This is really very frustrating!

By the way, there is another theory: millennial-scale changes in the Sun's strength have been hypothesized as a cause of millennial climatic oscillations, but the evidence really weak! We've not been collecting data for very long (precise measurements started in 1978), but there seems to be a cyclical variation in radiation of 11 years (Figure 5) (This cycle is supported by astronomical observations - but not precise measurements - over very many decades). Climatic models indicate that a change of 0.15% in the Sun's strength could alter global mean temperature by as much as 0.2^oC if it persisted for a long time. For an 11-year cycle, however, the 5.5- year interval between minimum and maximum does not allow the climate system enough time to register a full equilibrium response. So, it's unlikely that any variation in Sun activity produced the Little Ice Age.

We can't give up without looking at one other possible factor: an unusual series of El Niño events. As you know, El Niño is an ocean circulation pattern that interrupts the normal circulation pattern in the Pacific Ocean every 2 to 7 years. During non-El Niños years, surface temperatures along the coasts of Peru and Ecuador and in the eastern equatorial tropical Pacific are near 18oC in winter - far cooler than typical tropical water temperatures of 25oC or higher. This region typically has the coolest tropical surface water on Earth. Upwelling and strong winds in the southern hemisphere's winter (August) is the cause. Winds drive warm surface water away from the coast, and cooler water wells up from below (top of Figure 6). Near the equator, in the southern hemisphere's high-pressure regions, the winds turn westward, driving warm water ahead. The upwelling cool water near the coast brings nutrients with it, supplying food for fish, marine mammals and water birds.

El Niño years change all this. During El Niño winters, strong winds fail to blow in the eastern and tropical Pacific, upwelling does not occur, and the surface waters near the coast are warm. The resulting effects on sea life and humans along the South American coast are devastating. El Niño warming of surface waters near the coast also produces large amounts of moisture, leading to cloudbursts, flash floods, and a whole series of natural disasters. Because weather systems are global, the effects of El Niño appear throughout all oceans, but are strongest in the Pacific.

Historical records track past occurrences of El Niño. The records start in 1525 (the Spaniards began making environmental observations soon after their Inca empire conquest), and the time between successive El Niño events averages 4 years, but the actual timing varies widely. As shown in Figure 7, nine very severe events occurred in the interval of 1525 to 1980 (another very severe one occurred in 1983 and the most recent in 1998), but there's no correlation whatever with the timing of the Little Ice Age!

Humans and Greenhouse Gas Changes

Humans arrived late in Earth's long history. The first somewhat human creatures who walked on two legs and used stone tools appeared only within the last 4 million years. Over most of their time on Earth, humans have been affected by climate but have not had any measurable impact on the climate system (In fact, there are many scientists that insist it was changes in climate that caused humans to evolve as they have. They believe that warm, wet interglacial periods allowed minor population explosions and that cool, dry glacial periods then winnowed out those less fit to survive under severe conditions). Within the last two centuries, humans have begun to alter climate (through changes to the Earth's greenhouse), first at regional and then global scales, although the magnitude of our impacts remains uncertain (that's why there are question marks at the ends of the graphs in Figure 4).

Carbon Dioxide and Methane Variation

We all recognize carbon dioxide as the major greenhouse gas of our atmosphere; Figure 8 shows the increase in CO₂ since preindustrial times. We also all recognize that the increase is largely due to human activities. Throughout the late 18th century and most of the 19th, the main source of carbon was the clearing of forests to meet the need for farmland, for home heating fuel and to produce charcoal for the beginnings of the Industrial Revolution. After 1900, most of the extra carbon has come from burning fossil fuels. Figure 9 shows where the extra carbon goes.

Methane (CH₄) is another important greenhouse gas, and Figure 10 shows how its content has changed; most of that methane comes from our increase in artificial wetland agriculture (such as rice paddies). In the absence of oxygen (underwater), bacteria break down vegetation and extract its carbon, which combines with hydrogen to form methane. The increase from cattle simply reflects our increased need for dairy products.

Climate in the Next 100 to 1000 Years

Natural Climate Variation

There's no doubt that global warming is real - but what are the causes? Figure 11 presents three different scenarios (all three use the same observed warming trend): the case where all warming is superimposed upon a static natural system (graph A), the case of a natural process (or combination of processes) leading to warming in addition to our greenhouse contributions, and the case of greenhouse contributions being so large as to negate a natural cooling effect.

Many scientists believe that the third graph is closest to the truth; in other words, temperature in our present interglacial period had peaked just prior to the Little Ice Age, and, except for human intervention, we would be heading back into another glacial period. They refer to this effect as a 'superinterglacial' period (Figure 12), and suggest it will all come to a crashing end soon after we consume most of the world's fossil fuels. At that point, Earth's climate will resume its cooling trend.

Projected Temperature Change

When scientists discuss potential climate changes, they constrain their models with two limiting greenhouse gas curves: the curve obtained for an atmospheric CO₂-content that reaches 2 x CO₂ of preindustrial times, and that which reaches 4 x CO₂ levels of preindustrial times (Figure 13). Of course, atmospheric CO₂ levels of either 2x or 4x the preindustrial values would be without precedent in the last several years of Earth history. We think that CO₂ levels were last at the 2x level about 7 million years ago; concentrations as high as 4x preindustrial values have probably not existed since the Cretaceous greenhouse world of 100 million years ago.

 Can we simply match CO₂ contents with past times, thus use those past climatic conditions as analogs for future climate? Absolutely not! The future high-CO₂ pulse will not stay around long enough to bring all (or even most) parts of the climate system into equilibrium. The fast-response systems will quickly adjust but the slow systems (like ice sheets) won't have enough time before most of the excess CO₂ disappears into the oceans (and that will take place sometime after our fossil fuels run out). We expect the peak in CO₂ to last about one century, but levels will remain above preindustrial for one thousand or more years.

The greater warmth will cause melting along the ice margins, particularly in the warmer region around southern Greenland, but the bulk of the ice will survive (By the way, high latitudes are more responsive to climate change because of the positive feedback effects of ice and snow albedo). Climate in 2100 will, in some respects, be like that 5 to 10 million years ago.

Of particular interest to Canadians is the question: what will the Arctic be like in the future? Because sea ice and vegetation are relatively fast- responding parts of the climate system, we should expect similarly large transformations of polar ice, permafrost, tundra, and northern forests as climate warms [I've included an Appendix to this lecture that is a summary of an environmental study of the Mackenzie Basin, Canada. If you wish to learn more, the information comes from a study in Canadian Geographic, Nov/Dec issue, 1997].

Evaporation will increase worldwide because warmer temperatures will permit air to hold more water vapor. With more water vapor in the air, global average precipitation is also likely to increase, but in patterns that vary greatly from region to region. Very likely, those areas that already receive little precipitation will receive less.

There will be rapid melting of ice margins in Greenland, contributing to a rise in sea level. In the western Antarctic, ice melting will be even more rapid, significantly increasing ocean water levels. Of course, expansion of water as it warms will further cause oceans to rise.

Some Impacts on Humans

The 15 cm rise in sea level that has accompanied the 0.6^oC warming of the last 100 years should increase by a factor of about 3 over the next century. Although a sea level rise of 50 cm sounds small, remember that in places like Bangladesh most of the population lives on land that is within 1 meter of present-day sea level. Closer to home, think about the heavy coastal populations of Florida, Louisiana and Texas that already live behind sea walls.

More critical than a simple rise in water level will be the devastation due to water surges that accompany hurricanes and typhoons. There is also a reasonable probability that the number of hurricanes and typhoons will increase as waters warm.

Large-scale melting of permafrost and high latitude sea ice will open the regions to increased transportation, but will devastate the populations of caribou, polar bears, and the rest of the polar ecosystem - including the cultures of northern people. The rate of high latitude species extinction will dramatically increase.

Seasons will change (Figure 14). In northern mid-latitudes (here), summers will last for an extra two months, while winters will be shorter and less harsh (this is probably not the time to invest in a local ski resort!).

Water is already a scarce and precious commodity. In regions where evaporation increases but precipitation does not, human populations will face serious water shortages. Agriculture will become more dependent upon irrigation.

Climate in the Far Future

More than a millennium from now, when the excess CO₂ has been absorbed by the oceans and this huge experiment is finished, what will happen to Earth's climate? Left to function on its own, natural climatic processes will again regain control, but slowly. And just as during its past history, Earth's future climate will vary in different ways at different time scales.

On shorter time scales of centuries to millennia, changes in global temperatures of 1^oC or less will be driven by changes in the strength of the Sun and by whatever other processes cause millennial scale oscillations. Over thousands to tens of thousands of years, the probability is that a cooling of climate at high northern latitudes will gradually initiate the next glacial period. As noted before, the expansion of ice in the Little Ice Age is thought to have been the first step toward the next glaciation, but it was ended by the human effect of strengthening the greenhouse gases. Without those gas additions, the process will resume. Over still longer tectonic time scales (millions to tens of millions of years), Earth's climate will probably continue to drift toward colder temperatures and more intense glaciations. The relatively pleasant climatic interlude of the last several thousand years will certainly end.

Appendix: The Mackenzie Basin Impact Study

This is the best environmental study that has yet been done in the world as the superinterglacial begins. Three northern regions around the world have been termed "climate hot spots": Lake Baikal in Siberia, the northwestern region of Alaska, and parts of the Mackenzie Basin (Figure A1). Over the last century, temperatures have risen by three times the global average of half a degree in each of those areas.

The Mackenzie Basin Impact Study (MBIS) focuses on the 1.8- million square-kilometer drainage basin of the continent's largest northerly flowing river. The six-year, $950,000 effort documents rising temperatures, melted permafrost, new landslides, and increased forest fires. Then, using computer technology available, its authors project the future impact of continued global warming on the area's wildlife, vegetation and human inhabitants.

The 4,241-kilometre-long Mackenzie River draws its waters from as far south as the Alberta Rockies near Jasper (Figure A2). It is fed by the Liard, Peace and Athabasca rivers, as well as by three bodies of fresh water, Great Bear, Athabasca and Great Slave lakes.

These ecosystems have undergone significant changes. Over the last 35 years, there's been a substantial increase in temperatures - about a degree a decade - in the center of the basin. There has been glacial retreat, landslides, and a lowering of water levels, and the southern edge of permafrost in the Prairie Provinces has receded northward by about 100 kilometers during the past century. Fires, burning as much as three million hectares per year (3x normal), have been common.

Supported by, among others, the federal government, the Northwest Territories government, B.C. Hydro, the University of Victoria and Esso Resources Ltd., the study has already been recognized worldwide as one of the most significant bodies of work in the climate field. The long-awaited report, released in August 1997, provides unprecedented documentation of global warming changes and gives us a glimpse of what the future might look like if greenhouse gas emissions are not curtailed.