Climate Change: Natural and Human

Written by Chadwick Dearing Oliver and Fatma Arf Oliver

Figure 1. Glacier in Iceland. Farmhouse in lower center shows scale. (1) Courtesy of Cambridge University Press and Professor Roger Mesznik.

Figure 1. Glacier in Iceland. Farmhouse in lower center shows scale. (1) Courtesy of Cambridge University Press and Professor Roger Mesznik.

Climate change has always occurred naturally, and now is compounded by human causes as well. The climate change caused by people will probably be hotter and so more harmful than natural climate change; natural climate change, on the other hand, would likely be more spectacular because glaciers would return and sea levels would recede. We need to stop or slow human-induced climate change while simultaneously preparing for inevitable changes. �

When and in what directions more climate shifts will occur is uncertain.

Exactly when the climates in various regions reach thresholds and shift to different climate regimes is uncertain. Many grasslands have already shifted to deserts. Recent cold weather in upper latitudes may be foretelling of a shift there—or may be simply a random fluctuation. We also don’t know how the mixture of natural and human actions will change the climates, so, we need to be flexible.

When climates change, peoples commonly migrate from lands that are too cold, hot, or dry. About 2-3 thousand years ago, West Africa was not a desert and supported many more people in comfortable lifestyles, likely ancestors to the Bantu peoples who migrated to southern Africa.  Between one to two thousand years ago, many Huns, Mongols, and Turks left their drying homelands in central Asia for Europe for similar reasons (2). The migrations and resulting violence were much greater than the recent refugees moving to Europe from Africa or to the United States from Central America.

In the future, some peoples will need to move into land currently inhabited by others, but we don’t know who will be moving and who will be moved upon.  Before the “who’s and where’s” of migrations become known, the world should develop protocols so the migrations won’t be violent. Similar protocols for sharing water in transboundary rivers have worked in to various degrees in different world rivers (3).  Since currently the imposing and imposed populations are unknown, everyone will want protocols that ensure a fair treatment no matter which side of the migration pattern they are on.

When climates change, people commonly migrate. Protocols could keep migrations nonviolent.

Natural Climate Change

World climates have always changed in a natural cycle (Milankovitch Cycles) based on the Earth’s irregular orbit around the sun (1). These cycles are predictable. The changes are noticeable and will increasingly create problems if we’re not prepared.

17 thousand years ago, sunlight concentrated at the equator; upper latitudes were filled with glaciers.

The natural (Milankovitch) cycles shift sunlight concentration from the equator to the upper latitudes and back on approximately a 20-23 thousand year cycle (Figure 2). For several thousand years before 17 thousand years ago, sunlight was concentrated on the equator, which made it dry. The low sunlight concentration on the upper latitudes made them so cold that glaciers formed and flowed toward the equator, covering Canada and much of northern Europe.  The glaciers took so much water that ocean levels became about 120 meters lower than present.  The lower oceans exposed fertile land, especially in southeast Asia, southern South America, and southeastern United States. The air was drier, and even the tropics were probably cooler than now. Tropical forests dried to shrub- and grass-lands, and extreme deserts formed (Figure 3).

Figure 2. Changes in Antarctic temperatures (at 2 stations) and global ice volume (4).

Figure 2. Changes in Antarctic temperatures (at 2 stations) and global ice volume (4).

By 8 thousand years ago, most glaciers had melted and oceans had risen to near-present levels as sunlight concentration shifted to the poles. Both equatorial and upper latitudes were mild and moister than today. Forests expanded in many tropical areas and in previously glaciated areas. Deserts —Sahara, Arabian, Mojave—became grasslands.

Figure 3. Past and present Earth land covers.  Bordered white = glaciers; other white = deserts; gray=grass/shrubs; black=forests.  (1) Courtesy of Cambridge University Press.

Figure 3. Past and present Earth land covers. Bordered white = glaciers; other white = deserts; gray=grass/shrubs; black=forests. (1) Courtesy of Cambridge University Press.

During the past few thousand years, the sunlight has been shifting back to the equator, tropical and lower temperate areas are drying, and forests are shrinking.  The Sahara, Arabian, and Mojave grasslands have changed back to deserts. Sunlight has not reached its peak at the tropics, so drying and similar processes will continue.

Without human interference, we would expect the tropics to continue drying and upper latitudes to become cooler. Without human global warming, glaciers would form where they originated before (Figure 3), but they would not advance as far as 17 thousand years ago—nor would the

By 8 thousand years ago, sunlight concentration had shifted to upper latitudes; glaciers melted, oceans rose, and forests expanded.

sea level lower as far—perhaps one third to one half as much. The potential weaker glaciation is because the Milankovitch Cycles will be in less synchrony (Fig. 2); extreme glaciations followed by the very comfortable warm period we are just leaving happens about every 100 thousand years.

In the past few thousand years, sunlight is shifting back to the equator, grasslands are becoming deserts, and forests are shrinking.

Even with the slightly less extreme glaciation, sea level drop, and equatorial drying, we would expect human migrations as places become unfit for living—just as they happened before. The greater, exposed land should help accommodate them.

Human-Induced Climate Change

Human pollution—especially releasing CO2 and other greenhouse gases into the atmosphere primarily by burning fossil fuels—is also causing climate changes that are becoming noticeable. They will become more serious if we do not curb atmospheric CO2 and are not prepared.

Human-caused greenhouse gases are expected to prevent glaciation for one thousand years, but create other problems.

The human-caused greenhouse gases are predicted to prevent the glaciations for the next one thousand years (5). Even if glaciation does not occur, the upper latitudes may still become too cold for living, the equatorial areas much hotter and drier because the sea level will not drop, and little or no more land will be exposed for people to live on.  Consequently, people will probably concentrate on smaller, mid-latitude areas without the expanding agriculture land had the sea level receded.

References:

1. Oliver, C.D., and F.A. Oliver. Global Resources and the Environment. 2018. Cambridge University Press.

2. Frankopan, P. The Silk Roads. 2017. Vintage Books.

3. P. H. Gleick. Water and Conflict: Fresh Water Resources and International Security. International Security. 1993;18(1):79-112.

4. Free Software Foundation, Inc. 2000. https.commons.wikimedia.org.wiki.File:Ice_Age_Temperature.png#filelinks

5. T. Stocker, D. Qin, G. Plattner, M. Tignor, S. Allen, J. Boschung, et al. Climate Change 2013: The Physical Science Basis. 2013.