Climate Change: Course Preview

Consequences of Climate Change

by Dr. Ed Mathez

This essay was developed for Week 6 of the AMNH online course Climate Change, part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators. Explore more sample resources...

Potential consequences of climate change include loss of biodiversity, destruction of terrestrial ecosystems, spread of human infectious and respiratory diseases, changes in ocean chemistry that disrupt the marine food chain and destroy tropical coral reefs, extreme and unusual weather events, drought due to warming and changing weather patterns, decreased water supplies from reduced winter snow packs and loss of mountain glaciers, and rising sea level. The list could be much longer. Rather than catalogue potential consequences, however, this essay explores just two examples in order to illustrate the profound risks and uncertainties associated with climate change, and to extract some relevant lessons.

Sea level rise

The Dynamic Climate System
Figure 1: Glaciers in the UK
The Channel River system and ice sheets about 21,000 years ago, when glaciers were at their maximum extent and sea level was about 120 meters (400 feet) lower than today. At that time the British Islands and mainland Europe were connected. The dashed lines show the shorelines near where the Channel River drained into the Bay of Biscay at 21,000 and 14,000 years ago. The dashed lines within the ice sheet are contours in meters of ice thickness. (from Ménot et al., 2006) ©Science

The notion that sea level can dramatically change may seem unfamiliar because it occurs so slowly as to be barely perceptible over a lifetime. But imagine this: 21,000 years ago, at the height of the last glaciation-when sea level was 120 meters (400 feet) lower than it is now-some of our ancestors were probably eking out an existence fishing and trapping on the banks of the Channel River, which flowed in the broad valley separating what is now Great Britain and the rest of Europe (Figure 1). That river and valley no longer exist, of course, because the region lies deep beneath the waters of the English Channel.

During the 20th century, sea level rose nearly 20 centimeters (8 inches), but over the last couple of decades it has been rising at the accelerated rate of 2.6 ± 0.04 millimeters/year (26 centimeters per century) (Figure 2). Several factors have contributed. The oceans have warmed, which causes water to expand and ocean volume to increase. Second, the amount of meltwater, mainly from mountain glaciers and permafrost, has increased. The grave concerns in the 21st century are that sea level will rise much more rapidly than it has in the past because of ice loss from Greenland and Antarctica, and that it will do so far faster than we can practically adapt to.

The Dynamic Climate System
Figure 2: Global Sea Levels
A reconstruction of global sea level based on tide gauge records from 1700 to 2000. The shadow shows uncertainty in the reconstruction, and the curve is a fit to annual data. (from Jevrejeva et al., 2008) ©American Geophysical Union

The Greenland Ice Sheet (Figure 3) is sensitive to warming for two reasons. First, it receives a lot of sun and melts substantially in the summer, which is a normal seasonal effect. Second, it is losing significant mass at its margins due to a rapid flow of ice into the ocean. The cause of the latter is a subject of active research. Temperatures over Greenland and rates of flow of the marginal glaciers appear to be particularly sensitive to the temperature of the surrounding ocean surface. The warm waters presumably melt the bases of the glaciers where they extend out into the fjords, causing the points at which the glaciers are "grounded" on the submarine bedrock to recede toward the heads of the fjords. This reduces the friction between the glaciers and the underlying rocks, allowing the glaciers to flow more rapidly. An added complication is that the ocean surface temperature around Greenland is sensitive to the natural decadal variations in climate that affect the entire North Atlantic Ocean. So, changes in the Greenland Ice Sheet may depend on how global warming influences the natural rhythms of the climate system, which at this point is poorly understood. Regardless of the detailed mechanisms of ice loss, a particular concern is that the climate may warm to the point where the loss of a large proportion of the Greenland Ice Sheet becomes unavoidable.

The Dynamic Climate System
Figure 3: Greenland Ice Sheet
This data visualization shows the changes in elevation over the Greenland Ice Sheet from 2003-2006. The ice sheet covers 1.7 million square kilometers (656,000 square miles) and at its maximum is over 3,000 meters (9,900 feet) thick. It holds enough ice to raise sea level by seven meters were it all to melt; this is unlikely, and would in any case take centuries. Overall, the ice sheet is losing mass and contributing to sea-level rise, faster now than 10 years ago. The pink and red regions indicate a slight thickening, while the blue and purple shades indicate a thinning of the ice sheet. ©NASA / GSFC

West Antarctica comprises the part of the continent on the west side of the Trans-Antarctic Mountains and includes the Antarctic Peninsula. The West Antarctic Ice Sheet appears to be particularly sensitive to the state of the surrounding ocean because its base is largely below sea level and thus around its edges in contact with the ocean. Also, the Antarctic Peninsula extends far enough to the north to be affected by the relatively warm circumpolar ocean current. Again, we do not currently know enough about the specific mechanisms to predict the rate at which ice will be lost as warming progresses.

Nevertheless, we have been able to measure the changing masses of ice on Greenland and Antarctica by satellite. Between 2002 and 2009, the Greenland Ice Sheet lost ice at a rate of 230 ± 33 gigatons/year, and the Antarctica Ice Sheet at a rate of 143 ± 73 gigatons/year (Figure 4). The total loss for the period corresponds to an average sea level rise of 1.1 ± 0.2 millimeters/year. While this corresponds to only 11 centimeters of sea level rise in a century, the rates of ice loss have been increasing. Consequently, the rate of sea level rise has itself been rising by the substantial amount of 0.17 millimeters each year.

For the 21st century, plausible estimates indicate that sea level could rise as much as two meters. Most estimates are lower, however, and all are highly uncertain. To put them in perspective, we should keep in mind that 634 million people (as of 2006), or 10% of the world's population, live in coastal regions within a 10-meter (33-foot) elevation of sea level. Further, most of the world's largest cities are also at or near sea level. A sea level rise of just 0.5 meters (1.7 feet) in a century would expose most of these people and cities to inundation (mainly as a result of coastal erosion), as well as to flooding during storm surges and unusually high tides. Can we adapt? What will it cost? What will be the consequences of displacing hundreds of millions of people? Without knowledge of the rate and amount of sea level rise, we have no answers.

The Dynamic Climate System
Figure 4: Ice Sheets Losing Mass
The declining masses of the Greenland and Antarctic ice sheets as determined by the GRACE (Gravity Recovery and Climate Experiment) satellite mission. Note the accelerating mass loss for both ice sheets. (from Velicogna, 2009) ©American Geophysical Union

Food supplies

From 2006 to 2008, the world experienced a small-scale food crisis. It was brought on by a combination of increased demand, more crops being dedicated to animal feed and biofuels, and local droughts and crop diseases that reduced global supplies of rice and other grains. The situation was further exacerbated by export restrictions introduced by several countries seeking to protect domestic supplies. Could that mini-crisis be prophetic?

According to 2010 U.N. projections, world population will grow from 6.9 billion people at present to more than 9.1 billion by midcentury. Barring some global catastrophe, demand for food will therefore increase. Meanwhile, increased global temperatures will affect the food supply in several ways. First, warming changes large-scale climate patterns, bringing drought conditions to some places and more precipitation to others. In particular, warming is causing the downward limbs of the Hadley cells (which correspond to the global belts of deserts) to shift poleward. It is also shifting large-scale atmospheric circulation patterns through changes in sea surface temperature patterns. Second, warming is decreasing mountain snow packs and causing mountain glaciers to melt. When these glaciers disappear, the flows of some major rivers, such as the Indus and Brahmaputra of India, are likely to diminish significantly, which will jeopardize the food security of many millions of people. Third, by increasing water evaporation rates, warming increases the rate at which soils dry out (Figure 5). Fourth, warming reduces crop yields. For example, various studies indicate that a 1°C increase in seasonal temperature in the tropics and subtropics will cause major grain yields to drop by between 2.5% and 16%.

The Dynamic Climate System
Figure 5: Change in the Palmer Drought Severity Index, 1900 to 2002
The index is a measure of soil moisture as determined by records of precipitation and temperature. It is expressed as a departure from the long-term local mean condition. On the map, reds and oranges are drier and greens and blues are wetter than the mean; on the graph, blue is wetter than the mean and red is drier than the mean. ©NCAR / IPCC

We can gain insight into the future effects of warming on agriculture from looking at the effects of past heat waves, such as the one that struck Europe in the summer of 2003. In central France, that summer was 3.6°C above the 1900-2006 summer mean temperature, making it the hottest in at least 600 years (based on the timing of the grape harvest). Compared to the year before, the French maize crop yield declined 30% and the fruit harvest 25%. Even the harvest of winter wheat, which had not matured by the time the heat wave began, dropped 21%. These alarming statistics were overshadowed, however, by the estimated 52,000 people who died from heat stress throughout western Europe.

The 2003 summer was unusual, but as warming continues the unusual will become less so. Under the assumption that greenhouse gas emissions will grow at a moderate rate (Figure 6), climate models show that most late 21st-century summers will be warmer than that of 2003. Even more worrying is the likelihood of a few extremely hot summers, which would probably have a far greater impact on food production.

The Dynamic Climate System
Figure 6: Summer Temperatures, 1900-2006
Distribution of mean summer (June-August) temperatures for the years 1900 to 2006, compared to the distribution of summer temperatures for the years 2080 to 2100 projected from climate models, assuming emissions grow at moderate rates through the century. The temperatures are expressed as departures from the average 1900-2006 summer temperature, and the distributions for each time period are normalized to 100 summers. (from Battisti and Naylor, 2009) ©Science

Because the 2003 heat wave was only a regional event, its effect on the global food supply was limited. Climate models predict, however, that summer temperatures will increase significantly across nearly the entire globe. The regions most at risk are the tropics and subtropics, which are home to about half the world's population. The lessons are clear: in the absence of adaptations such as the development of more heat-resistant crops, food supplies will likely decrease through the 21st century at the same time that population growth increases demand. As warming affects more of the globe, it will become increasingly difficult to offset food shortages in one region with surpluses from another.

Some Lessons

These two examples yield some general lessons. First, while it is true that some consequences of climate change are upon us now (e.g., stresses on sensitive ecosystems such as coral reefs), the profound changes are unfolding only slowly, and their harmful effects on humanity remain in the distant future. At the same time, one can imagine them manifesting themselves in sudden, catastrophic events, such as Hurricane Katrina in 2005, which led to the flooding of much of New Orleans. It is also hard to project how reduced food supplies will play out across the globe, because it depends to a large extent on how society reacts. Unfortunately, it is all too easy to imagine that the exercising of national interests combined with volatility in global food markets could lead to chaos, even wars.

As we have learned in this course, abundant observations indicate that the climate is warming. But the consequences of climate change, especially the more distant ones, are generally uncertain. We do not know how the natural world will respond to warming, or what the extent of warming itself will be. In large part, warming will depend on how greenhouse gas emissions grow, which in turn depends on a whole host of social, political, and economic factors. So society is left with a conundrum. Preventing the most harmful consequences of climate change will require costly mitigation policies or adaptation strategies. Yet neither human nature nor our political institutions are designed to cope with consequences that seem so distant and uncertain.

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