In this chapter, we’ll review Unit 9 of the AP Environmental Science Course, Global Change. According to the College Board, about 15–20% of the test is based directly on the ideas covered in this chapter. If you are unfamiliar with a topic presented here, consult your textbook for more in-depth information.

By now, you’ve probably almost completed your AP Environmental Science course. What’s left? In this chapter, we zoom out to the global picture—first, with the biggest environmental problem facing humanity: global climate change. Then we’ll talk about the other major global problem: loss of biodiversity. Finally, we’ll get around to what steps we as global citizens can take in the face of these problems!

THE GREENHOUSE EFFECT

Remember in Chapter 6 when we discussed the greenhouse effect and the greenhouse gases that cause it? It looks something like this: the sun’s rays strike the Earth, and some of the solar radiation is reflected back into space; however, greenhouse gases in the troposphere intercept and absorb a lot of this radiation. This warms the atmosphere and the Earth’s surface. Remember: the greenhouse effect results in the surface temperature necessary for life on Earth to exist.

• The principal greenhouse gases are water vapor (H2O), carbon dioxide (CO2), and methane, but several other gases (mainly pollutants), such as nitrous oxide, ozone, chlorofluorocarbons (CFCs), and hydrofluorocarbons (HFCs) have some amount of greenhouse effect as well.

• As you will have noticed in the previous chapters, several human activities are responsible for the release of these gases in greater quantities than would naturally occur without us.

• While the Earth has undergone climate change throughout geologic time, with major shifts in global temperatures causing periods of warming and cooling as recorded with carbon dioxide data and ice cores, we have recently reached the unavoidable consensus that the global climate is undergoing a shift that is almost certainly in large part due to these human contributions: an idea referred to simply as climate change.
  

CLIMATE CHANGE

Scientists use very sophisticated computer models and make several thousand meteorological observations each day to monitor the daily temperature of the Earth’s atmosphere. Over the last several years, their observations have shown that there has been a slow but steady rise in the Earth’s average temperature. According to NASA, 2016 was the warmest year on record, and if El Niño and La Niña patterns were removed from the record, 2017 would have been the warmest year on record. Other qualified scientists have carefully documented a decrease in the size of glaciers and ice sheets, a slight rise in the average ocean level, and more severe rainstorms.

• In response to these phenomena, the Intergovernmental Panel on Climate Change (IPCC) gathered hundreds of scientists from around the world to study these problems.

• In a 2013 report, the IPCC stated that most of the observed increase in the global average temperature since the mid-20th century is very likely (greater than 95 percent) due to the observed increase in anthropogenic greenhouse gas concentrations.

• The three major gases are carbon dioxide (from pre-industrial levels of 280 ppm to 400 ppm in 2016), methane (from preindustrial levels of 715 ppb to 1,840 ppb in 2016), and nitrous oxide (from preindustrial levels of 270 ppb to 328 ppb in 2016).

• These gases absorb the infrared heat radiating from the Earth and thus heat the lower atmosphere. This warming is in addition to the normal warming of the atmosphere by the greenhouse effect.

• The impact of a particular greenhouse gas on global climate change can be thought of in terms of its global warming potential (GWP).

• Carbon dioxide, which has a GWP of 1, is used as a reference point for the comparison of the impacts of different greenhouse gases on global climate change. Chlorofluorocarbons (CFCs) have the highest GWP, followed by nitrous oxide, and then methane.

It should also be mentioned that while water vapor is a greenhouse gas and even accounts for the largest percentage of the greenhouse effect, it doesn’t contribute significantly to global climate change, because it has a short residence time in the atmosphere (an average of about nine days compared to years or centuries for other greenhouse gas molecules), and its levels have remained consistent for some time. However, water does respond to and amplify the effects of other greenhouse gases.

The increase in the Earth’s temperature will lead to a variety of changes to the Earth.

• Physical changes on Earth include continued rising temperatures, further melting of glaciers, ice sheets, and permafrost, changes in precipitation patterns (with wet areas getting more precipitation and dry areas getting less precipitation), an increase in the frequency and duration of storms, an increase in the number of hot days, and a decrease in the number of cold days.

While it’s clear that rising air and surface temperatures have widespread effects on the biosphere, it’s also true that ocean temperatures are increasing due to the increase in greenhouse gases in the atmosphere and the greenhouse effect they produce.

• The most far-reaching consequence of this is that the thermal expansion of water (warm water is less dense than cooler water, and therefore takes up more space) is one of the factors contributing to rising sea levels (along with the melting of glaciers and ice sheets mentioned above).

• Ocean warming will also likely cause changes in coastlines, ocean currents, sea surface temperatures, tides, the sea floor, and weather.

Complicating the picture of oceanic effects is the phenomenon of ocean acidification, the decrease in pH of the oceans that’s primarily another effect of the increased carbon dioxide concentrations in the atmosphere due to the burning of fossil fuels, vehicle emissions, and deforestation.

• As more carbon dioxide is released into the atmosphere, the oceans absorb a large part of that carbon dioxide.

• Climate change will also affect biota. While there will be increased crop yields in cold environments, this is likely to be offset by loss of croplands as other areas suffer droughts and higher temperatures. Cold-tolerant species will need to migrate to cooler climates or they may become extinct.

• As equatorial-type climate zones spread north and south into what are currently subtropical and temperate climate zones, pathogens, infectious diseases, and any associated vectors will spread into these areas (where these diseases have not previously been known to occur).

Entire coastal populations and ecozones will also be displaced by the rising oceans.

Marine ecosystems will also be affected.

• The change in sea level will have effects: some positive (for example, in newly created habitats on newly-flooded continental shelves) and some negative (for example, in deeper communities that may no longer fall within the photic zone of seawater). But the effects of ocean warming are likely to be even more drastic.

• Some habitats will be damaged or lost; some species are likely to adapt through metabolic and/or reproductive changes; and, as with sea levels, ocean warming may have positive effects on some habitats and organisms as well.

Ocean acidification has and will continue to have tangible effects on ocean biota as well. Some species are experiencing reproductive and metabolic changes, just as with ocean warming; and the greatest effects of acidification can be seen in organisms that make use of calcification (as in shells): for example, corals.

As we reviewed in Chapter 4, coral reefs are structures found in warm, shallow tropical waters that represent diverse and ecologically crucial ecosystems.

• Coral reefs are created by small marine animals (called cnidarians), which are involved in mutualistic relationships with photosynthetic algae called zooxanthellae.

• Reefs provide local populations with a great variety of seafood and are popular recreational areas for humans.

• Ocean acidification damages corals by decreasing their ability to calcify (due to loss of calcium carbonate), making it difficult for them to form shells. When you put this together with coral bleaching, the world’s coral reefs are under severe threat.

• Coral bleaching is what happens when coral polyps expel the symbiotic algae that live in their shells and are vital to their health—the algae provide up to 90 percent of the corals’ energy!

• Once a coral bleaches, it continues to live but begins to starve. Some corals recover, but many don’t. The leading cause of coral bleaching is rising water temperatures, but the list of contributing factors also includes:

increased sunlight exposure

increased sedimentation (due to silt runoff)

bacterial infections

increased or decreased salinity

herbicides

exposure (as in extremely low tides)

mineral dust carried in dust storms caused by drought

pollutants such as those commonly found in sunscreens

ocean acidification

oil and chemical spills

oxygen starvation caused by increase in zooplankton after overfishing

In many areas of the world, pollution, climate change, and exploitation have led to severe and irreversible damages to these reefs.

• Another set of effects we may see due to global climate change is changes to the larger patterns of climate. For example, remember our discussion of winds in Chapter 6—winds generated by atmospheric circulation help transport heat throughout the Earth.

• Climate change may change the circulation patterns of wind, because the temperature changes may impact Hadley cells and the jet stream. And the winds in turn affect the oceanic currents, which carry heat in the water just as the winds do in the air.

• When these currents change, it will likely have a great impact on the climate in many places, especially coastal regions.