Fires, Floods, and Droughts: Impacts of Climate Change in the U.S.

Unit Content 

The content that will be addressed in this unit is general climate change impacts, hydrocarbons and the combustion of fuels, fundamental chemistry of acids, bases, pH, and indicators, the chemistry of ocean acidification, biodiversity loss, and the graphing and analysis of relevant data. The goals of this unit are to provide basic knowledge about relevant chemistry concepts as they relate to ocean acidification, explain the connections between ocean acidification and biodiversity loss, and present student-friendly data that can be used to further investigate ocean acidification and its connection to climate change impacts. This unit serves to allow students to engage with these topics through a cross-curricular approach that enables teachers from various disciplines (chemistry, biology, ecology, environmental science, mathematics, or others) to use the content as they see fit. 

Climate Change Impacts

The impacts of climate change differ for different regions of the world. Within the United States, a range of different regions have begun to experience impacts of climate change, such as droughts, extreme weather events, biodiversity loss, air quality concerns, human health concerns, sea level rise, and ocean acidification.¹ The extent to which these impacts in specific regions will occur is unknown, with various models predicting different levels of impact at different confidence levels.⁵ Future projections are contingent upon different scenarios in the Representative Concentration Pathways (RCPs).⁵ These pathways represent the extent to which greenhouse gases will be emitted within this century, and they are based upon how we will respond to our greenhouse gas emissions. Cutting as many further carbon emissions as possible projects the best outcomes for our global temperature and climate change future.⁵

Hydrocarbons and the Combustion of Fuels

Carbon is an element on the periodic table with an atomic number of 6. Hydrogen is an element on the periodic table with an atomic number of 1. Carbon is known as one of the building blocks of life due to its chemical structure and ability to combine with hydrogen in order to form large molecules that are vital for everyday biological life. In addition to their role in biological life, carbon and hydrogen compounds, known as hydrocarbons, have been used as fuels. The process of combustion, or burning these hydrocarbon fuels in the presence of oxygen gas, produces a different carbon compound, carbon dioxide (CO₂) as well as water (H₂O) and a substantial amount of energy released as heat.⁶ The general chemical equation for the combustion process of a simple hydrocarbon fuel is as follows: C_nH_2n+2 + O₂ → CO₂ + H₂O + energy. The n in the equation could be any whole number, typically between 1 and 22. This chemical reaction jump-started the Industrial Revolution with its ability to take naturally occurring hydrocarbon fuels found in the Earth, such as methane, crude oil, and coal, and turn them into energy used to power factories, automobiles, homes, and many other facets of life.

The downside of producing all of this power was the production of CO₂ and its release into our atmosphere for over 100 years. CO₂ is a greenhouse gas, meaning that it is a gas that traps some of the heat energy radiated from the Earth within our atmosphere, rather than allowing it to be released into space.¹ The amount of CO₂ in the atmosphere has increased by approximately 40% since the Industrial Revolution.¹ As a result of this anthropogenic emission of CO₂, the global temperature of Earth has increased, bringing with it many other changes in our climate. As a result of higher atmospheric CO₂ levels, there are higher carbonic acid (H₂CO₃) levels and thus ocean acidification.¹

Ocean Acidification: an Impact of Climate Change

Ocean Acidification Chemistry

Many climate change impacts are the direct or indirect result of increasing atmospheric CO₂ levels. Ocean acidification is no exception to this phenomenon. Before the Industrial Revolution, the ocean pH was approximately 8.2, while currently ocean pH is approximately 8.1.⁷ This increase in ocean acidity results from higher levels of CO₂ present in the atmosphere. According to Henry’s Law, an increase in the partial pressure of a gas will increase the amount of gas dissolved in a liquid.⁸ As the amount of atmospheric CO₂ has increased since the Industrial Revolution, the partial pressure of CO₂ (pCO₂) has also increased. With this increase in pCO₂, more CO₂ has dissolved in the oceans.

The ocean acidification process is driven by additional CO₂ dissolved in seawater. This dissolved CO₂ reacts with water to produce carbonic acid (H₂CO₃).⁷ Carbonic acid is a weak acid that readily dissociates in the presence of water into bicarbonate ions (HCO₃^-) and hydronium ions (H₃O⁺). The general chemical equation for this is: H₂CO₃ + H₂O ↔ HCO₃^- + H₃O⁺. This equation is written with a double-sided arrow to indicate that it is a reversible reaction given the conditions of the solution. The forward reaction is favored in an ocean environment because the ocean pH is approximately 8.1, and higher concentrations of bicarbonate are favored with a pH value in this range. An increase in the forward reaction increases the concentration of H₃O⁺ ions, which results in a lower and more acidic pH.⁹

Chemistry of Acids, Bases, pH, and Indicators

Acids are substances with a pH less than 7, neutral substances have a pH of exactly 7, and bases are substances with a pH greater than 7.⁹ Pure water has a pH of exactly 7 due to the self-ionization of water into equal concentrations of hydronium (H₃O⁺) ions and hydroxide (OH^-) ions. Acids have a higher concentration of H₃O⁺ ions while bases have a higher concentration of OH^- ions.⁹ pH represents the negative log of the concentration of H₃O⁺ ions in a substance. Since the pH scale operates using the log base 10 system, a substance with a pH of 5 has a concentration of H₃O⁺ ions that is 10 times larger than a substance with a pH of 6.⁹ Acid-base indicators are chemical substances that change color based on their pH range.¹⁰ Indicators can also be made from natural products such as red cabbage.¹⁰ Different types of pH paper can also be made from indicators, the most common of which is universal indicator solution, which has a wide color range. The pH paper can be dipped in a chemical and it will change color according to its pH.¹⁰

The Role of Carbonate Ions in Ocean Acidification

The mechanism of ocean acidification also has a slightly different pathway when it occurs in the presence of carbonate ions (CO₃^-2). In this pathway, CO₂ reacts with water (H₂O) and carbonate ions (CO₃^-2) present in the water to form bicarbonate ions (HCO₃^-).¹¹ The general equation for this process is as follows: CO₂ + H₂O + CO₃^-2 ↔ 2HCO₃^-. This is also a reversible reaction, depending on the conditions of the ocean water solution. An increase in the forward reaction is caused by increased amounts of CO₂ dissolved in the water, which shifts the chemical equilibrium toward decreased concentrations of carbonate ions (CO₃^-2) that are naturally present in seawater.¹¹

Since carbonate ions play an important role in the mechanism of ocean acidification, one metric scientists have used to measure levels of these ions is the aragonite saturation.¹² Aragonite is a form of carbonate ions that is used by marine organisms to produce their calcium carbonate structures.¹² Climate scientists have been studying the relationship between ocean acidification and its effects on aragonite saturation and pH level. In the Chesapeake Bay from 1986-2015, there have been overall reductions in pH and aragonite saturation.¹³ Since aragonite saturation measures the amount of usable calcium carbonate in the water, a lower level of aragonite saturation indicates that there is less calcium carbonate that marine organisms can use to build their shells.

Biodiversity Loss of Marine Ecosystems

Ocean Acidification and Shellfish

Certain marine organisms, including many shellfish, such as oysters and mollusks, rely on carbonate ions in order to produce their calcium carbonate (CaCO₃) shells or skeletons.⁷ The general chemical equation for the formation of calcium carbonate in these organisms is CO₃^-2 + Ca⁺² ↔ CaCO₃. At a lower pH, bicarbonate ion production increases according to the following equation, CO₂ + H₂O + CO₃^-2 ↔ 2HCO₃^-. As a result of this, more carbonate ions must enter the water environment to re-establish the chemical equilibrium. The primary sources of these ions are the calcium carbonate shells and skeletons of marine organisms, so they begin to dissolve in the slightly more acidic water in order to produce carbonate ions.⁴ Ocean acidification negatively affects pteropods through this mechanism. As the pH of the water decreases, the shells of developed pteropods have the potential to dissolve, causing them to die.⁴ These small sea snails are significant parts of marine food webs. If their numbers are reduced, then the populations of marine creatures that eat them will also diminish over time. 

The limitation on the amount of carbonate ions and the more acidic pH in the ocean also negatively affects coral polyps and their ability to produce reef structures.⁷ Similar to how shellfish rely on carbonate ions in order to build their shells, coral polyps rely on carbonate ions in order to build their skeletons, which in turn become coral reefs. These coral reefs have the ability to become biodiverse marine ecosystems by providing homes for many other marine organisms.¹⁴ Approximately one fourth of the ocean’s fish rely on the health of coral reefs.¹⁵ Without healthy reefs, many species of marine life are at risk such as fishes, invertebrates, plants, sea turtles, birds, and marine mammals. 

As ocean acidification impacts shellfish and coral, this will in turn have an impact on fisheries and aquaculture.³ With declining ocean pH values, fishing and aquaculture systems will produce less healthy organisms. This will negatively affect humans through our inability to rely on these organisms as a stable source of food. It will also have a negative economic impact on the communities that rely on fishing and aquaculture for their livelihoods.³

Eutrophication and Hypoxia

Eutrophication and hypoxia are processes that work in conjunction with ocean acidification to negatively affect certain types of marine life.¹⁶ Eutrophication describes the process of a marine environment becoming enriched by additional nutrients that are not ordinarily present in those areas. This enrichment causes increased plant and algae growth.¹⁷ One well-documented location with observed eutrophication is the Gulf of Mexico.¹⁶ When additional sediments force their way into estuaries like the Gulf of Mexico, the amount of algae and plants increases as their habitat becomes more favorable.¹⁶ Algae and plant production have also benefitted from warmer water temperatures in this area. When algae and plants die, they sink to the bottom of the estuary. As the organic material at the bottom of the estuary decays, it removes O₂ from the water and produces CO₂.¹⁷ Hypoxia is the term that describes this resulting reduction in O₂ levels. When there are lower levels of oxygen in the water, this puts significant strain on marine organisms, especially those with limited mobility, such as mollusks.¹⁵

Hypoxia has several contributing factors related to other climate change impacts.¹⁸ One of these factors is the result of an increase in extreme precipitation events. During these events, nutrient-rich sediment loads have found their way into coastal waters at an increased rate.³ This process is likely to continue as more extreme weather events are projected to occur within the next century. In addition to these increased extreme precipitation events carrying nutrients, increased fertilizer use has also contributed to more nutrient-rich sediment entering coastal waters.¹⁸

Ocean Temperature Increases

The surface temperature of the oceans has been increasing by about 1.3℉ on average per century globally, which has already had significant impacts on certain marine ecosystems.³ Certain organisms thrive in cooler waters, so as the temperature of the water in these regions are increasing, they are not able to thrive in these environments. Large ecosystem changes are likely in areas where larger temperature increases are occurring. Additionally, since the saturation concentration of oxygen is lower in warmer water, some areas are experiencing deoxygenation, which limits the survival and reproductive capacity of marine organisms.³ The reduction of oxygen levels and the higher temperatures have had major impacts on fisheries and aquaculture and fisheries, and this is likely to continue as temperatures continue to increase in the future. It is worth noting that all three of these factors, ocean acidification, ocean temperature, and ocean deoxygenation, work in tandem to negatively impact various marine ecosystems.³

Data Retrieval and Analysis

Atmospheric CO₂ data

Given that this unit is about climate change and how it specifically impacts the oceans, it is important to consider the processes of data retrieval and data analysis. One metric that is concerned with both of these topics is the amount of atmospheric CO₂. These numbers have dramatically changed since the beginning of the Industrial Revolution.¹ Graphs of this data show clear trends and are easy to analyze, which makes this topic a good starting point for the data section of the unit. This graph shown in Figure 1a presents a great opportunity for students to practice understanding trends in data.¹⁹ They will be able to observe that the amount of atmospheric CO₂ has increased by about 100 ppm within the past 60 years. Stretching this concept a bit further, students can analyze a graph of atmospheric CO₂ amounts and CO₂ emissions, as shown in Figure 1b. This will encourage students to consider and speculate whether or not these two variables are related.

Figure 1a: The amount of atmospheric CO₂ has been increasing significantly since the 1960s. This image is composed of data from NOAA.¹⁹

Figure 1b: The amount of atmospheric CO₂ has increased as CO₂ emissions have increased.¹

Dissolved CO₂ and pH data

One important metric for measuring ocean acidification is the amount of dissolved carbon dioxide in water. This is sometimes referred to as the partial pressure of carbon dioxide in water, or pCO₂. There is good data about this topic from various sources, each over a long span of time, that is available from the United States Environmental Protection Agency (EPA).²⁰ One large data set with information about pCO₂ and pH that is geographically fairly close to Richmond is the data set from Bermuda through 1983-2016.²⁰ A graph of Year vs. pCO₂, shown in Figure 2a, displays the data in a clear manner showing that pCO₂ has increased over time. Using the same Bermuda data set obtained from EPA, one can also investigate the relationship between ocean pH and time. A graph of Year vs. pH, shown in Figure 2b, displays the data in a clear manner showing that the ocean pH near Bermuda has decreased slightly over time. Using this same data set, a graph of pCO₂ vs pH can be created, as shown in Figure 2c. This graph shows a very clear trend, and the R² value of 0.984 indicates a strong relationship between these two variables. In agreement with the chemistry presented above, this graph shows what we would predict, that pCO₂ and pH are inversely related. When pCO₂ is higher, pH is lower and when pCO₂ is lower, pH is higher.

Figure 2a: Graph of Bermuda pCO₂ data acquired from 1983-2016.²⁰

Figure 2b: Graph of Bermuda pH data acquired from 1983-2016.²⁰

Figure 2c: Graph of Bermuda pCO₂ vs. pH data acquired from 1983-2016.²⁰