Unit 8. Climate: Atmosphere and Oceans

Earth’s atmosphere is key to the origin and evolution of life on our planet, which distinguishes our planet from others in the solar system. The original atmosphere was formed in a very hot and turbulent early Earth, from outgassing of trapped volatiles. Several processes cooled our planet’s surface, allowing the formation of oceans that, in turn, permitted the evolution of O-producing life. The diversity of life is intimately coupled with atmospheric and ocean circulation, which explain today’s regional climate patterns and biomes distribution. Today’s atmosphere is being altered by the rapid increase of greenhouse gases, leading to new challenges for modern life. These topics are explored in this unit.

How did Earth’s atmosphere and oceans form?

Following accretion into a large planet-sized object during the early years of the solar system, Earth’s first major atmosphere was formed by the release of gases trapped in the restless interior, a process that still goes on today in volcanoes (Figure 8.1). These early years are marked by swirling oceans of hot magma that no longer exists today on any planet in our solar system. Extreme volcanism in Earth’s early history occurred in response to this energetic motion of then-molten mantle material. As planetary material violently overturned, volatile gases from the interior, especially Carbon dioxide (CO₂), Carbon monoxide (CO), Hydrogen (H₂), Nitrogen (N₂) and water vapor (H₂O), were released, and accumulated in a gaseous surface layer that was trapped by gravitational forces. Radiation from the nearby Sun swept lighter gases as H and He away, leaving only heavier molecules in this early atmosphere. Chemical reactions in the hot surface layer formed other simple atmospheric compounds, such as methane (CH₄) and ammonia (NH₃). While far less abundant, the latter compounds are highlighted as they are key components of amino acids, which are the fundamental building blocks of life’s proteins. Note also that Oxygen (O₂), key to the survival of many forms of modern life, was not present in the early atmosphere.

Figure 8.1 Volcanic outgassing.

Water vapor is the dominant form of gas released in outgassing, so much that Earth’s early atmosphere became saturated with water, leading to an era of continuous rain on the planet. These rains contributed to cooling of the Earth’s surface and the formation of the first oceans, which today cover two thirds of the surface. Another contributing process to the surface heat budget was the permanent removal of CO₂ from the atmosphere by the formation of carbonate rocks (consisting mostly of the mineral calcite, CaCO₃), which reduced the atmosphere’s greenhouse potential and lowered the surface temperature.

The continuous rains and atmospheric CO₂ removal cooled our planet and created a surface where water could eventually be present in all three phases: vapor, liquid and solid. Ocean formation made Earth habitable for life at about 4000 million years ago, leading to today’s highly complex organisms. The origin of life remains heavily debated, but the changes in organism over billions of years would not have been possible without surface conditions that persist today.

Life began to have a major impact on the environment once simple photosynthetic organisms, like blue-green algae, evolved in the oceans. These organisms fed off solar energy and CO₂, and produced oxygen (O₂) as a waste product. For hundreds of millions of years, the oxygen that the algae produced was dissolved in ocean water and reacted with iron rich sediment in a process called oxidation, but O did not build up as a gas in the atmosphere. It was not until approximately 2 billion years ago that the reservoirs of oxidizable material became saturated and free oxygen built up to significant amounts in the atmosphere. Today, the outgassing process continues as volcanism, releasing mainly H₂O, CO₂, SO₂ and lesser amounts of gases such as N₂, albeit in much less dramatic form, while the O budget is maintained by surface life.

Why does Earth have water in all three physical states?

The distance at which each planet orbits the Sun influences the surface temperature. For example, Venus being closer to the Sun, receives a higher amount of solar radiation than Earth, thus the surface temperature of Venus is higher than Earth. Mars is therefore cooler. However, in addition to a planet’s distance from the Sun, the atmospheric composition of each planet has major impacts on the resulting planetary surface temperature.

Venus has a surface pressures that is much too high and temperatures that are too warm for liquid water to develop, because of a high concentration of CO2 in its atmosphere (Figure 8.3). Any water on Venus will be in the form of water vapor, because a surface temperature of >450^o C and a high atmospheric surface pressure are too high for liquid water to form. Vapor saturation of H₂O on Venus is never reached, so the atmosphere continues to accumulate H₂O. Today’s atmosphere of Venus is more than 98% CO₂, with no oxygen or liquid water, which led to the concepts of the runaway greenhouse. The thick atmosphere of CO₂ and water vapor trap incoming solar radiation and make the surface of the planet extremely hot, without a cooling process to provide a negative feedback.

Figure 8.3 H2O phase diagram and occurrences on the planets Venus, Earth and Mars

Modern Earth is at the right distance from the Sun (150 x 10⁶ km) and with a suitable atmosphere, so that H₂O is able to form in all three phases: gaseous (vapor), liquid (water) and solid (ice). After the differentiation and outgassing period in Earth’s early history, the water vapor that accumulated in the atmosphere changed to torrential rains that lasted millions of years, leading to the first oceans. The formation of oceans was likely complemented by the impact of icy comets that brought water to the surface. Once liquid water became available at the surface, the atmospheric composition of Earth changed further, as atmospheric species that were soluble in water dissolved into the rain and oceans. Because CO₂ is soluble in water it was slowly removed from the atmosphere leaving the relatively scarce but non-reactive nitrogen (N₂) to build up in the atmosphere, to today’s value of 78%. This is one of the key differences between Earth’s atmosphere and those of Venus and Mars that are dominated by CO₂. Earth’s average surface temperature is today ~15^o C and the surface pressure is approximately 1,000 mb, which allows the formation of H₂O in liquid form, gaseous water vapor in the atmosphere, and ice at the colder poles (Figure 8.3).

In comparison to Earth, Mars is too cold for liquid water to form, as it is farther from the Sun (at 228 x 10⁶ km) than Earth. Therefore, Mars receives less solar radiation than Earth and Venus. The relatively low Martian surface temperatures of –60^o C and low surface pressure at 6 mb results in water vapor forming ice. The thin Martian atmosphere is currently made of 96% CO₂, 2.5% oxygen and has only trace amounts of water, but in its earlier history likely had flowing water that formed the famous Martian channels.

Why is Earth’s atmosphere dynamic?

Earth’s atmosphere has evolved through volcanic outgassing and from the evolution of oxygen producing organisms approximately 3.5 billion years ago, which changed the composition of the atmosphere from an oxygen-less planet to one that could support complex life. Today’s dominant changes in the atmosphere are occurring due to human activities, such as fossil fuel combustion and land use change, which are altering the global climate through changes in greenhouse gas concentrations, reflectivity of the atmosphere, and alteration of precipitation patterns. Currently, the atmosphere consists of approximately 78% nitrogen, 21% oxygen, 0.9% argon, close to 0.04% carbon dioxide, and a plethora of other elements, compounds, and particulate aerosols.

Earth’s atmosphere is held to the planet by gravity and from thermal movement of the air molecules. Radiative energy from the Sun, the rotation of the Earth and the distribution of land and water, are the drivers for Earth’s physically dynamic atmosphere. Incoming solar radiation is at the highest angle and therefore strongest at the equator, and highly oblique and therefore weakest at the Earth’s polar regions. This causes uneven heating of atmospheric air parcels, where warm air at the equator rises and cooler air at the poles descends (a north-south gradient). However, the Earth’s rotation also pushes the air parcels around the Earth, but in an east-west manner. The temperature gradients and the rotation of the Earth together create convective cells of atmospheric circulation on the planet. These processes produce prevailing winds and latitudinal belts of low and high air pressure around the globe.

Figure 8.2 Global atmospheric circulation. http://www.emc.maricopa.edu‑/faculty/farabee/BIOBK/deserts.gif

Temperature is a measure of the thermal energy or the movement of molecules and refers to the heat of an object: temperature equals the mean atomic speed squared: T = v². Air parcels with higher energy, meaning warmer, are less dense so they tend to rise. Pressure is a measurement of the force per unit area. Atmospheric pressure is the weight of the atmosphere per unit area, above the point of measurement. Therefore, as you move away from the Earth’s surface toward space, the atmospheric pressure decreases, i.e. the weight of the atmosphere is less because there is less air mass above you. In addition to temperature and pressure, the amount of water vapor that is present in the atmosphere plays a role in air movement, because it affects the temperature of the atmosphere.

Climate and weather patterns are strongly related to the position of the convective atmospheric cells and belts of atmospheric pressure around the planet. For example, many of the world’s hot deserts, regions of very low precipitation, reside near 30^o N and 30^o S latitude (Figure 8.2). This is because these locations correspond to areas of high pressure where cool and dry air from the atmosphere descends and picks up moisture from the Earth’s surface. As the air warms and picks up moisture, it moves toward the low pressure-belts of the equator and near 50-60^o N and S latitude, and in the process releases much of the moisture as rainfall. As the rising air parcel releases the rain, it cools and cycles back toward higher latitudes and repeats the loop. This process of releasing moisture over the equator determines why much of the equatorial region is tropical with high amounts of rainfall. The circulation of the Earth’s atmosphere, and the high and low pressure belts transfer heat and precipitation around the world. The wide spectrum of moisture and temperature gradients that result from this dynamic atmosphere adds to the great diversity of organisms that are found on Earth.

What is the function of the great ocean conveyer belt?

The Earth’s oceans cover approximately 71% of the globe and contain 97% of the water on the planet. Oceans also move absorbed solar radiation in the form of heat around the world; thus, oceans are extremely important for climate regulation on Earth. For example, the oceans take up over half of the solar radiation absorbed on Earth, with ocean currents then redistributing this energy. A key attribute of ocean water is its ability to hold dissolved ions in solution. In addition to other solutes, seawater has approximately 3.47% dissolved salt, which differs slightly by ocean region. As the salinity of water increases, the density of the liquid also increases, which has profound effects on ocean circulation and nutrient transfer rates.

Unlike the atmosphere, which is characterized by turbulent weather systems, the ocean is a fairly stable because it is heated from above, in contrast to the atmosphere, which is heated from below. The relatively warm surface waters, therefore, tend to remain floating on top of the colder and denser deeper waters. This tendency generally leads to a stable situation that inhibits turbulence. If the deep ocean were never to move, however, conditions would quickly become anoxic or void or oxygen. Because ocean waters are not usually anoxic there must be another mechanism for deep-ocean mixing.

When surface ocean water absorbs radiation, it heats up and eventually evaporates, leaving behind dissolved salts. This evaporative process leaves the remaining water with a high solute concentration and thus more dense, than the water below it. The dense, saline water on the surface tends to sink to the ocean floor, causing ocean mixing and deep ocean currents. In addition to the evaporative driver for changes in ocean water salinity, sea ice formation also contributes to saline polar waters. As sea ice forms, solutes are not incorporated into the ice crystals, thus the solutes are left behind and increase the salinity of the surrounding water.

The sinking action of saline water produced by a combination of temperature and salinity is called thermohaline circulation, and drives large-scale ocean circulation over century long time scales. Large-scale circulation of the oceans is deemed the great ocean conveyer belt and involves a deep current, which carries more than 30 times the volume of all the rivers of the world combined (Figure 8.4). The conveyor belt is an immense ocean current that transfers heat from the warm tropics to the cold, polar-regions. For instance, cold, salty, North Atlantic waters sink deep into the ocean and travel south around Africa. The water either rises to the surface near the tropical waters of India, or the cold saline water continues to move along Antarctica and travels up the Eastern side of Australia to the Southern coast of Alaska where the current rises toward the surface along the west coast of North America. This warmer shallower current then moves through the tropical equatorial waters of Indonesia, then back around the southern tip of Africa and up through the Atlantic Ocean back to the North Atlantic where it sinks and begins the cycle again.

In addition to the transfer of heat around the globe, the great ocean conveyor belt moves nutrient-rich water around the ocean. The ocean conveyor belt also moderates the climate in regions such as Northern Europe, which have a much warmer climate than then would be predicted from using latitudinal location alone. Scientists are currently concerned that climate warming could cause a greater input of fresh water from glacier melting, excess precipitation, or runoff into the North Atlantic Sea, and this may cause the ocean conveyor belt to weaken or even shut down, which would have major consequences for global climate.

Figure 8.4

The great ocean conveyor belt that circulates warm and cold ocean water around the globe.

http://science.hq.nasa.gov/oceans/images/CONVEYOR.jpg

Why is the global climate changing?

In a nutshell, humans are altering the radiative balance of the Earth. The balance between incoming energy and outgoing energy dictates the temperature of the Earth. Temperatures increase when more energy is received than lost. The Earth's surface absorbs shortwave radiation from the Sun. This energy is then redistributed by atmospheric and oceanic circulation and radiated back to space at longer, infrared wavelengths. On average for the Earth as a whole, the incoming solar radiation energy is balanced approximately by the outgoing terrestrial radiation. Global climate can be affected by any factor that alters the radiation received from the Sun or lost to space, or that alters the redistribution of energy within the atmosphere and between the atmosphere, land, and ocean. A change in the net radiative energy available to the global Earth-atmosphere system is termed radiative forcing. Positive radiative forcing tends to warm the Earth’s surface and lower atmosphere, whereas negative radiative forcing tends to cool them.

Some amount of positive radiative forcing is essential for life to exist on Earth. Without the natural amount of radiative forcing or the natural greenhouse effect, the temperature on Earth would be too cold to support life. This natural greenhouse effect warms Earth to conditions that are conducive to support life, and should be distinguished from the anthropogenic greenhouse effect that is currently causing so much concern. Human activities are currently changing the amount of greenhouse gases in the atmosphere, thereby increasing positive radiative forcing and warming the planet.

Since the industrial revolution, humans have increased the amount of greenhouse gases in the atmosphere due to the burning of ancient carbon stored in the Earth, in the form of coal, oil, and natural gas, which releases CO₂, an important greenhouse gas. In the last one hundred years, humans have increased the CO₂ concentration in the atmosphere by more than 100 parts per million (ppm; Figure 8.5). In the next 100 years scientists estimate that concentrations will increase by another 200 ppm or more to greater than 600 ppm. The current concentration of CO₂ in the atmosphere at 388 ppm is greater than we have seen in the last 650,000 years! Furthermore, in addition to increases in CO₂ from fossil fuel combustion that is increasing greenhouse warming, there are many other greenhouse gases on the rise, such as methane and nitrous oxide, which are also contributing to global warming.

Figure 8.5 CO2 concentration over time http://images.wri.org/chart_scan_cni_f04b.gif

An Increase in the concentration of greenhouse gases reduces the efficiency with which the Earth’s surface radiates energy back to space. As the amount of greenhouse gases increases, more of the outgoing terrestrial radiation from the surface is absorbed by the atmosphere and re-emitted at higher altitudes and lower temperatures. This process results in positive radiative forcing that tends to warm the lower atmosphere and Earth’s surface (Figure 8.6). Because less heat escapes to space, this is the enhanced greenhouse effect – an enhancement of an effect that has operated in the Earth’s atmosphere for billions of years due to the presence of naturally occurring greenhouse gases: water vapor, carbon dioxide, ozone, methane and nitrous oxide.

Figure 8.6

Temperature record for the past 1000 years

The amount of positive radiative forcing depends on the size of the increase in concentration of each greenhouse gas, the radiative properties of the gases involved (indicated by their global warming potential), and the concentrations of other greenhouse gases already present in the atmosphere. Further, many greenhouse gases reside in the atmosphere for centuries after being emitted, thereby resulting in a long-term commitment to positive radiative forcing. For example, CO₂ has an atmospheric residence time of 100 years. Therefore, the CO₂ that you emitted to the atmosphere today will still be warming the planet when your great grandchildren are born.

How is the composition of the troposphere changing?

The troposphere is 10-12 km thick and is the layer of the atmosphere that is closest to the Earth’s surface. The troposphere contains 90% of the Earth’s atmosphere and is the layer in which weather occurs. In addition, the troposphere is the layer in which human beings and other living organisms inhabit. Because humans live in the troposphere, this layer of atmosphere is also the layer that receives the majority of atmospheric wastes such as CO₂, nitrogen oxides, and sulfur oxides. The temperature of the troposphere decreases as one moves away from the Earth’s surface towards the stratosphere, which is the second layer of the Earth’s atmosphere. The boundary between the troposphere and the stratosphere is deemed the tropopause, which acts like a lid to trap moisture in the troposphere.

Human activities have substantially altered the composition of the troposphere in the last 200 years. The burning of fossil fuels is one of the main drivers of tropospheric changes. Fossil fuel emissions have increased the concentration of CO₂, CO, methane, nitrogen oxides, sulfur oxides, and chlorofluorocarbos (CFCs) in the atmosphere. Many of these gases are greenhouse gases meaning that they selectively absorb radiation at certain wavelengths and trap heat from escaping into space, altering the radiative equilibrium of the planet. In addition to the increase in greenhouse gases, human activities have significantly increased the amount of dust, smoke, and particulate aerosols in the troposphere, which alters the albedo or reflectivity of the atmosphere and plays a role in cloud formation. Changes in the reflectivity of the atmosphere and the amount of cloud cover can also play major roles in regional and global changes in temperature.

The massive increase in atmospheric pollution resulting from human activities has major consequences for the health of ecosystems and people alike. For example, noxious tropospheric smog and ozone develop over urban areas from the combination of nitrogen oxides interacting with sunlight (Figure 8.7). Photochemical smog, tropospheric ozone, and particulate aerosols can cause major health consequences such as asthma, cancer, and respiratory and heart problems in humans and animals. In addition, atmospheric pollution can cause acid rain and can damage photosynthetic systems in plants and decrease agricultural yield. In addition, because of the dynamic nature of the Earth’s atmosphere, noxious air pollution formed in urban and industrialized areas can travel to many other regions of the globe, which can cause far-reaching climatic and health consequences for people and ecosystems.

Another major driver for changes in the composition of the troposphere is land use change. Land use changes like deforestation and conversion of forestlands to agricultural lands impact surface albedo, as well as the amount of dust, aerosols, and particulates that are present in the atmosphere. Atmospheric dust and particulates can increase due to erosion and desiccation of soil from land use change. Removal of forest vegetation, especially tropical forest vegetation, also has a major impact on the amount of water vapor that is present the in the atmosphere. Vegetation removal reduces evapotranspiration from the Earth’s surface, creating warmer, dryer atmospheric conditions, which has consequences for local, regional, and global weather patterns. For example, in the Amazon basin, more than half of the rainfall occurring within the basin is a result of evapotranspirative recycling from within the basin itself. Experts predict that major deforestation within the basin could cause profound changes in the regional weather patterns and potentially prevent rainforest regeneration.

Figure 8.7 Tropospheric photochemistry. http://www.uwm.edu/Course/416-120/Chapter02/fg03_011.jpg

Last updated: 8/30/2006 10:32 AM