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Climate Change Reading

Climate Change

In 2005, the Intergovernmental Panel on Climate Change (IPCC) published a report in 2005 stating that the 1° F global temperature increase in the previous century was "very likely due to anthropogenic greenhouse gases" and "unlikely to be caused by natural changes alone." These unambiguous statements generated much interest and skepticism among people around the world. To what extent do humans effect our global environment? Are any or all of these effects reversible? How does this current climate shift compare to natural climate changes that have occurred in Earth’s past? What might be some of the possible consequences of climate change, and when will these consequences manifest themselves?

Before analyzing modern shifts in climate, it is important to recognize global climate has changed many times prior to the Industrial Revolution in the 1800s. Climate change can occur gradually over long time periods and abruptly over shorter time spans. Over geologic time (meaning millions, not thousands, of years), many variables can contribute to changes in climate. Here are just a few:

Position of the Continents: Based on our understanding of plate tectonics, we know that continents can drift further from the Equator and closer to the poles, increasing the chances for Ice Ages. Look at a world map- current continental configurations have70% of Earth’s landmass in the Northern Hemisphere. The frequency of cooler summers increases with proximity to the poles, and when more snow falls in the winter than melts in the summer over a period of decades to centuries, glaciers can grow into ice sheets.

Volcanic Activity: Extensive, long-lived volcanic eruptions can cause long term warming through the release of volcanic gases, primarily CO_{2, H2O, and CH4, all of which are considered greenhouse gases. Greenhouse gases are those molecules in the atmosphere which regulate Earth’s climate by retain a some of the heat re-radiated by Earth. During periods of extensive and prolonged volcanism, greenhouse gases can accumulate in the atmosphere, retaining more heat and causing an increase in Earth’s average temperature.}

Land Uplift: During plate convergence, either material from the upper mantle and crust melt and erupt at Earth’s surface in the form of lava or folded mountain belts form during the collision of two continental plates. Exposures of rock at higher elevations are likely to be rapidly weathered. Precipitation is naturally slightly acidic, containing small amounts of carbonic acid, H2CO3. Carbonic acid reacts with rock, weathering it to clay, and the carbon in the acid incorporated into of the clay, ultimately taking it out of the atmosphere. During periods of extensive mountain building, such as the development of the Himalayan Mountains, rapid drops in the concentration of CO2 in the atmosphere may have contributed to the resurgence of continental ice sheets on Earth.

Formation of Fossil Fuels: Fossil fuels are formed from the burial of ancient life whose remains did not fully decay, but were rapidly buried. The most common fossil fuels utilized by industry today include oil, natural gas, and coal. As this organic material was buried, carbon was removed from the ocean and atmosphere and stored underground, contributing to global cooling. Because of the geologic setting and environmental conditions, the Permian, Pennsylvanian, Mississippian and Cretaceous were periods of significant fossil fuel formation.

Natural shifts in climate can also occur over shorter (thousands to hundreds of year) time spans. Some of the factors that can contribute to natural climate change include:

Milankovitch Cycles: The Milankovitch Hypothesis named after Serbian mathematician and engineer Milutin Milankovitch, states that short term shifts in climate can occur due to variations in Earth’s orbit and orientation over time. Specifically, Milankovitch referred to three parameters, eccentricity, precession, and obliquity, discussed below.

As the Earth orbits the Sun each year, its orbit varies from mostly circular to slightly more elliptical. This change is referred to as Earth’s eccentricity. As Earth’s orbit becomes more ellipitical, its distance from the Sun increases and the amount of solar radiation received decreases (Figure 5.1). This can be most important when summers are cooler in the Northern Hemisphere. As summers become cooler, at some latitude, more snow will fall in the winter than will melt in the summer. Over years, this can lead to growth of glaciers and gradual cooling of climate. One eccentricity cycle lasts approximately 100 ky.

The Northern Hemisphere currently is located furthest from the Sun, a position known as aphelion, during its summer. Conversely, when the Northern Hemisphere is closest to the Sun, or in a position of perihelion, it is winter. So then why is it cold in the Northern Hemisphere at perihelion? The amount of solar radiation received by the Northern Hemisphere is dependent upon the tilt of the Earth, and at perihelion, the Northern Hemisphere is tilted away from the Sun, and the Southern Hemisphere oriented towards the Sun, producing a southern hemisphere summer and northern hemisphere winter.

Earth is not always tilted in this particular astronomical configuration, however. Approximately every 13,000 years, the orientation of the Earth relative to the Sun reverses. This means that at perihelion, the Northern Hemisphere would be tilted towards the Sun, and would experience northern summer. Consider that most continental landmasses are in the Northern Hemisphere. Therefore, cold northern summers (northern summers near aphelion) create conditions that increase the likelihood of an Ice Age, whereas warmer northern summers (northern summers near perihelion) create conditions that do not favor the development of Ice Ages. This change in the orientation of Earth relative to the Sun is called precession, and a full precession cycle lasts approximately 26 ky (Figure 5.1). Precession occurs because of a series of the gravitational attractions between the Earth, Moon, and Sun because the Earth slightly bulges near the Equator and is not perfectly spherical.

The Earth is currently tilted 23.5° from vertical, but this tilt is not constant. Earth’s tilt varies from 22.1° – 24.5° approximately every 40 ky, a cycle known as obliquity (Figure 5.1). At 22.1°, Earth is oriented closest to vertical, and solar radiation is more evenly distributed to the poles and tropics. As Earth’s tilt increases, polar regions receive less solar radiation, causing temperatures to drop and increasing the likelihood of Ice Ages.

When Milankovitch cycles are plotted with estimated polar temperatures recovered from ice cores in Greenland and Antarctica, there is a good correlation between the eccentricity cycle and Ice Ages. This indicates that eccentricity plays an important role in shorter-term climate changes on Earth (Figure 5.2)

Transient Events: On shorter time scales, several factors can impact climate. Ash particles emitted by volcanic eruptions can reflect solar radiation in the atmosphere back into space, cooling global temperatures. The 1883 VEI 6 eruption of Mt. Krakatoa in Indonesia corresponds with a global cooling of over 1° F, and a causal relationship between the two events has been speculated.

Rapid dissociation of methane hydrate deposits on continental slopes have also been associated with abrupt, transient increases in global temperature as a result of large inputs of methane gas, especially at the Paleocene-Eocene Thermal Maximum, a transient warming event that occurred nearly 55 mya. For a more thorough discussion of the PETM, see chapter 6.

There is little debate among reputable scientists that humans are impacting current climate change. When looking at the past millennium, the 20^{th century is unique for both its increased rate and magnitude of warming. Though it can be challenging to determine exactly how much of the 1° F increase in global temperature in the past century is due to human activity, called anthropogenic impact, coupled global circulation models (computer models which simulate global and regional changes in the atmosphere an oceans) estimate that about half of the warming in the first half of the 20th century and nearly all of it in the latter half of the century are attributable to human activities (Figure 5.3). A number of behaviors and industries contribute to the anthropogenic effect on climate by releasing high concentrations of greenhouse gases, primarily in the form of carbon dioxide, CO2, and methane, CH4.}

The combustion of fossil fuels, primarily oil, natural gas, and coal, is the largest source of anthropogenic carbon. Fossil fuels are derived from the ancient remains of organisms whose bodies were not fully decomposed, and so the carbon within them was buried and heated over millions of years. Eventually these chemical reactions resulted in the creation of fossil fuels, whose energy is released in combustion engines. A by-product of energy release is carbon, usually as CO2 and smaller amounts of CH4, and water vapor. These by-products are greenhouse gases, and their abnormal influx into the atmosphere via fossil fuel combustion has more than doubled the natural concentrations of greenhouse gases that likely existed prior to the Industrial Revolution. Between the years of 1958 and 2004, atmospheric concentrations of CO2 increased from 315 ppm to 378 ppm, an increase of 34%. While CO2 concentrations in the atmosphere are highest, methane gas has a significantly greater ability to retain heat.

Deforestation is another large contributor to anthropogenic warming of the atmosphere. Although deforestation rates have been gradually declining for the past 30 years, the IPCC maintains that it is the source of nearly 1/3 of anthropogenic carbon emissions. Deforestation is the permanent removal of established forests, primarily to create land for urbanization or for logging enterprises. Plants provide a natural removal mechanism for CO2 during photosynthesis, which is reduced as mature forests are destroyed. Further, slash-and-burn techniques common in poor countries with food shortages or intending to promote cattle ranching, burn the timber, releasing CO2.

Climate models differ in terms of the timing and magnitude of predicted changes on Earth, but most suggest similar changes due to climate shift in the next few centuries. With temperatures gradually rising, sensitive polar climates will be affected first. In fact, while global temperature has risen about 1° F in the past century, polar temperatures have risen 4° F. Warmer temperatures cause accelerated melting of the Antarctic and Greenland ice sheets. A decline in ice and snow cover causes a decrease in Earth’s albedo, the amount of solar radiation reflected from Earth into space. As Earth reflects less of the Sun’s rays, it absorbs more, causing further melting and warming.

As ice sheets and mountain glaciers melt, sea level rises. Globally, sea level increases on average about 1.8 mm/year (0.07 in/year) due to melting ice and thermal expansion of water. Rising sea level will have the largest impact on low-lying areas, such as the Mississippi River delta and much of the southeastern United States and the country of Bangladesh in Southeast Asia.

Current climate models project a 3° F increase in temperatures in Greenland in the next century. Some scientists fear this would mark the beginning of long term melting for its ice sheet, which over several centuries, would result in sea level rising more than 7 m (23 ft). A 7 m rise in sea level would dramatically alter the coastline of the eastern US, as well as many other regions throughout the world. Furthermore, more than 2% of all water on Earth is incorporated into the Antarctic Ice Sheet, which would cause sea level to rise more than 70 m (230 ft) if it were to melt completely. However, most climatologists do not believe that our current warming, even if its rate and magnitude were to increase, would cause complete melting of Antarctica.

Climatologists are particularly concerned about the West Antarctic Ice Sheet (WAIS). Antarctica is bisected by the Transantarctic Mountains into east and west Antarctica, which differ significantly. The East Antarctic Ice Sheet is older, frozen to bedrock, and above sea level. It is so cold in East Antarctica that a warming of the atmosphere is predicted to actually produce increased precipitation in this frozen desert, thickening the ice sheet and producing a sea level drop of nearly 0.5 m (1.65 ft), roughly equal to the sea level rise that would occur if all mountain glaciers on Earth were to melt.

The WAIS, however, is younger and has melted away completely in previous interglacial periods. Since much of it is in contact with ocean water, warmer sea surface temperatures could increase rates of melting. Additionally, fast-moving regions within the WAIS, called ice streams, can flow more than 0.8 km/yr (0.5 mi/yr), relative to regions frozen to bedrock, which flow only a few feet per year. While ice streams are not completely understood, ice core samples indicate that they flow on a layer of water a few millimeters thick, which causing it to flow at a more rapid pace than surrounding ice. The source of this water may partly be due to melting of ice by active volcanoes within the Transantarctic Mountains. Ice streams can therefore transport higher volumes of ice to the ocean to be melted or broken from the ice sheet. Complete melting of the WAIS would produce a sea level rise of nearly 7 m (23 ft).

Assuming that model predictions of sea level rise and shifting weather patterns are accurate, the habitats of many species will likely be destroyed in the coming centuries. While species may have the ability to adapt to changing environmental conditions, climate will likely change at a much greater rate than evolution. Organisms in polar and tropical regions and coral reefs are particularly threatened by climate change. Deforestation can greatly contribute to habitat loss and a decline in biodiversity, particularly in rainforests. Some studies predict by as early as 2050, more than 35% of species on Earth will be extinct, largely from loss of habitat.

As CO2 concentrations and temperatures in the atmosphere continue to rise, over centuries, the oceans will absorb much of the CO2 and heat. Over the past two centuries, the ocean has uptaken approximately 40% of anthropogenic emissions, slowing the rise of global temperatures. When CO2 dissolves in seawater, it forms a weak acid called carbonic acid, or H2CO3. This can lower the pH of seawater, making it more acidic. Microorganisms, including coral, foraminifera, and other plankton that form the basis of many food webs make small shells, called tests, from dissolved CaCO3 in seawater. CaCO3 dissolves readily in acidic solutions, and ocean acidification could threaten important species in ecological communities.

In 2007, the IPCC reported that as a result of climate change, an increase in the frequency of heat waves and heavy rainfalls was "very likely," (>90%), and an increase of droughts and tropical storms "likely" (>66%). As more heat is delivered to the tropics, the atmosphere redistributes the excess heat to colder regions, increasing the frequency of tropical storms and hurricanes in the Atlantic Ocean and typhoons in the Pacific. Since warm air can hold more moisture than cold air, more precipitation will be held in the atmosphere as water vapor, causing increased incidence of drought. Warmer temperatures will also encourage the transference of tropical diseases to higher latitudes, such as malaria.

Currently, the Gulf Stream, a powerful wind and air current, transports warm Caribbean water and air masses north along the east coast of the US. Near North Carolina, the current diverges eastward and warms the shores of Western Europe, creating a warmer climate than other cities at the same latitude.

Off the southeast coast of Greenland, sea ice currently forms on the surface of the ocean. As saltwater turns to ice, only nearly-pure H2O is removed from the ocean water; the salt is left behind. As a result, the surface water is some of the saltiest and coldest in the oceans, and therefore, fairly dense. Dense water masses sink from the surface water until everything above it is less dense and everything below it is denser. Ocean deep water masses form off the coasts of Antarctica and Greenland. These deep water currents power a series of surface and deep currents that operate like a conveyor belt, a process referred to as thermohaline circulation (thermo= heat, haline = salinity).

Deep water masses form because surface water is both very cold and very salty. However, sea surface temperatures are increasing globally, limiting the extent to which sea ice currently forms, and increased melting of the Greenland ice sheet has begun to decrease the salinity of coastal waters. As surface waters in this region become warmer and fresher, the difference in temperature between the tropics and poles will lessen, creating a more sluggish ocean circulation. As ocean circulation continues to slow, some climatologists fear the Gulf Stream will deliver so little heat to the poles that they may begin to cool again. If cooling occurs, ice sheets will begin to grow, increasing the albedo in polar regions and reflecting more solar radiation back into space. As more solar radiation is reflected, temperatures will continue to decline, causing ice sheets to grow still larger and increasing albedo even more. This positive feedback cycle, caused by a slowing or stopping of thermohaline circulation, illustrates how runaway global warming might cross a climate threshold, and in doing so, lead the Earth into its next Ice Age.