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Lesson 18.4: Climate Change

Lesson Objectives

Explain the mechanism of the greenhouse eﬀect.

Recognize that the greenhouse eﬀect maintains an equilibrium.

Compare greenhouse conditions on Earth to those on Mars and Venus.

Explain the extent of current increases in the Earth’s temperature.

Review past changes in the Earth’s temperatures.

Summarize the evidence and support for greenhouse gases as the cause of recent global warming.

Discuss the signiﬁcance of global warming for Earth’s ecosystems.

Relate global warming to current global stability.

List the atmospheric gases that absorb the Earth’s thermal radiation, and their sources.

Evaluate possible solutions to the problem of global climate change.

Recognize the tradeoﬀs required by nuclear power plants: reduced emissions vs. ra- dioactive fuels and waste

Introduction

On December 10, 2007, the Intergovernmental Panel on Climate Change (IPCC) and former US Vice President Al Gore received the Nobel Peace Prize “for their eﬀorts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change.” The Peace Prize is designated “to the person who shall have done the most or the best work for fraternity between the nations, for the abolition or reduction of standing armies and for the holding and promotion of peace congresses.” A high honor, the award also announced to the world that climate change (Figure 18.49) is a critical issues for the future of the Earth and its people. What is climate change? What are its causes? How do its eﬀects relate to world peace? What are “the foundations for the measures that are needed to counteract such change”? Can individuals like us help? These are the questions we will explore in this last lesson about human ecology.

Figure 18.49: Temperature variations from 1940-1980 averages show that most of the Earth warmed signiﬁcantly in just a single decade. The average temperature change across the en- tire globe for this period is 0.42oC (0.76 oF). Over the past 100 years, surface air temperatures have risen 0.74 ± 0.18 °C (1.33 ± 0.32 °F). (29)

What is the Greenhouse Eﬀect?

The Greenhouse Eﬀect is a natural feature of Earth’s atmosphere – yet another ecosystem service. Without the Greenhouse Eﬀect, Earth’s surface temperature would average -18oC (0oF) – a temperature far too cold to support life as we know it. With the Greenhouse Eﬀect, Earth’s surface temperature averages 15oC (59oF), and it is this temperature range to which today’s diversity of life has adapted.

How does this ecosystem service work? The Greenhouse Eﬀect is summarized in Figure

18.50. Of the solar radiation which reaches the Earth’s surface, as much as 30% is reﬂected back into space. About 70% is absorbed as heat, warming the land, waters, and atmosphere (you may recall that only about 1% is converted to chemical energy by photosynthesis). If there were no atmosphere, most of the heat would radiate back out into space as infrared

radiation. Earth’s atmosphere, however, contains molecules of water (H2O), carbon dioxide (CO2), methane (CH4), and ozone (O3), which absorb some of the infrared radiation. Some of this absorbed radiation further warms the atmosphere, and some is emitted, radiating

back down to the Earth’s surface or out into space. A balance between the heat which is absorbed and the heat which is radiated out into space results in an equilibrium which maintains a constant average temperature for the Earth and its life.

Figure 18.50: Without greenhouse gases, most of the sun’s energy (transformed to heat) would be radiated back out into space. Greenhouse gases in the atmosphere absorb and reﬂect back to the surface much of the heat which would otherwise be radiated. (6)

If we compare Earth’s atmosphere to the atmospheres which surround Mars and Venus

(Figure 18.51), we can better understand the precision and value of Earth’s thermal equi- librium. Mars’ atmosphere is very thin, exerting less than 1% of the surface pressure of our own. As you might expect, the thin atmosphere cannot hold heat from the sun, and the average surface temperature is -55oC (-67oF) – even though that atmosphere is 95% CO2 and contains a great deal of dust. Daily variations in temperature are extreme, because the atmosphere cannot hold heat.

Figure 18.51: The thickness of a planet’s atmosphere strongly inﬂuences its temperature through the Greenhouse Eﬀect. Mars (left) has an extremely thin atmosphere, and an average temperature near -55oC. Venus (right) has a far more dense atmosphere than Earth, and surface temperatures reach 500oC. (4)

In contrast, Venus’ atmosphere is much thicker than Earth’s, exerting 92 times the surface pressure of our own. Moreover, 96% of the atmosphere is CO2, so a strong Greenhouse Eﬀect heats the surface temperature of Venus as high as 500oC, hottest of any planet in our solar system. The thick atmosphere prevents heat from escaping at night, so daily variations are minimal. Venus’ atmosphere has many layers which vary in composition, and scientists have identiﬁed a layer about 50 km from the surface which could harbor liquid water and perhaps even life; some scientists propose that this would be a reasonable location for a space station. Near this altitude, pressure is similar to the Earth’s sea level pressure, and temperatures range from 20oC to 37oC. Nitrogen, though only 3.5% of Venus’ atmosphere, is present in the same overall amounts as on Earth (because the density on Venus is so much greater); oxygen, however, is absent, and sulfuric acid would present challenges.

Considering the extremes of Greenhouse Eﬀects on Mars and Venus, we can better appreciate the precise balance which allows our own atmosphere to provide temperatures hospitable to liquid water and life. Inevitably, we must also ask this chapter’s repeating query: how have human activities aﬀected this equilibrium? This leads us back to the 2007 Nobel Peace Prize, and an evolving consensus that our species is responsible for signiﬁcant global warming.

Global Warming

Global warming refers to the recent increase in the Earth’s average near-surface and ocean temperatures (Figure 18.52). During the past 100 years, surface air temperatures have risen ± 0.18 °C (1.33 ± 0.32 °F). Multiple sources agree that the two warmest years since the introduction of reliable instrumentation in the 1800s were 1998 and 2005.

Figure 18.52: Global warming refers to the increase in Earth’s average near-surface temper- atures over the past 100 years. “Anomalies” measure deviation from 1961-1990 averages. (50)

This recent increase contrasts with relatively stable temperatures shown by scientiﬁc data for the previous two millennia. Multiple sets of temperature data inferred from tree rings, coral growth, and ice core samples are compiled in Figure 18.53. Warmly debated exceptions to the stability include a warm period during the Middle Ages and a “Little Ice Age,” attributed to decreased solar activity and increased volcanism.

According to paleoclimatologists, on a scale of millions of years Earth’s temperatures have varied almost regularly (over time intervals of roughly 140 million years) from those which support global tropics to continental glaciations (Figure 18.54). Scientists estimate the global average temperature diﬀerence between an entirely glaciated Earth and an ice-free Earth to be 10oC.

The causes of Ice Ages are not completely understood, but greenhouse gases, especially CO2 levels, often correlate with temperature changes (Figure 18.55). Rapid buildup of green- house gases in the Jurassic Period 180 million years ago correlates with a rise in temperature of 5oC (9oF). Similar changes have been hypothesized as causes for the dramatic Permian Extinction 250 million years ago and the Paleocene-Eocene Thermal Maximum (one of the

Figure 18.53: Global temperatures compiled from tree ring, coral growth, ice core analysis, and historical records, show relative stability over the last 2,000 years before about 1850, interrupted by a debatable Medieval warming and a more recent cooling termed the “Little Ice Age.” Colored lines indicate diﬀerent published data sources. For more detail on the increase since 1850, refer to Figure 3. (55)

Figure 18.54: Paleoclimatological measures of global temperatures show dramatic ﬂuctua- tions in temperature. Graphs should be read from right (past) to left (present). Ice core data for temperature is recorded in oxygen isotope units rather than oC. (22)

most rapid and extreme global warming events recorded in geologic history) 55 million years ago. Paleoclimatologist William Ruddiman proposes that human activities began to aﬀect global CO2 levels as long ago as 8,000 years, when agriculture and deforestation began. Ruddiman argues that without this early contribution to greenhouse gases, cycles indicate the Earth would already have entered another Ice Age.

Figure 18.55: Over the past 450,000 years, temperature changes (blue) correlate closely with changes in atmospheric CO2 (green) and dust levels (red). (51)

Others dispute Ruddiman’s “overdue-glaciation” theory, but most scientists today agree that recent global warming since 1850 is caused by an unprecedented rise in atmospheric CO2 (Figure 18.56) which resulted from human activities – primarily burning of fossil fuels, but also continuing deforestation and changes in land use. Fossil fuels burn organic compounds in the same way your cells burn glucose to make ATP: a product of both reactions is CO2. Deforestation and other land use changes contributes to the CO2 levels from the opposite direction – a decrease in photosynthesis, which would have removed CO2 from the atmosphere. Slash-and-burn destruction of tropical forests combines the worst of both worlds; burning adds CO2 to the atmosphere, and the loss of layers of vegetation decreases CO2 use.

Two additional greenhouse gases having anthropogenic (human activity) sources are methane (CH4) and nitrous oxide (NO). Agriculture adds both of these to the atmosphere; cattle production is responsible for much of the methane, a powerful greenhouse gas. Land use changes, waste processing, and fossil fuel production, which we’ve already implicated in CO2 increases, are other anthropogenic (human-caused) sources. A last but important contributing factor is secondary to these primary causes; triggers of “runaway greenhouse eﬀects” will be discussed below.

Figure 18.56: Since the Industrial Revolution began, the burning of fossil fuels has dramat- ically increased atmospheric concentrations of CO2 to levels unprecedented in the last 400 thousand years. The graph on the right integrates recent measurements with paleoclimato- logic data. (41)

Although the causal connections between fossil fuel combustion, deforestation, greenhouse gases, the greenhouse eﬀect, and global warming have been strongly debated in the past, the majority of the world’s scientiﬁc organizations now support these relationships, and many use the term “consensus.” (See “Scientiﬁc Opinion about Climate Change” in Further Reading.) The awarding of the Nobel Peace Prize to the organization which focuses most directly on climate change, the IPCC, highlights this consensus. Alternative hypotheses include variation in solar activity; several references are included in Further Reading. The IPCC projects future temperature increases ranging from 1.1 °C to 6.4 °C (2.0 °F to 11.5

°F) between 1990 and 2100. Predictions from multiple models which incorporate connections between greenhouse emissions and global warming are summarized in Figure 18.57; all show signiﬁcant rises in temperature by 2100.

Once again, then, we “have met the enemy” and “he is us.” What have we done? What are the environmental and socioeconomic consequences of this human disruption in atmospheric equilibrium?

A partial list of eﬀects of climate change includes:

Direct Physical Eﬀects

Melting of glaciers and a consequent rise in sea level, already documented (Figure 18.58)

Sea level rise of 18-59 cm predicted by 2100

River ﬂooding followed by drought

Coastal ﬂooding and shoreline erosion

Melting permafrost, leading to release of bog methane (CH4) increasing warming via positive feedback*

Figure 18.57: Various models of climate change which include “business-as-usual” increases in greenhouse gas emissions predict continuing increases in global temperature; this graph compares the projected increases to temperatures during the year 2000. (45)

Figure 18.58: Glacial melting (left) and a rise in sea level (right) are two consequences of global warming. The left image shows the Larsen Ice Shelf B, which broke up during February of 2002 after bordering Antarctica for as long as 12,000 years. Excluding polar ice caps, 50% of glacial areas have disappeared since the turn of the century. Although sea levels have risen since the end of the last Ice Age, rates increased by a factor of 10 beginning about 1900. (56)

Changing patterns of precipitation

Regional drought

Regional ﬂooding

Ocean warming, leading to increased evaporation

Increasing rainfall

Increasing erosion, deforestation, and desertiﬁcation

Release of sedimentary deposits of methane (CH4) hydrates – positive feedback*

Ocean acidiﬁcation: 0.1 pH unit drop already documented; 0.5 more predicted by 2100

Loss of corals

Loss of plankton and ﬁsh

Temperature extremes

Increasing severity of storms such as tropical cyclones, already documented (Figure 18.59)

Further reductions in the Ozone Layer (due to cooling of the stratosphere)

Figure 18.59: The proportion of hurricanes reaching category 4 or 5 increased from 20% in the 1970s to 35% in the 1990s. The EPA and the World Meteorological Organization connect this increase to global warming, and NOAA scientists predict a continuing increase in frequency of category 5 storms as greenhouse gases rise. (13)

Ecosystem Eﬀects

Contributions to the Sixth Extinction reaching as much as 35% of existing plant and animal species

Decline in cold-adapted species such as polar bears and trout

Increase in forest pests and ﬁres

Change in seasonal species, already documented

Potential increase in photosynthesis, and consequent changes in plant species

Loss of carbon to the atmosphere due to

Increasing ﬁres, which together with deforestation lead to positive feedback

Increasing decomposition of organic matter in soils and litter

Socioeconomic Threats Result From Some of the Above Changes

These include:

Crop losses due to climate and pest changes and desertiﬁcation

Increasing ranges for disease vectors (e.g., mosquitoes – malaria and dengue fever)

Losses of buildings and development in coastal areas due to ﬂooding

Interactions between drought, desertiﬁcation, and overpopulation leading to increasing conﬂicts (Figure 18.60)

Figure 18.60: A camp in Sudan houses refugees from the far western province of Darfur, who ﬂed from genocide intensiﬁed by severe drought. The Darfur conﬂict echoes predictions that global warming may increase drought and desertiﬁcation in overpopulated regions and result in more such tragedies. (59)

Costs to the insurance industry as weather-related disasters increase

Increased costs of maintaining transportation infrastructure

Interference with economic development in poorer nations

Water scarcity, including pollution of groundwater

Heat-related health problems

Threats to Political Stability

Migrations due to poverty, starvation, and coastal ﬂooding

Competition for resources

Note that at least three(*) of the direct physical eﬀects – melting permafrost, ocean warming, and forest ﬁres/deforestation - can potentially accelerate global warming, because tempera- ture increases result in release of more greenhouse gases, which increase temperatures, which result in more greenhouse gases – a positive feedback system aptly termed a “runaway greenhouse eﬀect.” Here’s how it could work: rising temperatures are warming the oceans and thawing permafrost. Both oceans and permafrost currently trap huge quantities of methane – beneath sediments and surface – which would undergo massive releases if tem- peratures reach a critical point. Recall that methane is one of the most powerful greenhouse gases, so the next step would be further increase in temperatures. Warmer oceans and more thawed permafrost would release more quantities of methane – and so on. These compound- ing eﬀects are perhaps the most convincing arguments to take action to reduce greenhouse gas emission and global warming.

What measures have been considered?

Preventing Climate Change

Basically, greenhouse gases are products of fossil fuel combustion; according to the EPA, more than 90% of U.S. greenhouse gas emissions come from burning oil, coal, and natural gas. Therefore, energy use is the primary target for attempts to reduce future global warming. In Figure 18.61 you can see the sources of emission for three major greenhouse gases in 2000, when CO2 was 72% of the total, CH4 18%, and NO 9%. Chloroﬂuorocarbons (CFCs, HCFCs, and HFCs) are also greenhouse gases; refer to the lesson on The Atmosphere for more information about them.

Knowing the causes of climate change allows us to develop potential solutions. Direct causes include combustion of fossil fuels, deforestation and other land use changes, cattle production, agriculture, and use of chloroﬂuorocarbons. Runaway eﬀects can result from temperature- dependent release of methane from permafrost and ocean sediments, and forest ﬁres or intentional burning. Unfortunately, the best way to avoid runaway eﬀects is to prevent temperature increases. Prevention, then, should address as many of these causes as possible. A partial list of solutions being considered and adopted follows.

Reduce energy use.

Switch to cleaner “alternative” energy sources, such as hydrogen, solar, wind, geother- mal, waste methane, and/or biomass.

Increase fuel eﬀiciencies of vehicles, buildings, power plants, and more.

Increase carbon (CO2) sinks, which absorb CO2 - e.g., by planting forests.

Figure 18.61: Global greenhouse emissions during 2002 show sources for each of the three ma- jor greenhouse gases. Knowing the causes makes ﬁnding solutions clear, but not necessarily easy! (24)

Cap emissions release, through national and/or international legislation, alone or in combination with carbon oﬀset options (see below).

Sell or trade carbon oﬀsets or carbon credits. Credits or oﬀsets exchange reduc- tions in CO2 or greenhouse emissions (tree-planting, investment in alternative energy sources, methane capture technologies) for rights to increase CO2 (personally, as for air travel, or industry-wide).

Key urban planning to energy use, e.g., eﬀicient public transportation.

Develop planetary engineering: radical changes in technology (such as building solar shades of dust, sulfates, or microballoons in the stratosphere), culture (population con- trol), or the biosphere (e.g. iron-seeding of the oceans to produce more phytoplankton to absorb more CO2).

Legislate Action: International agreements such as the 2005 Kyoto Protocol (which the US has not yet ratiﬁed), or national carbon taxes or caps on emissions. Interestingly, in the U.S., some States and groups of States are taking the lead here.

Set goals of carbon neutrality: in 2007, the Vatican announced plans to become the ﬁrst carbon-neutral state.

Support developing nations in their eﬀorts to industrialize and increase standards of living without adding to greenhouse gas production.

Every potential solution has costs and beneﬁts which must be carefully considered. Human health, cultural diversity, socioeconomics, and political impacts must be considered and kept in balance. For example, nuclear power involves fewer greenhouse gas emissions, but adds the new problems of longterm radioactive waste transport and storage, danger of radiation exposure to humans and the environment, centralization of power production, and limited supplies of “clean” uranium fuels. Studies of costs and beneﬁts can result in solutions which make eﬀective tradeoﬀs and therefore progress toward the goal of lowering greenhouse gases and minimizing future global warming.

We have reached the point where we understand how and the extent to which our activities have destabilized the Earth’s atmosphere and reduced and threatened its ecosystem services. Now we need to move one step further, and put our knowledge to work in the form of action.

What will you do to help?

Lesson Summary

The awarding of the 2007 Nobel Peace Prize to the Intergovernmental Panel on Climate Change (IPCC) and former US Vice President Al Gore recognizes the potential impact of global warming on the economic, social, and political welfare of the world.

The greenhouse eﬀect is an ecosystem service which warms the Earth to temperatures which support life.

The greenhouse eﬀect involves water, carbon dioxide, methane, and ozone, which ab- sorb heat that would otherwise be radiated out into space.

Earth’s atmosphere maintains an equilibrium between heat added by sunlight and heat lost by radiation.

The atmosphere of Mars is too thin to hold heat, and that of Venus is so thick that temperatures reach 500oC.

In 2000, the major greenhouse gases were CO2, CH4, and NO; CFCs and H2O con- tribute, as well.

Global warming refers to an increase in the Earth’s temperature of 0.74°C (1.33°F) within the past 100 years.

Paleoclimatologists document changes in the Earth’s temperature over millions of years which cycle between tropical and ice age extremes – a variation of 10oC.

Greenhouse gases, especially CO2 levels, often correlate with temperature changes.

Deforestation and agriculture – by reducing levels of CO2 uptake – may have initiated warming 8,000 years ago.

Most scientists today agree that fossil fuel combustion, deforestation, and agriculture contribute to greenhouse gases and the greenhouse eﬀect.

Global warming can cause physical changes for the Earth: melting of glaciers and permafrost, changes in precipitation patterns, temperature extremes, warming and acidiﬁcation of the oceans, and ozone depletion.

Melting of oceans and permafrost can release methane, resulting in a “runaway green- house eﬀect.”

Ecological eﬀects may include loss of biodiversity and addition of still more CO2 to the

atmosphere.

Socioeconomic threats include crop losses, increased disease, water scarcity, and coastal ﬂooding.

Population growth and socioeconomic factors (especially interference with third world development) can combine to produce political instability and conﬂict.

Most greenhouse gases are products of fossil fuel combustion, so reduced use, increased eﬀiciency, and alternative fuel development are primary means of prevention of climate change.

CO2 uptake can be increased by eliminating deforestation, reforestation, and green

roofs technology.

Legislation from local to international levels can cap emissions and develop carbon oﬀset trading.

Careful urban planning can increase the eﬀiciency of transportation and energy use.

Planetary engineering could enact radical changes in technology, culture, or the bio- sphere.

Support of third world eﬀorts to develop without adding greenhouse gases could im- prove global stability.

Every potential solution has costs and beneﬁts which must be carefully considered; tradeoﬀs are necessary.