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Climate Change
Briefings From Southern Africa
By Bob Scholes, Mary Scholes, Mike Lucas Wits University Press
Copyright © 2015 Bob Scholes, Mary Scholes, Mike Lucas
All rights reserved.
ISBN: 978-1-86814-921-6
CHAPTER 1
SECTION 1
Earth system science: The processes that underlie climate change
INTRODUCTION
1.Why is Earth habitable?
Sidebar: Heat radiation
2.How do greenhouse gases regulate Earth's temperature?
3.Is water vapour the most important greenhouse gas?
4.Why are clouds the wild card in climate change?
5.Is climate change just part of a long-term natural cycle?
Sidebar: Milankovic cycles
6.Are climate variations just due to volcanoes or other Earth processes?
7.How do El Niño and La Niña events affect South African weather?
8.How hot might it get in South Africa this century?
9.How might the rainfall in Southern Africa change in the twenty-first century?
10.Are extreme weather events related to climate change?
11.How do land-use changes and deforestation affect global warming?
12.What is South Africa's contribution to global warming?
13.What happens to carbon dioxide emissions?
14.Can ecosystems keep sucking up carbon dioxide from fossil fuel burning?
15.Could ocean currents slow down or change direction?
16.Is there any chance of runaway global warming?
INTRODUCTION
Global warming is a symptom of a much wider set of changes in the metabolism of our planet. It refers to the increase in the average air temperature near Earth's surface, measured over the past century-and-a-half by weather stations on land and on ships all around the world. Warming is the result of a set of connected changes in the global climate system, which consists not only of the atmosphere, but also the oceans, the cryosphere (the frozen parts of Earth) and the land. Scientists refer to this broad set of disturbances as 'global change', which is brought about largely by human activities. They include changes to the composition of the atmosphere caused by emissions of various gases and particles from industry, vehicles, domestic fires and agriculture; changes in how reflective and aerodynamically smooth the land surface is due to the spread of farmlands and cities into previously natural vegetation; changes in the ocean due to pollution, fishing, warming and the increased uptake of carbon dioxide (CO) from the atmosphere; and the melting of glaciers, ice caps, permafrost, snow cover and sea ice. The symptom these changes produce, acting together, is an unsteady rise in air temperature. There are other signals of a changing world, such as shifts in the seasons, in the amount of rainfall and in the height of sea level relative to the land, as well as increasing acidity of the ocean and severe storms occurring more frequently.
Underlying these worldwide changes is the unprecedented rise to global dominance of our species. Human numbers have grown exponentially since the Neolithic Revolution when we 'invented' agriculture by domesticating plant and animal species. Agriculture only became viable after Earth emerged from the last ice age about 10 000 years ago, and entered the era of relatively warm and stable climates known as the Holocene. The next important event in terms of the global impact of Homo sapiens was the Industrial Revolution, when we learned how to exploit the energy in fossil fuels such as coal and later in oil and gas. The industrial era began gradually and its starting date is usually given as about 1750. We only have regular, accurate and direct measurements of Earth's climate since about 1850, and of Earth's atmospheric composition from about 1950. The most dramatic changes have been in the past four decades. By the start of the twenty-first century, humans had transformed about a quarter of Earth's land surface to crop-lands, plantations or habitations; were using a fifth of the world's fresh water; were diverting about half of global primary production to their purposes; and had fished most of the global fish stocks to the limit.
Humans are also altering the way Earth's system works – the 'global metabolism' introduced above. We are changing the climate system, the cycling of nutrients and water, and the diversity and abundance of many species at given locations.
Currently, the growth in the effect we have on the global ecosystem is about one-quarter due to the continuing (but slowing) growth of the human population, and three-quarters due to the increasing consumption of resources per person. There are social changes under way everywhere. Democracies are unsteadily prevailing over despotisms; market economies over centrally planned economies; global over local trade; and worldwide, near-instant communication over the town crier. The social changes both drive the spread of human influence and offer a way of limiting it.
This section describes the basic science of the global climate system. It aims to explain why the climate is changing, and how we can be quite sure that it will continue to change unless we urgently do something about it.
1 Why is Earth habitable?
Earth is just the right distance from the sun and has the right atmospheric gas composition to support life. This results in an average global temperature of 15°C, which is maintained by a balance between incoming solar radiation and outgoing reradiated heat energy.
Our brightly burning sun radiates unimaginably large amounts of energy into space, warming Earth sufficiently to make it a habitable planet. By contrast, Venus, our neighbouring planet closer to the sun, is far too hot for life, while Mars, our neighbour more distant from the sun, is far too cold. So Earth, at 150 million kilometres from the sun, is neither too close nor too distant. But this is not the whole story. The temperature on Venus is hot enough to melt lead, partly because its atmosphere consists mostly of heat-trapping carbon dioxide (CO2). Conversely, distant Mars is so cold partly because its thin atmosphere does not retain much of the solar energy it receives.
Without the naturally warming effect of our atmosphere, Earth's temperature would be about 33°C cooler than it is. In other words, the global average would be a freezing –8°C rather than a pleasant +15°C. The heat-trapping properties of the atmosphere are due to the so-called greenhouse gases. These gases, and other substances in the atmosphere, alter the amount of solar radiation entering and leaving Earth's atmosphere. Some of these substances are tiny airborne particles known as aerosols, some of which have a cooling rather than warming effect. Most radiatively-active substances are gases, and the most important greenhouse gases (see Sec. 1, Q2) are water vapour (H2O), CO2, methane (CH4) and nitrous oxide (N2O). Most of the atmosphere consists of nitrogen (N2) and oxygen (O2), which have little direct effect on Earth's radiation balance. The greenhouse gases are all minor constituents of the atmosphere – less than 1% by volume – but they make an enormous difference to Earth's heat budget.
Earth's overall temperature remains more or less in balance over long periods of time because it is controlled by a negative feedback loop (see Fig 1.16.1 and Glossary), rather like a natural thermostat. If Earth warms up, more energy is lost by re-radiation of heat until the incoming and outgoing energy balance is restored. If Earth cools down, the reverse happens. The temperature at which this balance is struck depends on the composition of the atmosphere.
The amount of solar radiation reaching the outer edge of Earth's atmosphere, on the sunny side and on a flat surface perpendicular to the sun's rays, is 1 360 Watts of energy per square metre (Wm-2). This is called the solar constant, but in fact it varies by about 6.3% over the year since Earth's orbit around the sun is elliptical. The amount also varies slightly over roughly decadal cycles due to sunspot activity. This constant translates to a global average of about 342 Wm-2 at the outside of the atmosphere, once the day–night cycle, the seasonal cycle and the spherical shape of Earth are taken into account.
As the incoming solar radiation passes through the atmosphere towards Earth's surface, about 20% (68 Wm-2) is absorbed by the atmosphere, which warms the air. Another 23% (79 Wm-2) is reflected back into space by clouds and aerosols. The remaining 57% (195 Wm-2) reaches Earth's surface, but here about 9% (30 Wm-) is immediately reflected back into space. This is an average taken over highly reflective surfaces such as snow and ice and far less reflective surfaces such as vegetation and water. Any heat not re-radiated is absorbed by Earth's land and ocean surfaces, thereby warming the surfaces and causing evaporation of water. Clouds that form from the evaporated water reduce the amount of incoming radiation, forming a further negative feedback that helps regulate Earth's temperature.
The atmospheric temperature is controlled from both above and below. The overlying atmosphere (the troposphere) is warmer in its lowest parts, where it is heated by the warm Earth, but cooler at higher altitudes (see Sec. 1, Q4). Above the troposphere, the stratosphere is warmed by incoming solar radiation from above, so it is warmer with increasing altitude. Consequently, the boundary between the troposphere and the stratosphere is marked by a temperature minimum, which is closer to Earth's surface in high latitudes (that is, towards the poles) than at the Equator, since Earth's warming influence at the Equator is far greater. Incidentally, the cooling of the stratosphere over the past decades is an argument sometimes used by sceptics to discredit the idea of global warming, but it is exactly the expected pattern based on the physics of the atmosphere.
To balance the heat budget and therefore maintain Earth's temperature equilibrium, Earth must lose as much energy back into space as it receives at the atmosphere's boundary. The energy is dissipated into space at the atmosphere's outer boundary as long-wave infrared radiation. An increase in the concentration of greenhouse gases over the past 250 years has reduced the amount of infrared radiation emitted into space, causing the atmosphere to warm, which has in turn increased the amount of energy it radiates. Eventually the energies will balance once again, but at a higher global average temperature. This is the fundamental cause of global warming.
HEAT RADIATION
Any warm object – such as the sun, Earth, rocks or even oceans – radiates heat energy to its cooler surroundings. The range of wavelengths at which heat is radiated depends on the temperature of the object, according to an equation known as the Stefan–Boltzmann law. That is, the hotter the object, the shorter the wavelength at which it radiates energy. The temperature of the sun's surface is nearly 6 000°C, so most of its energy is radiated in the short-wavelength band of 0.2–4.0 µm (micrometres), which includes the part of the spectrum visible to humans. Earth's surface temperature averages about 15°C, so most of its energy is radiated in the long-wavelength band of 4–100 µm, which includes the infrared bands. Our eyes cannot see infrared radiation unaided, but we can feel it as a warm glow. Earth's atmosphere is almost transparent to incoming short-wave radiation, but is partially opaque to outgoing long-wave infrared re-radiation.
2 How do greenhouse gases regulate Earth's temperature?]
Earth is warmed by short-wave radiation from the sun, while it cools by re-radiating heat back into space as long-wave radiation. Until recently these gains and losses were more or less balanced, but the effect of accumulating greenhouse gases is tipping Earth's radiation balance by trapping more heat within the atmosphere than Earth is losing back to space.
Up until about two-and-a-half centuries ago, the gases within Earth's atmosphere comprised 78% nitrogen (N2), 21% oxygen (O2), and a long list of trace gases, including 0.93% argon (Ar), 0.03% carbon dioxide (CO2) and less than 0.001% methane (CH4). The density of the atmosphere decreases with altitude. In other words, there are fewer molecules per cubic metre higher up, but the relative proportions of the gases remain similar.
The molecules of greenhouse gases such as CO2 absorb and re-emit radiation in all directions in several narrow bands within the infrared range. On average therefore, half of this energy is reradiated back towards Earth, while the other half is re-radiated towards space. This process of absorption and re-radiation continues throughout the atmosphere, until eventually some of the upwardly re-radiated infrared radiation is lost to space because there are too few molecules to trap it (see Sec. 1, Q1). When additional greenhouse gas molecules are added to the atmosphere, the amount of radiation emitted back towards Earth increases, as well as the amount radiated into space. However, until the atmospheric temperature reaches a new, warmer equilibrium, the amount trapped by successive absorptions and re-radiations in the atmosphere slightly exceeds the amount leaving the atmosphere.
A crucial question to ask is how much extra warming is caused by a given additional quantity of greenhouse gas? For example, a doubling of the CO2 concentration from the pre-industrial level of about 285 parts per million (ppm, equivalent to 0.03% by volume) to 570 ppm (0.06%) could be reached this century. By itself, this would lead to an average global warming of about 1.2°C. However, the total warming resulting from a doubling of CO2 would be higher than this because of the amplifying effect of greenhouse gases such as water vapour that are naturally present, but increased by warming (see Sec. 1, Q3). In addition, other greenhouse gases such as methane and nitrous oxides are increasing, for the same fundamental reasons as CO2 : human activities. The many feedbacks and interactions in the climate system make the rise in global mean temperature per unit increase in greenhouse gases (known as the climate sensitivity) somewhat uncertain. Accounting for everything we can, there is a 90% chance that average global warming for a 570 ppm CO2 world is 2–6°C, with a most likely global average value of 3.5°C.
Polar latitudes are more affected by greenhouse gas warming than equatorial latitudes. This is because the re-emission of infrared radiation from CO2 in cold polar latitudes (back radiation) returns relatively more heat to Earth than would be the case for the same amount of CO2 in tropical latitudes. By the same reasoning, re-emission of infrared radiation from CO2 at high altitudes in the atmosphere, where the air is cold, contributes more to global warming than re-emission of infrared radiation at lower, warmer altitudes. This partly explains why the greenhouse effect of high-altitude clouds and aircraft contrails is greater than the greenhouse effect of low-altitude clouds. Low clouds have a net cooling effect because of their heat-reflecting (albedo) effect on incoming radiation (see Sec. 1, Q4).
To compare the warming effects of different greenhouse gases, climatologists apply an approximate conversion factor known as the Global Warming Potential (GWP). This is the warming, over a specified time period (usually 100 years), of one tonne of a given greenhouse gas relative to the warming caused by a tonne of CO2. The values attributed to GWPs have changed slightly over the past decades as scientists have learned more about atmospheric processes. The current values used are given in Table 1.2.1. They are not precise constants, but an approximation agreed to by scientists from many countries to allow relatively simple comparisons. Some scientists believe we should use a different or additional system of comparison, for instance by comparing the contributions of the different gases to observed atmospheric temperature increases.
(Continues...)
Excerpted from Climate Change by Bob Scholes, Mary Scholes, Mike Lucas. Copyright © 2015 Bob Scholes, Mary Scholes, Mike Lucas. Excerpted by permission of Wits University Press.
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