Nick Read: May 2017
The Theology of Climate Change — 1. The Science
The Earth's Energy Balance and the Greenhouse Effect
The sun radiates energy as electromagnetic radiation. Generally, the hotter something is the shorter the wavelength of the energy radiated. The energy that reaches the earth is equivalent to 1,360 watts/m2 (a lightbulb uses between 40-100 watts). However, taking into account the curvature of the earth and the fact that only half the surface is facing the sun at a given moment, and this figures averages out at 340 watts/m2. It is this energy which acts as the earth's heat engine driving the processes of atmospheric circulation (such as evaporation, convection, rainfall and winds) and ocean circulation. Of the energy that reaches earth:
29% is reflected back into space, primarily by clouds but also by other material with high reflective properties (albedo) such as the cryosphere (ice and snow).
23% is absorbed by gases, dust and particles in the atmosphere
48% is absorbed at the planet's surface
Since the atmosphere and the earth's surface absorb 71% of the incoming solar radiation, they must also radiate an equivalent amount back into space if the temperature is to remain stable (otherwise the earth would simply get hotter).
The surface energy balance
At the surface, this energy balance is maintained through three processes:
Evaporation — Liquid water molecules absorb the incoming solar energy and change from liquid to gas. The heat energy remains latent in the water vapour and is released into the atmosphere when the vapour condenses back into rain. Evaporation from tropical oceans is the primary driver of the atmospheric heat engine, producing the movement of gases in the lowest layer of the atmosphere that are responsible for wind and weather. This mixing gives this layer of the atmosphere its name — troposphere (Greek tropos, mixing or turning).
Convection — Air in contact with the sun-warmed ground becomes warmer and more buoyant. Warm air rises taking heat energy away from the surface.
Emission of thermal infrared energy — All matter emits energy and the wavelength of the energy emitted from the earth's surface is different from that of the incoming solar radiation. It is infrared (i.e. it has a longer wavelength). In the atmosphere are particles which are capable of absorbing some of his longwave radiation, including water vapour, CO2, methane and some trace gases. These are called the Greenhouse Gases (GHGs).
When GHG molecules absorb energy their temperature rises and they also radiate energy. Some of this heat is radiated upwards and absorbed by GHGs higher in the atmosphere, but will eventually radiate into space when the concentration of molecules in the atmosphere becomes too low to interact with each other. Some radiated energy goes back down and will contact the earth's surface, so that the surface becomes warmer than it would be if it were only being heated by direct solar radiation. This supplemental heating effect is the natural greenhouse effect. Because of this effect the earth's temperature is approx 15 degrees higher than it would be if GHG gases were not present, and more than 30% warmer than if we had no atmosphere at all (a difference of 33 degrees C).This makes life possible.
Things that change the balance between incoming and outgoing energy (expressed in watts m2) are referred to as "forcings". Forcings may be natural, for example:
Changes in the sun's brightness, variations in the axis of the earth's orbit and large volcanic eruptions that inject particles into the atmosphere
Or man-made, such as:
Particle pollution (aerosols) which absorb or reflect energy, deforestation and other significant land use changes which alter the albedo, and greenhouse gases
Radiative forcing was very small in the past and the earth's average temperature was very stable. For convenience, therefore, researchers choose a pre-industrial baseline year (either 1750 or 1850) and compute any forcing in relation to this base-year. A positive forcing induces warming whilst a negative forcing induces cooling. It is possible to determine the radiative forcing of many GHGs accurately because we know their atmospheric concentration, their spatial distribution and the physics of their interaction with radiation. The radiative forcing depends on the ability of these particles to absorb or emit longwave radiation. However, some substances have more complicated effects, for example aerosols (small airborne particles in the atmosphere) can have contradictory effects in that bright aerosols, e.g. sulphates emitted from burning coal, tend to cool the earth, whilst black aerosols (e.g. carbon particles from diesel exhausts) lead to warming.
Changes due to forcings will not occur right away because of the heat capacity of the ocean which provides thermal inertia to the system. This does not stop change from happening, but it slows down the rate of change so that even if all GHG emissions stopped tomorrow, there would still be a temperature rise until a new energy balance occurred. However, if GHG emissions are not checked then the level of absorbed energy will continue to rise and so will the earth's temperature. Calculations suggest that doubling the concentration of CO2 in the atmosphere from pre-industrial levels i.e. from 280 parts per million (ppm) to 56 ppm, will lead to a temperature rise of approx. 3 degrees. This may not sound much but the difference between today and the last ice age was an average global temperature of only 5 degrees less than today. A 3 degree rise represents a vast amount of extra energy in the earth's climatic system.
The most significant greenhouse gases that we need to control are:
Symbol Name Common Sources
CO2 Carbon Dioxide Fossil fuels, forest clearing, cement production
CH4 Methane Landfill, natural gas and petroleum, fermentation from ruminants, rice cultivation, fossil fuels
N2O Nitrous Oxide Fossil fuels, fertilisers, nylon production, manures
HFCs Hydrofluorocarbons* Refrigeration, aluminium smelting, semiconductors
PFCs Perfluorocarbons Aluminium production, semiconductor industry etc.
SF6 Sulphur Hexafluoride Electrical transmissions and distribution systems, circuit breakers, magnesium production etc.
HFCs replaced CFCs (Chloroflurocarbons) which were also effective greenhouse gases but were removed because of their role in destroying the ozone layer.
GHGs are indexed according to their Global Warming Potential (GWP). This is the ability of a GHG to trap heat in the atmosphere relative to that of an equal amount of Carbon Dioxide (the table means that 1 tonne of methane has a forcing equivalent to 21 tonnes of CO2). Therefore, all GHGs can be expressed as Carbon Dioxide Equivalents (written as CO2e).
Greenhouse Gas Global Warming Potential (Carbon Dioxide Equivalent)
PFCs 6,500 — 9,200
CO2 is the most important of these GHGs because although it is the least powerful it is present in increasing quantities due to the release of carbon when we burn fossil fuels. Coal deposits, the fossilised remains of forests, are largely from the carboniferous period (300-360m years ago) and oil was formed when microscopic sea creatures and plant material was subject to pressures that turned tissue into liquid. If no effort is made to cut carbon emissions it is predicted that global warming could reach between 2-7 degrees by the end of this century. The daily average concentration of CO2 in the atmosphere is monitored at Manua Loa Observatory in Hawaii ( and this recorded a concentration in excess of 400ppm on 10th May 2013.
What complicates predicting the effects of global warming are various feedback loops. One of the most important is the interactions of the cryosphere. Cryosphere is the collective name for all the snow, river and lake ice, sea ice, glaciers and ice caps, ice shelves, ice sheets and frozen ground. The cryosphere on land stores about 75% of the world's freshwater. Ice permanently covers about 10% of the land surface and an average of 7% of the ocean. In midwinter, snow covers approximately 49% of the land surface of the northern hemisphere. Ice and snow have a high surface albedo, reflecting 90% of solar radiation. As the temperature rises and snow and ice begin to melt, the melting may reveal darker land and water surfaces that lay underneath the ice. These will absorb more of the sun's energy (they have a lower albedo and therefore reflect less), which will in turn further increase the temperature and therefore the rate of melting. This positive feedback is called the ice-albedo feedback. A global temperature rise may also lead to more precipitation falling as rain rather than snow, which means that the ice is not replenished to the same extent each season, further exacerbating the positive feedback.
Other feedback loops affect marine ice sheets such as the West Antarctic Ice Sheet (WAIS). As sea levels rise more of the ice at the edge of the sheet begins to float, which reduces the forces holding the ice together. Consequently, the ice flows more rapidly leading to rapid disintegration or collapse at the edges of the sheet. Warm ocean water also acts to melt the underside of the ice sheet and this increases its disintegration. If WAIS melted it would raise global ocean levels by an average of 6m. Satellite measurements show that some of the ice is thinning and that this is due to the dynamic changes taking place in the ice sheet.
The cryosphere not only has a critical role in the albedo effect and sea levels, but also freshwater availability. For example, the Himalayas represent the largest concentration of glaciers outside the poles and are the headwaters of major river systems within Asia which affect millions of people.
Indicators of climate change
The weather is the conditions of the atmosphere over a short period of time and in a particular location; the climate is this behaviour measured over a much longer period of time (by convention scientists tend to use 30-year averages). Many people will argue about climate change when they are really talking about the weather, not the climate! Therefore, whilst one extreme weather event, such as a tropical storm, does not prove that climate change is taking place, the trend in these events (e.g. are they becoming more frequent and more intense) will indicate whether the climate is changing.
Many parameters show that the climate is changing:
Readings are taken daily at thousands of stations and combined with sea surface readings to give a global figure. The last decade has seen the highest global average surface temperatures since records began, about 0.8 degrees C above pre-industrial levels. 1983-2013 was the warmest 30-year period for 1400 years. In practical terms, the global temperature increased in the period 1850 to 2005 by 0.76 degrees C.
Sea level and ocean heat
Global sea temperatures are approximately 1 degree C higher now than they were 140 years ago. Sea level rises occur because water expands as it becomes warmer and because water from melting glaciers and ice sheets are added to the oceans. Satellite observations show that the sea level rose by 3.1mm (plus or minus 0.7mm) per year from 1993-2003.
Snow and ice cover
This is especially important in the Northern Hemisphere as this has the largest land mass. Part of the Arctic Ocean is permanently covered by ice, though the extent is typically smallest in September after the summer melt. The annual minimum extent of Arctic ice is reducing and September 2012 was the smallest on record. The length of the melt has increased and the width of ice has decreased, making it more vulnerable to further loss. The Greenland and the Arctic ice sheets are losing more ice and snow, and spring snow cover in the northern hemisphere is decreasing.
These include precipitation levels, wind and biomass. "Extreme" weather events are becoming more frequent (they occur more often), and of greater intensity (they are more severe), they are also of longer duration and they occur over a longer period in the year.
Climate Change Denial
Whilst there is no controversy that GHG concentrations are rising and that the average global temperature is rising, some people contend that this is not due to human (anthropogenic) activity. This is "climate change denial". Scientists adopt the language of probability when making statements about climate change and this is reflected in the precise terminology used in official reports by the Inter-Governmental Panel on Climate Change (IPCC) (see next session's notes). For example:
Verbal terms used in the IPCC report Likelihood (probability expressed in percentages)
"Virtually certain" 99 — 100
"Extremely likely" 95 — 100
"Very likely" 90 — 100
"Likely" 66 — 100
Over the years, the terminology in each successive report has grown more robust:
The 2001 IPCC Report (AR3) concluded that it was "likely" (more than 66% probability) that most of the warming since the mid twentieth century was attributable to human actions
The 2007 Report (AR4) raised this to "Very likely" (more than 90% probability)
The 2014 Report (AR5) concluded that it was "Extremely likely" (more than 95% probability).
In other words, the more robustly we examine the subject, the more certain we are that man induced global warming (because of GHG emissions) is leading to climate change.
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