A Primer on Climate Change and Global Warming – Part I

The climate is changing. The earth is warming up, and there is now overwhelming scientific consensus that it is happening, and human-induced. With global warming on the increase and species and their habitats on the decrease, chances for ecosystems to adapt naturally are diminishing. Many agree that climate change may be one of the greatest threats facing the planet. Recent years show increasing temperatures in various regions, and/or increasing extremities in weather patterns. This 3-part article looks at (1) what causes climate change, (2) what the impacts are and (3) what we can do to mitigate it.
What is global warming and climate change?

The climate system is a complex, interactive system consisting of the atmosphere, land surface, snow and ice, oceans and other bodies of water, and living things. Climate is often defined as ‘average weather’. Climate is usually described in terms of the mean and variability of temperature, precipitation and wind over a period of time, ranging from months to millions of years (the classical period is 30 years). The climate system evolves in time under the influence of its own internal dynamics and due to changes in external factors that affect climate (called ‘forcings’). External forcings include natural phenomena such as volcanic eruptions and solar variations, as well as human-induced changes in atmospheric composition.

Fig 1: Schematic view of the components of the climate system, their processes and interactions. (Source: IPCC 4th Assessment Report)

Solar radiation powers the climate system. There are three fundamental ways to change the radiation balance of the Earth: 1) by changing the incoming solar radiation (e.g., by changes in Earth’s orbit or in the Sun itself); 2) by changing the fraction of solar radiation that is reflected (called ‘albedo’; e.g., by changes in cloud cover, atmospheric particles or vegetation); and 3) by altering the longwave radiation from Earth back towards space (e.g., by changing greenhouse gas concentrations). Climate, in turn, responds directly to such changes, as well as indirectly, through a variety of feedback mechanisms.

Global warming and climate change refer to an increase in average global temperatures. Natural events and human activities are believed to be contributing to an increase in average global temperatures. This is caused primarily by increases in “greenhouse” gases such as Carbon Dioxide (CO2) and methane (CH4).

What is the Greenhouse Effect? What do we have to do with it?

{sidebar id=34} The Sun’s warmth heats the surface of the Earth, which in turn radiates energy back to space. Some of this radiation, which is nearly all in the infrared spectrum, is trapped in the atmosphere by greenhouse gases. For instance, water vapour strongly absorbs radiation with wavelengths between 4 and 7 micro-metres, and carbon dioxide (CO2) absorbs radiation with wavelengths between 13 and 19 micro-metres.

The trapped radiation warms the lower atmosphere, or troposphere. Some heat then finds its way back down to the Earth’s surface, making it hotter than it would otherwise be. This is the “greenhouse effect”.

Figure 2: The Green-house Effect (Source: Wikipedia)

Greenhouse gases are a natural part of the atmosphere and is important for supporting life on Earth. The problem we now face is that human actions—particularly burning fossil fuels (coal, oil and natural gas), agriculture and land clearing—are increasing the concentrations of the gases that trap heat. This is the enhanced greenhouse effect, which is contributing to a warming of the Earth’s surface.

Water vapour is the most abundant greenhouse gas. Its concentration is highly variable and human activities have little direct impact on its amount in the atmosphere. Humans have most impact on carbon dioxide, methane and nitrous oxide. Various artificial chemicals such as halocarbons also make a small contribution to the enhanced greenhouse effect.

Six main greenhouse gases are carbon dioxide (CO2), methane (CH4) (which is 20 times as potent a greenhouse gas as carbon dioxide) and nitrous oxide (N2O), plus three fluorinated industrial gases: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6). Water vapor is also considered a greenhouse gas.

Is the Earth’s climate really warming up?

The global average surface temperature has increased since reliable measurements began in the late 1800s. During the past 100 years, global average surface temperature increased by about 0.7°C. Tree rings and other records tell us that average Northern Hemisphere temperatures during the second half of the 20th century were likely to have been the highest in at least the past 1300 years. The five warmest years over the last century occurred in the last ten years. They stack up as follows: the warmest was 2005, then 1998, 2002, 2003 and 2004. In addition to warming of the Earth’s surface, there has been an increase in heat waves, warming of the lower atmosphere and deep oceans, fewer frosts, retreat of glaciers and sea ice and a rise in sea level during the 20th century of approximately 17 cm. Many species of plants and animals have changed their location or the timing of seasonal activities in ways that provide further evidence of climate change.

Figure 3: Rising Global Temperatures (Source: IPCC 3rd Assessment Report)

This graph indicates how the Earth’s surface temperature has increased since the mid 19th century. Scientists have projected a range of possible temperatures based on a number of future greenhouse gas emission scenarios. Scientists believe that the Earth’s average temperature will rise by 1.1 to 6.4 ºC by 2100 if nations around the world do not act to control greenhouse emissions (IPCC 3rd assessment report).

Although many natural factors influence the Earth’s climate, a majority of the world’s scientists have determined that greenhouse gas increases were the main factor contributing to climate change since the 1950s. In its Fourth Assessment Report released in 2007, the Intergovernmental Panel on Climate Change—an international body that assesses the latest science of climate change—stated that ‘Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in [human produced] greenhouse gas concentrations”. Scientists consider that warming will continue as a result of past, current and future emissions.

How do we know that most recent global warming is attributable to human activities rather than natural causes?

The present atmospheric concentration of carbon dioxide has not been exceeded for the past 650,000 years, and possibly not for 20 million years. Ice core records show that carbon dioxide levels in the atmosphere varied between 180 and 280 parts per million (ppm) due to glacial cycles. For the past 10,000 years global atmospheric carbon dioxide has been quite stable at between 260 and 280 ppm, and level at about 280 ppm from 1000 to 250 years ago. However, since the beginning of the Industrial Revolution, some 250 years ago, the concentrations of greenhouse gases in the atmosphere have increased dramatically. Human activities, such as burning fossil fuels (coal, oil and gas), land clearing and agricultural practices have increased carbon dioxide concentrations by more than a third (to approximately 380 ppm), nitrous oxide levels by about 19 per cent and methane concentrations have more than doubled. The rate of increase in carbon dioxide during the industrial era is very likely to have been unprecedented in more than 10,000 years.

Figure 4: Global atmospheric concentrations of 3 greenhouse gases.

The observed changes in climate, especially temperature increases since about 1970, cannot be explained solely by natural causes such as solar activity. Reconstructions of climate data for the past 1000 years indicate that this recent warming is unusual and is unlikely to have resulted from natural causes alone. Scientists use computer models to simulate past and future climate variations. Simulations of the 20th century have been driven by observed changes in various factors that affect climate. When only natural factors, such as volcanic aerosols and solar activity, are included in the models, the simulations do not explain the observed warming in the second half of the century. The warming in the second half of the century can only be explained if human-induced changes in greenhouse gases are included in the models.

What is the carbon cycle? How does human activity contribute to the carbon cycle?

Carbon, in various forms, continuously circulates between the living world, the atmosphere, oceans and the Earth’s crust. There are many different processes by which carbon is exchanged between these locations. Events, such as fires, which release carbon dioxide into the atmosphere, are known as ‘sources’. The oceans and growing trees remove carbon dioxide from the atmosphere and are known as ‘sinks’.

Figure 5: Carbon Cycle. Source: NASA

Each year human activity adds several billions of tonnes of carbon in the form of carbon dioxide to the atmosphere. A little over half of this carbon dioxide remains there, while the rest is absorbed by plants and the oceans (and ultimately some of this is returned to the Earth’s crust). More than 120 billion tonnes of carbon are exchanged each year between all living things during photosynthesis and respiration. Plants absorb about 61 billion tonnes of carbon and respire about 60 billion tonnes. Plants grow by absorbing carbon dioxide from the air or water and converting it to plant tissue through photosynthesis. Some of this carbon is used to supply the plant with energy. This process, known as respiration, releases carbon dioxide back into the atmosphere. The carbon from carbon dioxide absorbed by a tree may be stored as wood for hundreds of years. Or the carbon may become part of a leaf that dies and decomposes, with the carbon returning to the atmosphere relatively quickly.

The surfaces of the oceans release about 90 billion tonnes of carbon to the atmosphere and absorb about 92 billion tonnes each year. This absorption occurs when carbon dioxide in the air dissolves in the top layer of sea water and through photosynthesis by marine plants. The amount of carbon dioxide that people add to the atmosphere may seem very small in comparison to the amounts being added and absorbed by natural processes, but it only takes a small change to upset the balance.

The burning of fossil fuels by humans adds about 6.5 billion tonnes of carbon each year in the form of carbon dioxide. Land clearing, reduced soil humus and the erosion of topsoil account for one to two billion tonnes of carbon a year. Proof that more carbon dioxide is being added to the atmosphere than removed is the fact that concentrations of the gas continue to rise. Furthermore, scientific techniques reveal that this additional carbon dioxide originates from fossil fuel combustion. However, most of the observed warming over the past 50 years is very likely to have been due to the human-induced increase in greenhouse gas concentrations.

In the next part, we will explore the impacts of anthropogenic emissions on global warming and what this means to our ecosystem. We will also learn where the scientific consensus lies.


1) IPCC 4th Assessment Report: Intergovernmental Panel on Climate Change, Climate Change 2007: The Physical Science Basis – WGI contribution to the IPCC Fourth Assessment Report www.ipcc.ch/

2) IPCC 3rd Assessment Report: Intergovernmental Panel on Climate Change, Climate Change 2001: The Scientific Basis – WGI contribution to the IPCC Third Assessment Report www.ipcc.ch/

3) NASA: http://earthobservatory.nasa.gov/Features/CarbonCycle/

4) Wikipedia: http://commons.wikimedia.org/wiki/Image:Greenhouse_Effect.svg

5) http://www.nasa.gov/vision/earth/environment/2005_warmest.html

6) Environmental Protection Agency: http://www.epa.gov/climatechange/science/

7) National Oceanic and Atmospheric Administration/ Earth System Research Laboratory: http://www.esrl.noaa.gov/

8) United Nations Environment Programme: www.unep.org

9) The Australian Greenhouse: www.greenhouse.gov.au

Author’s note: Recognising the problem of global climate change, in 1988 the World Meteorological Organisation (WMO) and the United Nations Environment Programme (UNEP) established the Intergovernmental Panel on Climate Change (IPCC). The role of the IPCC is to assess the scientific, technical and socio-economic information relevant to understanding the threat of human-induced climate change. The IPCC has produced comprehensive assessment reports on the status of global climate change, with the Fourth Assessment Report released in 2007. Hundreds of the world’s leading climate scientists, contributed to the production of these reports, which provide the authoritative, consensus account of global climate change.

About the author:
Dr. Mita Das has a PhD in Atmospheric Science and has studied “Air Pollution” and “Acid Rain” for several years.

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