         
Original article by Alison Colls, Environmental Change Institute.
Read this popular and comprehensive overview of climate change, written by experts at Environmental Change Institute, Oxford University.
Introduction Climate change is an entirely natural process that occurs over a wide range of timescales, from a few years to hundreds of millions of years. The challenge faced by climate scientists is not so much detecting the changes in climate as attributing them to a specific cause - and importantly, making the distinction between natural climate variability and climate change which has been forced by human activity. The following article offers a starting point for an understanding of an incredibly complex process. Read the ‘Where Next?' section for guidance on how to probe further.
Global climate is affected by many different processes. Some of these are external such as changes in the earth's orbit, or output of energy from the sun. Other processes are internal and involve changes and interactions between the atmosphere, oceans and land systems. Photo: NASA. Sections in this article What causes climate change? Long term climate variability Short term climate variability Detecting climate change Observing Climate Change Attributing climate change to a specific cause Future climate change Uncertainties in projecting future climate change Some excellent books Where Next? What causes climate change? The general state of the Earth's climate is controlled by the balance of energy the Earth receives from the Sun and the amount of energy the Earth releases back to space. Causes of climate change involve any process that can alter this global energy balance.
Energy from the sun drives the Earth's weather and climate. Photo A. Colls. The processes that force the climate to change can be separated into internal and external processes. External processes take place outside the Earth, and include changes in the Earth's orbit around the Sun, and changes in the amount of energy the sun emits. Internal processes occur within the Earth's climate system (i.e., within the oceans, atmosphere, ice sheets, biosphere and geological systems) and include changes in the global energy balance due to changes in ocean circulation or changes in the composition of the atmosphere. Other processes affecting climate include the impacts of large volcanic eruptions and changing surface characteristics such as albedo (surface reflectivity) through, for instance, the growth/decline of ice sheets. Long term climate variability Long-term (millions of years) climate changes are associated with the changing locations of continents and oceans (plate tectonics) that affect atmospheric and oceanic circulation patterns. Over approximately the last 2.5 million years, the earth has experienced a series of ice ages and has shifted between cold intervals (glacials) and warmer intervals (interglacials). During glacials, mid-latitude mean temperatures were about 7-10°C lower than today, ice sheets expanded and sea levels dropped by up to 120m. The last glacial period ended about 10,000 years ago. Since then, the earth has been in a particularly stable, warm interglacial state. During interglacials, the global climate is mild enough for deciduous forests to develop in Europe. These glacial-interglacial cycles are related to variations in the earth's orbit around the sun, called Milankovitch cycles after the astronomer who identified them. A series of predictable changes in the earth's orbit affect the amount of solar energy the earth receives, the primary source of energy in the climate system.
The graph shows that there have been major fluctuations in climate over the last two million years. It is derived from oxygen isotope ratios of small oceanic organisms, which are an index for the amount of water stored as ice on land. (From R.T. Patterson Climate Change: a geological perspective. www.carleton.ca.) Short term climate variability The global climate also varies over much shorter timescales (years, decades, centuries and millennia). Short-term changes in climate occur in response to changes in the output of solar energy, the number of volcanic eruptions, the natural circulation of the oceans and concentration of aerosols and greenhouse gas concentrations in the atmosphere. All of these factors alter the earth's radiation budget by increasing, or decreasing the amount of energy that the earth receives from the sun, or affecting the way it is redistributed within the climate system. The composition of the atmosphere has changed naturally as a result of complex feedback mechanisms within the climate system. Cooler glacial periods, for example, were characterised by lower concentrations of carbon dioxide (an important greenhouse gas) and higher levels of atmospheric dust (which block incoming sunlight) which both lead to global cooling. More recently, however, humans have changed the composition of the atmosphere by increasing the concentration of greenhouse gases in the atmosphere, primarily as a result of burning fossil fuels, but also as a consequence of deforestation and agricultural activities. The concentration of atmospheric carbon dioxide in the atmosphere has increased by 31%, since 1750, to a level which has not been exceeded during the past 420,000 years and likely not during the past 20 million years. In addition to carbon dioxide, we are emitting other greenhouse gases such as methane via the combustion of fossil fuels, landfills, agricultural activities (cattle and rice cultivation) and manufactured greenhouse gases such as CFCs. Increasing the amount of greenhouse gases in the atmosphere increases the strength of the natural greenhouse effect, which in turn increases the earth's average surface temperature.
Levels of carbon dioxide in the atmosphere have increased dramatically since the start of the industrial revolution. Direct measurements of CO2 have been taken since the 1950s. Before that CO2 concentrations are determined from measurements of air trapped in Antarctic ice cores. From CSIRO, after Etheridge (1999). Detecting climate change We can detect recent changes in climate using the instrumental record of daily, monthly and annual changes in rainfall, temperature, humidity and other weather phenomena. These records provide a detailed, albeit short, history of climate change. Instrumental records of recent climate show that the global mean temperature has increased by 0.6±0.2ºC over the last century. They also indicate that, globally the 1990s was very likely the warmest decade, while 1998 and 2001 were the warmest and the second warmest years respectively since the start of the instrumental records in 1861.
The global mean temperature has increased by 0.6±0.2°C over the last century. The IPCC (2001) have said this is likely to have been the largest increase in temperature of any century in the past 1000 years. (Climate Research Unit, University of East Anglia: Jones, P.D., New, M., Parker, D.E., Martin, S. and Rigor, I.G., 1999: Surface air temperature and its changes over the past 150 years. (Reviews of Geophysics, 37, 173-199). Observing Climate Change In addition to the increase in average global temperature, a range of wider changes have been observed in our climate system. Mountain glaciers have retreated dramatically in non-polar regions, while evidence from tide gauge data shows that global average sea level has risen between 0.1 and 0.2 metres during the 20th century. In addition, the number of heavy rainfall events has increased in the mid latitudes in the northern hemisphere, while parts of Africa and Asia have experienced increases in the frequency and intensity of droughts. El Nino events have also been more frequent, persistent and intense since the mid 1970s compared with the previous 100 years.
Mountain glaciers have retreated dramatically as a result of the warming in the second half of the 20th century. Photo: NOAA However global warming has not been observed everywhere and much debate has surrounded the observation that some large areas of the atmosphere have actually been cooling. This cooling in the atmosphere is now believed to be a side effect of ozone depletion in the stratosphere, which is reducing the effect of global warming. Over longer timescales (hundreds, thousands and millions of years), we can deduce past climate change from a range of sources. Among the most important of these records are cores taken from ice and ocean sediments, tree rings, records of coral growth and wind-blown glacial sand (‘loess'). These records preserve detailed information about past climates and the natural variability of climate that can be analysed and dated using a wide range of techniques. This long-term perspective is essential for putting modern climate changes in context. Attributing climate change to a specific cause Although it is relatively easy to detect a change in climate, attributing a change in climate to a particular cause is much harder. The majority of climate scientists agree that the global climate has warmed over the last century. Their challenge has been to determine whether this warming is attributable to: (a) natural climate variability (e.g., caused by changes in solar output of energy), (b) mankind increasing the atmospheric concentration of greenhouse gases and enhancing the natural greenhouse effect, or (c) a combination of both natural and anthropogenic climate forcing. Scientists use complex computer models of the global climate to try and distinguish between natural and anthropogenic (human-induced) climate change. Coupled atmosphere-ocean general circulation models (AOGCMs) are the most complex of these climate models, since they attempt to represent the main components of the climate system in three dimensions. A series of experiments are run on these climate models which generate climate change scenarios (possible representations of how the climate will evolve) based on a combination of different natural or human forcing factors (i.e. causes of climate change).
A coupled ocean-atmosphere climate model was used to simulate the temperature changes that occur both from natural and anthropogenic causes. The simulations in (a) were done with only natural forcings: solar variation and volcanic activity. Those encompassed in (b) were done with anthropogenic forcings: greenhouse gases and an estimate of sulphate aerosols, and those encompassed by the band in (c) were done with both natural and anthropogenic forcings included. From Climate Change 2001: The Scientific Basis: The Intergovernmental Panel on Climate Change. The model simulations for different scenarios are compared with the observed records of actual climate change. The results of these climate model experiments show that natural climate variability may have contributed to the observed warming in the first half of the 20th century but does not, by itself, explain the warming in the second half of the 20th century. The best agreement between model simulations and observations over the last 140 years is found when anthropogenic and natural forcing factors are combined. Over the last 50 years, the IPCC (Working Group I: The Scientific Basis 2001) have said that there is ‘new and stronger evidence that most of the warming observed is attributable to human activities.' Future climate change Climate models are used to project how the climate might change during the 21st century. They calculate how the global climate might respond to the enhanced greenhouse effect, using a range of estimated increases in atmospheric greenhouse gas concentrations. Based on a range of climate model projections, the IPCC (2001) project an increase in the globally averaged surface temperature of 1.4 to 5.8°C over the period 1990 to 2100. Even the lowest projected rate of warming is much larger than the warming we have seen during the 20th century and is very likely to be unprecedented during at least the last 10,000 years. Uncertainties in projecting future climate change Model projections of future climate change contain large uncertainties and are based on a simplified simulation of the global climate. This is partly due to the level of computing resources available but mainly due to our poor understanding of certain (non-linear) feedback mechanisms within the climate system. Our understanding of how water vapour, clouds and anthropogenic aerosols will influence global warming is still incomplete and there is still uncertainty about the natural variability of the climate over decadal timescales.
Some of the largest uncertainty in model estimates of global warming is due to clouds. Our knowledge and understanding of global climate is continually evolving. Longer instrumental records and continued model development will help quantify and reduce these uncertainties, although the complex and non-linear feedbacks within the internal system mean it may never be possible to precisely model the global climate. There is limited confidence, for example, in the ability of the models to predict how ENSO events, the North Atlantic Oscillation, and the Asian monsoon will respond to global warming. Despite uncertainties about the actual response of the global climate to increasing greenhouse gas emissions, most scientists agree that the global warming trend of the 20th century will continue through the 21st century. This warming is projected to have many adverse impacts during the 21st century, especially on populations that are already vulnerable through famine, poverty etc. In terms of earth's history, a changing climate is not unusual; the global climate has fluctuated over millions of years and will continue to do so in the future. The ‘problem' is that the earth currently hosts a (increasing) population of 5.8 billion people, whose ecosystems, agriculture and settlement structures are based on the stable climatic conditions that have prevailed for the last 10,000 years. Some excellent books Houghton, Sir John T. (1997). Global Warming: The Complete Briefing 2nd Edition. Cambridge University Press, Cambridge. Very clear briefing on the background from one of the world's most respected atmospheric scientists, who is the Chair of Working Group I of the IPCC. Goudie, A.G., Stokes, S. (2002) Environmental Change 4th edition. Oxford University Press, London. An excellent book for those interested in the study of environmental changes over the last three million years. It contains a clear and helpful explanation of the causes of climate change. Imbrie, J. & Imbrie, K.P. (1984). Ice Ages: Solving the Mystery. Oxford University Press, London. An interesting and accessible account of the history of the discovery of the ice ages and the astronomical theory for their cause suggested by Milankovitch. Where Next? On ClimateX.org Leading UK political thinker Ian Christie looks at the challenge of transition in ‘Hearts, minds and habits'. A personal perspective on taking action is given in ‘Fighting Climate Change: a lifetime's commitment'. Read more about the carbon cycle which is impacted by human activity in ‘The Carbon Cycle and Climate Change: A Beginner's Guide'. At a very pragmatic level, does climate change affect economic performance? Look at ‘Climate change hits your pocket' and ‘Flooding: an introduction'. External links The Stern Review examines the economic case for acting to combat climate change: download the report from http://www.hm-treasury.gov.uk/independent_reviews/stern_review_economics_climate_change/sternreview_index.cfm Encyclopaedia of Atmospheric Environment - an excellent source of information on all aspects of climate change and global warming. Climatic Research Unit, University of East Anglia - An excellent site with clear information sheets on a range of climate change topics. Climate Change 2001: The Scientific Basis. Contributions of Working Group I to the third assessment report of the Intergovernmental Panel on Climate Change National Oceanic and Atmospheric Administration's (NOAA) Palaeoclimatology Program is a central location for palaeoclimate data, research, and education. Article by
Alison Colls, Environmental Change Institute
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