Canadian Climate Change Scenarios Network - National Node

EC - GC

Scenarios: Introduction

Climate Change Concepts

• Climate change is defined as a difference over a period of time (with respect to a baseline or a reference period) and corresponds to a statistical significant trend of mean climate or its variability, persistent over a long period of time (e.g. decades or more). Climate change may be due to both natural (i.e. internal or external processes of the climate system) as well as anthropogenic forcings (ex. increase in concentrations of greenhouse gases);
• Climate variability is defined as a deviation from the overall trend or from a stationary state, and refers to variations in the mean state and other statistics (such as standard deviations, the occurrence of extremes, etc.) of the climate on all temporal and spatial scales. Climate variability can be thought of as a short term fluctuation superimposed on top of the long term climate change or trend. Cycles of high and low values of weather events (drought, floods) are not climate change unless prolonged over many decades. Low frequency variability refers to phenomena such as the North Atlantic Oscillation or El Niño, which occur at a decadal scale or longer, and high frequency variability refers to meteorological events and their distribution (for example, frequency, duration and intensity) at yearly, seasonal or monthly timescales;
• Reference periods are typically 3 decades long (30 years, i.e. 1961-1990 is often used as a climatological baseline period in impacts and adaptation assessments and to quantify the anomalies in the future). These periods are of sufficient length to adequately represent the climate of the period, and are used to compare fluctuations of climate between one period and another. Also, given the substantial inter-decadal climatic variability exhibited by most GCMs, it’s often difficult to distinguish a climate change signal from the background noise (i.e. the internal variability of the model or the model’s representation of natural variability). For this reason, the IPCC (2001a) has recommended to use at least 30-year averaging periods for GCM output data. Conventionally, these reference period differences (future climate minus baseline climate) are used for model scenario comparison of most climate variables. The differences are also often expressed as ratios (future climate/baseline climate), or percentage differences between periods. Typically, a number of fixed time horizons in the future are produced from model output, e.g., the 2020s (2010-2039), the 2050s (2040-2069), and the 2080s (2070-2099).

Graph illustrating the climate change, climate variability and reference period concepts (Source: courtesy of Elaine Barrow).

 

Definitions and Climate Change Background

1. A climate change scenario is:
• a coherent, internally consistent and plausible description of a possible future state of the world…” [from Parry &Carter, 1998, and Parry, 2002], and
• a plausible future climate that has been constructed for explicit use in investigating the potential consequences of anthropogenic climate change…” [from IPCC TAR, 2001].


2. A climate change scenario is not a prediction of future climate!



3. Why do we need climate change scenarios?
   • To provide data for Vulnerability, Impacts and Adaptation (VIA) assessment studies;
   • To act as an awareness-raising device;
   • To aid strategic planning and/or policy formation;
   • To scope the range of plausible futures;
   • To summarize our knowledge (or ignorance) of the future;
   • To explore the implications of policy decisions.


4. What are the benefits of developing climate scenarios?
   • Simple to obtain, interpret and apply;
   • Provide sufficient information for VIA assessments;
   • Physically plausible and spatially compatible;
   • Consistent with the broad range of global warming projections;
   • Reflect the potential range of future regional climate change, i.e., be representative of the range of uncertainty in projections.


5. DISTINCTION between a climate scenario and a climate change scenario (see IPCC, Chapter 13, 2001)
   • A climate change scenario = “a plausible future climate….” strictly refers to a representation of the difference between some plausible future climate and the current or control climate (usually as represented in a climate model, i.e. monthly/seasonal/annual values). This concept can be viewed as an interim step toward constructing a climate scenario;
   • A climate scenario = the combination of the climate change scenario and the description of the current climate as represented by climate observations (i.e. via a scrupulous analysis of the climate regime).
     

Depending upon the study of interest, different temporal data must be used. For example, when considering extreme event changes and variability, daily data which can capture the changes of high frequency events of the climate regime must be explicitly taken into account. Often the investigation of extreme events is of utmost importance for impacts and adaptation studies. However, most climate change scenarios derived from GCM output are generally based on changes in monthly or seasonal mean climate although recently, daily quantities of model output are being archived by climate modeling centres. Even though model output is being made available at finer temporal resolutions, it does not mean that it is any more meaningful than the output at monthly or seasonal time scales.

 

Further Reading

Barrow E., B. Maxwell and P. Gachon (eds.), (2004): Climate Variability and Change in Canada: Past, Present and Future, ACSD Science Assessment Series No. 2, Meteorological Service of Canada, Environment Canada, Toronto, Ontario, 114 p. (available from AIRD).

IPCC (2001a): Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Dai, X., Maskell, K. and Johnson, C.A. (Eds.)]. Cambridge University Press, Cambridge, U.K. and New York, N.Y., U.S.A., 881pp. See http://www.ipcc.ch/.

IPCC (2001b): Climate Change 2001: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change, McCarthy, J.J., O.F. Canziani, N.A. Leary, D.J. Dokken and K.S.White (Eds.)] Cambridge University Press, Cambridge, U.K. and New York, N.Y., U.S.A., 1032pp. See http://www.ipcc.ch/.

IPCC (2001c): Climate Change 2001: Mitigation. Contribution of Working Group III to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, U.K. and New York, N.Y., U.S.A. See http://www.ipcc.ch/.

IPCC-TGCIA (1999): Guidelines on the use of scenario data for climate impact and adaptation assessment. Version 1. Prepared by Carter, T.R., Hulme, M. and Lal, M. Intergovernmental Panel on Climate Change, Task Group on Scenarios for Climate Impact Assessment, 69pp. See http://ipcc-wg1.ucar.edu/wg1/wg1_tgica.html and available for download from http://ipcc-ddc.cru.uea.ac.uk/guidelines/ggm_no1_ v1_12-1999.pdf

Mearns, L.O., F. Giorgi, P. Whetton, D. Pabon, M. Hulme and M. Lal, (2003): Guidelines for use of climate scenarios developed from Regional Climate Model experiments. Data Distribution Centre of the International Panel of Climate Change, 38 pp. Available for download from http://ipcc-ddc.cru.uea.ac.uk/guidelines/dgm_no1_v1_10-2003.pdf.

Parry M. (2002): Scenarios for climate Impact and Adaptation Assessment. Global Environmental Change 12, 149-153.

Ruosteenoja, K., T.R. Carter, K. Jylhä and H. Tuomenvirta, (2003): Future climate in world regions: an intercomparison of model-based projections for the new IPCC emissions scenarios. The Finnish Environment 644, Finnish Environment Institute, 83 pp. Available for download from http://ipcc-ddc.cru.uea.ac.uk/sres/scatter_plots/scatterplots_home. html.

Smith, J.B. and M. Hulme, (1998): Climate Change Scenarios. In: UNEP Handbook on Methods for Climate Change Impact Assessment and Adaptation Studies [Feenstra, J.F., Burton, I., Smith, J.B. and Tol, R.S.J. (eds.)]. United Nations Environment Programme, Nairobi, Kenya and Institute for Environmental Studies, Amsterdam, pp. 3-1 to 3-40. See http://www.ipcc.ch/.

Wilby, R.L., S.P. Charles, E. Zorita, B. Timbal, P. Whetton and L.O. Mearns, (2004): Guidelines for use of climate scenarios developed from statistical downscaling methods, available from the DDC of IPCC TGCIA, 27 pp. Available for download from http://ipcc-ddc.cru.uea.ac.uk/ guidelines/dgm_no2_v1_09_2004.pdf.

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