Concepts and Terms
Weather and Climate
Although the terms weather and climate are commonly used interchangeably, there is an important distinction. The term ‘weather’ describes the day-to-day and hour-by-hour changes in atmospheric conditions at a given location. ‘Climate’ is the synthesis of these day-to-day variations into a set of average conditions, often based on a statistical summary such as average 30-year record of weather observations.
Traditionally it has been assumed that the current climate is fixed, and we have experienced in recent history all climate conditions that will occur in the future. Many of the decisions we make as a society are based on similar assumptions (GSC 2001). However, we now know that these assumptions are false.
By examining climate variability and climate change as dynamic, we can improve both operational decision making and long term planning processes. This concept is discussed in more detail below.
Climate Change and Climate Variability
‘Climate change’ and ‘climate variability’ are sometimes used interchangeably just as ‘weather’ and ‘climate’ are. Climate is the mean conditions over a period of decades, whereas climate variability refers to oscillations and changes experienced on shorter time scales; climate change refers to changes on longer time scales.
These definitions are not universal. When discussing a time scale which is a millennium or longer, hundreds of years could be used to represent variability rather than change. For instance, during the last million years or so, glacial and interglacial periods alternated due to variations in the Earth’s orbit. Studies of the Northern Hemisphere over the past 1000 years show a cooling trend. Variability within this 1000-year period has occurred, a warm period between the 11th and 13th centuries and a cooler period between the 16th and 19th centuries followed by warming towards the end of the 20th century (IPCC, Scientific Basis, 2001 page 91-92).
When one talks about climate change, the inevitable question of attribution comes up — what caused the change? Prior to the industrial revolution any ‘climate change’ would be ascribed wholly to natural causes. Today, both natural and anthropogenic (human-induced) influences are at work simultaneously.
Within the scope of this document, there is no distinction made between natural and human influenced climate change. From the point of view of adapting to observed and projected climate change, we need to consider climate change regardless of its cause.
Paleoclimate and Climate Trends
The paleoclimatic record is based on terrestrial and marine records such as fossils, tree rings, pollen, sand dunes, and other information preserved in sediment. Such records, which are affected by climate, can be used to provide evidence about the climate that existed prior to the recording of instrumental observations. Paleoclimate analysis can provide useful information about realistic ecosystem responses to climate change and variability that have already occurred.
Climate trends are derived from statistical analysis of climate observations (historical trends) or from analysis of paleoclimate records prior to recording of direct measurements. Climate trends provide a picture of how much change has occurred, generally over a period of 50-100 years. Individual climate trends cannot be taken as evidence of human-induced climate change – a trend measures the change in climate from all influences over the period of analysis. Furthermore, a trend is not necessarily an indication of what future climate might be expected because the climate is comprised of overlapping climate variability on different time scales as well as long term climate change, which itself is not necessarily linear or gradual.
Forecasts, Predictions, Projections and Scenarios
Each of these terms refer to estimates of future weather and/or climate: forecasts, predictions, projections, and scenarios. Although there is some variation in the use of these terms, in this FAQ we will consider forecasts and predictions as interchangeable, though we will exclusively use the term predictions. A prediction refers to a specific value for a site or region generated by relationships between climate at that site or region, and other observed parameters that can be measured in advance. These methods are suitable for predicting average seasonal conditions up to a year in advance, so they are normally referred to as seasonal climate predictions.
Climate projections are derived from global climate models (GCMs), sometimes using regional climate models (RCMs) and other techniques to ‘downscale’ results to a higher resolution. A projection refers to an individual set of results for temperature, precipitation, or other parameters from a given climate model run under a specified set of assumptions about the amount of greenhouse gases released into the atmosphere (the standard set of assumptions used are SRES – from the IPCC’s Special Report on Emissions Scenarios). Projections are generally run over the 21st century and results analysed on 30-year average periods: to 2020s (average of 2010-2039), 2050s (2040-2069), and 2080s (2070-2099).
There is considerable uncertainty regarding the amount and timing of greenhouse gas emissions worldwide, and there is considerable uncertainty from model results as the models are imperfect. In addition, there is spatial and temporal uncertainty — models can only provide a probable future, not a certain one. For these reasons, it is important to avoid projections that apply specific predictions for a specific location and time. Instead, ‘climate scenarios’ are used — a set or range of climate projections from different climate models that are run with different emissions scenarios — to get a range of plausible future outcomes.
Vulnerability, Impacts, Adaptation, and Mitigation
Without an understanding of vulnerability, information on physical climate impacts is only of limited use for educational or decision-making purposes. Assessment of vulnerability is a means of identifying the sensitivity of socio-natural-economic sectors to climate change, which allows identification of relevant impacts. Future climate impacts are the consequences of climate change on the environment and society. Adaptation is the process of incorporating physical climate information into (operations and) long term planning. Mitigation is the attempt to reduce future climate impacts by reducing atmospheric greenhouse gas concentrations.
PCIC is focused on past climate variability and change as well as scenarios of projected future climate impacts. Past climate variability gives us a reference for normal interannual change; climate change in conjuction with an understanding of variability give us a frame of reference to understand the relative magnitude and importance of future climate impacts within the context of past variability. Adaptation is likewise informed by assessments of future climate impacts. Likewise, mitigation strategies are more effective if information on future climate impacts is incorporated into their planning and implementation.
GSC (Geological Survey of Canada) 2001. Weathering the Changes: Climate Change in Ontario. Miscellaneous Report 73.
Intergovernmental Panel on Climate Change: Assessment Reports.
