Climate Variability and Climate Change
Climate variability can be explained by the deviation in the mean and other statistics of the climate for any temporal and spatial extent, excluding odd weather events. In the common parlance of human induced ‘Climate Change’ studies, ‘Climate Variability’ often talks about anomalous climate behaviour within shorter timescales (e.g. a month, season or year) (WMO, 2019). Thus, it can be assessed by observed variations for the same calendar period compared to the long-term climate statistics (Figure 1).
Figure 1. Sample graph illustrating a difference between Climate Variability and Climate Change.
Variability can be categorized as an internal variability caused by internal natural processes or an external variability caused by outside natural or man-made factors. El Nino Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) can be considered as an example of internal climate variability, while short term cooling of climate due to a volcanic eruption (e.g. the eruption of Mount Pinatubo in the Philippines, June 15, 1991) can be considered as an example of external climate variability (NASA, 2000).
Climate change:
Climate change can be defined as a measurable shift in the mean state of the climate or in its variability, which can prevail for a prolonged period of time, may be for decades or longer.
Sometimes, an unprecedented event or a series of events occur, but it does not happen again in the next 30 years, then, we cannot take such events as an indication of climate change; rather we could term it as an exceptional season. Therefore, only a steady period (i.e. long term trend, as shown in Figure 1) of anomalous climate behavior with regards to local climatic parameters (such as temperature, precipitation, sea surface temperature etc.) can indicate a significant change in climate has happened. One could claim the persistence of anomalous behavior, when a rare event happens quite frequently, or vice versa. Thus, any unusual single event, such as a severe typhoon, can’t be ascribed to climate change. Changing Climate can be better understood by employing various statistical tools such as a frequency distribution curve depicting the probability of occurrence of specific weather events. Under changing climate scenarios, the curve gets altered in amplitude and/or moved to a new mean (Figure 2). We could conclude from such statistical observations that rare and extreme events would become more frequent and more severe. For example, under the changing climate scenarios some regions around the world would experience more intense rain events, and fewer light and moderate rains.
Figure 2. Severe weather events which were rare before become more frequent due to changing climate.
The United Nations Framework Convention on Climate Change (UNFCCC) attributes ‘Climate Change’ to human induced changes to the composition of global atmosphere, and which is in excess to the climate variability led by natural causes (UNFCCC, 1992). Thus, UNFCCC differentiates between ‘Climate Change’ caused by humans, and the ‘Climate Variability’ caused by natural factors. Through its series of assessment reports, Intergovernmental Panel on Climate Change (IPCC) is determining the anthropogenic evidences of climate change causing several climate hazards such as droughts, flash floods, hurricanes etc. (IPCC, 2019).
References
WMO (World Meteorological Organization). Frequently Asked Questions: Climate. Retrieved September 30, 2019. https://public.wmo.int/en/about-us/FAQs/faqs-climate
IPCC (Intergovernmental Panel on Climate Change). Reports. Retrieved September 30, 2019. https://www.ipcc.ch/reports/
UNFCCC (United Nations Framework Convention on Climate Change). Article 1 (Definitions). Retrieved September 30, 2019. https://unfccc.int/files/essential_background/background_publications_htmlpdf/application/pdf/conveng.pdf
NASA Earth Observatory. (2000, September). Articles: Volcanoes and Climate Change. Retrieved September 30, 2019. https://earthobservatory.nasa.gov/features/Volcano
