Climate change
The climate of a given region is defined by the statistical characteristics of several variables such as temperature, precipitation, wind, snow, etc. These statistics are based on at least three decades’ worth of observed or simulated data.
Climate change is any lasting difference in one or more of these statistical characteristics when a comparison is made between the climate of a future period and that of a historical reference period, for one or more variables in a given region. Climate change can consist of a change in the average or variability, as well as in the frequency, intensity or duration of extreme events.
Throughout Earth’s history, many climate changes have been caused by a combination of phenomena intrinsic to the planet, such as changes in its orbit, continental drift and variations in volcanic activity, and external phenomena such as changes in solar intensity or meteorite impacts. These changes are natural and will continue to occur.
Today, however, what we mean by climate change refers to changes in the characteristics of the climate induced by human activity, such as emissions of greenhouse gases and airborne particles (aerosols). They are mainly due to the increased use of fossil fuels since the start of the Industrial Revolution in the 19th century. Anthropogenic climate change is adding to the changes of natural origin, but is taking place at much faster time scales than anything the planet has ever experienced.
Using numerous climate model simulations, future climate change is estimated by comparing a projected climate for a future period (e.g. 2041-2060, or 2081-2100, as chosen for the 6th IPCC Report) with a climate of the past (e.g. 1995-2014, used in the 6th IPCC Report). By its very nature, the climate is highly variable, as it results from the combination of several phenomena with lifetimes or cycles covering a wide range of time scales, which may also interact with each other. For example, thunderstorms last a few hours, depressions a few days, monsoons a few weeks, El Niño-La Niña a few years, and fluctuations in ocean currents from a few decades to a few centuries. Since natural climate variability arises from fluctuations over the course of years or even decades, it’s important to consider periods of at least 20 years (like the IPCC) to 30 years (like the WMO) to detect real changes, and ideally 30 to 50 years to properly assess long-term trends.
Climate change alone cannot explain an intense weather event that occurs over the course of a few days or even weeks (e.g. a severe storm, an icy spell, a drought, etc.). What it’s responsible for are changes in the probability of a particular type of weather event of this intensity occurring in a given region. For example, an event of 75 mm of rainfall in 24 hours, which normally occurs once every 25 years, could increase in the future to once every 10 years. But recently, thanks to the advent of attribution techniques, it has become possible to establish whether climate change contributed to the magnitude of an extreme event. It has been demonstrated, with a high confidence level, that several recent extreme events would not have reached such intensities if it weren’t for climate change.
Despite the expected overall increase in temperatures, an abnormally cold season will still occur from time to time in the future. This is simply the result of natural climate variability and in no way calls the existence of climate change into question. Unfortunately, it’s feared that the speed and intensity of human-induced climate change will exceed the adaptive capacity of many natural and human systems.
