The overwhelming majority of scientific opinion supports the view that human activities are changing the Earth’s climate. These changes present a serious threat to human society and the natural environment. Without action to reduce greenhouse gas emissions, global temperatures will continue to rise creating risks of abrupt and possibly irreversible changes to our environment.
How does climate change work?
The word climate is used to describe the ‘averaged weather’. That is, the average temperature, pressure, wind speed and variability, precipitation, evaporation – plus seasonal variations of these parameters – over a given area of the Earth’s surface. The Earth’s climate has changed frequently over the long periods of the Earth’s history in response to a number of natural factors. These include the strength of the sun, the shape and tilt of the Earth’s orbit around the sun, the position and shape of the continents and the composition of gases in the Earth’s atmosphere.
The Earth's climate has been relatively stable since the last glaciation which ended about 10,000 years ago, but it is now changing. When the term 'climate change' is used in current contexts, it is often used to describe the shifts in temperature that have occurred over the past 100–200 years or so, and those that are expected the future due to the enhancement of the greenhouse effect.
What is the greenhouse effect?
The ‘greenhouse effect’ occurs naturally (see the box opposite). Without it our planet would be over 30ºC cooler and largely uninhabitable. However, since the industrial revolution human activities such as large-scale agricultural practices, the burning of fossil fuels and deforestation have led to an increase in the atmosphere of long-lived greenhouse gases such as carbon dioxide and methane. Carbon dioxide is the most important greenhouse gas – in the UK it contributed to over 85% of total greenhouse gas emissions in 2004.
How the world is being affected
The average global temperature is rising. The 20th century was probably the warmest century in the past 1000 years. The ten warmest years globally since instrumental records began in 1861 have all occurred since 1994. 1998 was the single warmest year in the 142-year global instrumental record, and 2005 was almost as warm. There is evidence that rainfall patterns are changing, sea levels are rising, glaciers are retreating, arctic sea-ice is thinning and the incidence of extreme weather is increasing in some parts of the world. Without action to reduce greenhouse gas emissions global temperatures are expected to rise by between 1.5 and 6ºC by 2100 and sea level could also rise by between 1 and 2 metres.