What is Climate?
We are all familiar with the topic of weather. Climate and weather are different but related. Weather is the state of the atmosphere at a particular time, for example, today's rain in Reading. Weather changes from day to day and hour to hour. Climate is the average range of weather experienced at a place or in a region over a long period of time, for example, rainfall averaged over 30 years in Reading or in the UK. Commonly used climate quantities include temperature, rainfall, snow and wind averaged over seasons, years, decades, centuries or more.
Our climate is powered by the Sun and modified by the atmosphere, which is made up of various gases. Some of the gases in the atmosphere allow sunlight to pass through, and prevent the heat from escaping back out into space, similar to glass in a greenhouse. This is called the natural greenhouse effect (Figure 1). The gases such as water vapour, carbon dioxide and methane are responsible for this effect and are called greenhouse gases. The Earth's climate is a result of complex processes and interactions between the atmosphere (including the greenhouse effect), oceans, land, ice, plants and animals (including humans).
Figure 1. Without the greenhouse effect the average temperature of the Earth would be -19oC, about 33oC colder than the present average temperature. (Image courtesy, World Meteorological Organization)
What is climate change?
Climate change is an identifiable change in the climate that persists for an extended period, typically decades or longer.
Climate can vary naturally
The Earth's climate is not fixed and in the past has changed many times in response to natural processes such as variations in the Sun's radiation, the Earth's orbit around the Sun, or the composition of the atmosphere due to volcanic eruptions.
Climate can also vary naturally because of interactions between the atmosphere and ocean. The El Nino/La Nina warming and cooling cycles in the tropical Pacific ocean are one of the best known examples of natural climate variability.
Humans are now also affecting global climate
In the early 1900s, our climate started changing due to a persistent increase in the amount of greenhouse gases in the atmosphere. This change has been particularly rapid in the last few decades. For example, the amount of carbon dioxide in the atmosphere has increased by about 35% in the industrial era, and this increase is known to be due to human activities, primarily the combustion of fossil fuels and changes in land use by removal of forests and agriculture.
Humans have hugely altered the chemical composition of the global atmosphere with substantial implications for climate, which is known as man-made climate change. The warming of the planet due to the increased greenhouse effect is known as global warming.
Climate change affects the environment, natural resources, economy and other aspects of life in all nations of the world. In 1988, an international body of climate scientists, the Intergovernmental Panel on Climate Change (IPCC), was established by the United Nations Environment Programme (UNEP) and World Meteorological Organisation (WMO).
The IPCC produces regular reports on the state of climate change research. Members of the panel study all the published research on climate change, and based on all the available evidence, make assessments of how the climate is changing and will change in the future. They also assess the extent to which the changes are human-induced, natural variations or a combination of both factors.
The fourth assessment report concluded that the warming of the climate system is evident. For their work, the IPCC authors were awarded the 2007 Nobel Peace Prize. The fifth IPCC report, produce by scientists around the world, was published in 2014.
Some important facts on climate change from the IPCC Fourth Assessment Report:
- The Earth's surface has warmed by about 0.75°C on average since around 1900 and by around 0.4°C since the 1970s.
- More than 30 billion tonnes of carbon dioxide are emitted globally each year by burning fossil fuels such as coal and oil.
- Average global temperatures may rise between 1.1°C and 6.4°C above 1990 levels by the end of this century.
In addition to observed temperature increases, evidence of recent climate change is also shown by :
- an increase in water vapour in the atmosphere
- continuing sea level rise
- an increase in the heat stored in the oceans
- a major decline in Arctic sea ice volume and summer extent over recent decades
- an increase in extreme weather events like storms, floods, heat waves and droughts
More information on the facts on climate change mentioned above is available at the IPCC Fifth Assessment Report, or via the Other Resources menu on the left