Climate science: The basics →

What is climate change?

‘Climate change’, or ‘man-made climate change’, is a shorthand term for the changes currently happening worldwide as the Earth warms, affecting the atmosphere, oceans, land and nature.

The main cause of modern climate change is believed to be the extra heat trapped in the Earth system by greenhouse gases such as carbon dioxide. And the main reason why greenhouse gas concentrations are rising is the burning of fossil fuels such as oil, coal and gas.

Natural and unnatural

The climate has always changed naturally. For example, over at least the last 800,000 years, the Earth has repeatedly swung between Ice Ages and warmer ‘interglacial’ periods, triggered by small changes in the Earth’s orbit around the Sun. The climate changes naturally across shorter periods too; and on the scale of a decade, natural factors have more influence than greenhouse gases. These natural factors include volcanic eruptions, slight changes in the Sun’s output, and climatic cycles such as the El Nino Southern Oscillation.

Since the dawn of the Industrial Revolution, the influence of human activities, in particular the accumulation of greenhouse gases in the atmosphere, has grown progressively more important. The Intergovernmental Panel on Climate Change (IPCC), the UN organisation charged with assessing scientific evidence, concludes that it is at least 95% probable that human activities are the dominant cause of warming since 1950.

Satellites measure the energy arriving at the Earth from the Sun very accurately. Measuring the energy that the Earth loses back into space is not so easy, but studies conclude that the Earth is receiving more energy from the Sun than it loses. The balance is trapped in the climate system by greenhouse gases, and is driving changes across the planet’s surface and in the ocean.

More than 90% of the extra energy is stored in the ocean. The majority of the remainder goes into melting ice in the Arctic and elsewhere, and heating the land. Only about 1% is stored in the atmosphere.

How does extra heat change our world?

The additional stored energy causes a huge variety of changes across the Earth, some more predictable than others. The most obvious is that the average air temperature at the Earth’s surface is rising – over the last century, it has risen by about 0.8ºC. Water in the ocean and in lakes is also warming. Ice on land and sea is melting. Sea level is rising, mainly because water expands as it gets warmer, but also because melting ice on land is increasing the volume of water in the oceans.

Other changes are harder to detect and forecast. Picking out long-term changes in weather patterns, for example, is difficult because weather varies so much from year to year anyway, and historical records are far from perfect. But there is some evidence that weather patterns are changing, with more heatwaves and stronger rainfall detected in various regions.

The natural world is also changing. Generally, species are moving away from the Equator towards the poles, and/or moving to higher altitude, in order to remain within the climatic conditions to which they are best suited.

These changes are happening unevenly across the world. For example, the Arctic region and the Antarctic Peninsula – the strip of land pointing towards the tip of South America – are warming at least twice as fast as the global average. The sea level is currently rising faster on the western side of the Pacific Ocean than on the east.

Future climate change

Greenhouse gas concentrations are likely to continue rising, and so further climate change is highly likely. But precise forecasts are difficult. Scientists do not understand the climate system perfectly; nor is it possible to forecast how human society will change in terms of population growth, economic development and future use of energy.

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The effect of clouds is one of the largest problems in climate forecasting. Image: NASA

In the main, projections of future climate are made using computer models. These use complex sets of equations to represent processes within the climate system, such as how seawater responds to warming or how plants respond to increasing levels of carbon dioxide.

Some of these processes are well understood, but others are not. For example, clouds can either warm the Earth by providing an insulating ‘blanket’, or cool it by reflecting solar energy back into space. So if a warming climate produces more clouds, that could either amplify the amount of warming or moderate it.

Much of the carbon dioxide emitted from fossil fuel burning, the main cause of man-made climate change, endures in the atmosphere for hundreds of years (with 15-40% staying in the atmosphere for 2000 years). So a certain amount of further climate change is inevitable because of CO2 already emitted into the atmosphere. Also, even if emissions were stopped overnight, it would take many decades for changes to stabilises across the various parts of the climate system, such as the atmosphere, surface ocean and deep ocean (technically, to ‘come into equilibrium’ with each other).

However, in general, the sooner emissions are reduced, the less severe impacts are expected to be, and the lower the risk of abrupt and irreversible change.

The range of computer models used by the IPCC suggests that strong constraints on greenhouse gas emissions – so that they reach a peak within about a decade, and fall to zero around 2070 – would probably limit warming this century to between 0.3ºC and 1.7ºC. However, if emissions continue to rise at the current rate, the warming is forecast to be between 2.6ºC and 4.8ºC by 2100.

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