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This content is taken from the University of Reading's online course, Our Changing Climate: Past, Present and Future. Join the course to learn more.

Skip to 0 minutes and 8 seconds I’m Richard Allan. I’m professor of climate science in the meteorology department at the University of Reading. Climate has always been changing. Over billions of years, the sun has gradually become brighter, and the atmosphere of the earth has evolved in symbiosis with life. Over hundreds of millions of years, the continents and the positions of the oceans have moved over time. And that’s led to vast swings in climate over these very long time scales. The last few million years, there’s been other changes in climate that relate to regular and predictable cycles in Earth’s orbit around the sun. These initiate glacial periods where ice sheets extend across northern hemisphere like North America and Eurasia, and milder interglacial interludes that we’re currently enjoying today.

Skip to 0 minutes and 57 seconds We’ve progressed from last glacial period. The last glacial maximum of which was around 20,000 years ago. And since then, climate has been changing in response to changes in the brightness of the sun to volcanic eruptions and also to very slow fluctuations in the ocean. There’s a vast spectrum of changes that affect climate. More recently, human societies and their activities have actually contributed to a change in our climate, for example through emissions of greenhouse gases like carbon dioxide, which are gradually heating our planet up. Earth’s climate depends on a balance between the absorbed sunlight it receives and the continuous cooling to space through infrared radiative emission.

Skip to 1 minute and 44 seconds Warmer bodies emit more of this radiation, so in terms of the earth, it actually receives a certain amount of radiation from the sun. At a particular temperature, it can exactly balance the amount of radiation it receives through infrared emission to space. And this balance is actually attained at a global average temperature of around 15 degrees Celsius. Our planet rotates each day about a line, which passes through each pole. This line of rotation is tilted at around 23 and 1/2 degrees, relative to the orbit of the earth around the sun. In June currently, the northern hemisphere including North America, Europe, and most of Asia is tilted towards the sun, so it receives more sunlight.

Skip to 2 minutes and 29 seconds Whereas in December, the northern hemisphere is tilted away from the sun, and so less sunlight is received. So this gives us our seasons. The warm tropical regions receive more sunlight than the cold polar regions. This uneven distribution of heating across the planet combined with the position of the continents, the mountains, and the oceans, and also combining with the rotation of the planet each day, conspires to produce an atmospheric and oceanic circulation. All these factors combine to produce our current climate and to produce our current climate zones, which include the warm tropical rain forest belts, the dry desert regions, and also the more temperate climates we have in middle latitudes. Our climate is extremely complex.

Skip to 3 minutes and 18 seconds Throughout the rest of the course, we’ll be learning about how we know about past climate through observations, how we use physics and bring those together with observations to build simulations of Earth’s climate, and to make projections about how we think climate will change in the future. And crucially, we’re going to look at what are the implications for water resources, cities, and life of altering Earth’s energy balance through emissions of greenhouse gases such as carbon dioxide.

The climate system

In this video, Professor Richard Allan looks at how Earth’s climate has changed naturally in the last million years. He will cover factors such as the Sun’s radiation, the Earth’s energy balance and an overview of how the climate system works.


We recommend that you keep a link to this IPCC glossary to hand as you go through the course. We will include a link to the glossary at the start of each week.

Glossary Source: IPCC, 2013: Annex III: Glossary [Planton, S. (ed.)]. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Further reading

Throughout the course there will be a series of optional reading resources. Although it might help further your knowledge on the subject of climate change, don’t feel obliged to read through all of them.

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This video is from the free online course:

Our Changing Climate: Past, Present and Future

University of Reading