The Sun’s energy is absorbed by the Earth’s surface and atmosphere, transferring the energy into the climate system. Because the Earth is (roughly) spherical, solar energy isn’t distributed evenly around the globe. At higher latitudes, sunlight hits the surface at progressively more oblique angles, so that the same amount of light covers a larger area. At the Equator, sunlight hits the surface almost at a right angle, so the light is more intense. It also travels a shorter distance through the atmosphere before reaching the ground, so less is absorbed by atmospheric gases and aerosols. The result is that less solar energy is absorbed per square metre at higher latitudes than near the Equator, creating a temperature gradient between the Equator and the poles.

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• The tropical convection cells
• Tropical rainfall
• The effect of a reduction in sunlight on regional climates

The Earth’s axis is slightly tilted, with its angle fixed in space. This causes changes in the amount of sunlight received by certain areas of the planet throughout the year. In one half of the Earth’s orbit, from late March to late September, the North Pole is tilted towards the Sun. This results in longer hours of sunlight and higher temperatures in the northern hemisphere, reaching a maximum in late June at the summer solstice. In the other half of the orbit, the North Pole is tilted away from the Sun, causing the northern hemisphere’s winter and the southern hemisphere’s summer. Seasonal changes are more pronounced at higher latitudes than in Equatorial regions, with the poles going from 24-hour daylight in summer to permanent darkness in winter.

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• Response of the climate to seasonal sunlight variations

Water has a higher heat capacity than air, so the ocean surface can absorb a relatively large amount of energy before changing temperature. The oceans also contain a huge volume of water and extend to a global average depth of more than 3 km, so energy can be gradually transferred from the surface to the deeper layers. As a result, the oceans absorb and release heat at a slower rate than the land and atmosphere. This effect is known as high thermal inertia and has a moderating influence on the climate. For example, the oceans absorb heat from the more direct summer sunlight and warmer summer air and then release the stored energy into the cooler winter air, making seasonal temperature variations less pronounced than they would otherwise be.

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• Land, ocean and monsoons
• Different rates of warming around the world
• Increasing ocean heat energy

About half the sunlight reaching the Earth is absorbed by the planet’s surface, of which about 30% is covered by land and 70% by ocean. In response to the absorption of solar energy, the land surface changes temperature at a faster rate than the ocean surface. This is partly because water has a higher heat capacity than land and partly because the ocean waters can distribute energy to deeper layers. Most of the Earth’s landmasses are located in the northern hemisphere, with a greater expanse of ocean in the southern hemisphere. This causes the northern hemisphere to warm and cool at a faster rate, leading to more pronounced seasonal temperature variations than in the southern hemisphere.

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• The effect of continental positions on climate circulation
• Different rates of warming around the world

The temperature difference between the Equator and the poles drives large-scale circulation in the atmosphere and oceans. Warm air rises at the Equator and travels to higher latitudes, carrying energy around the climate system. The resulting circulation patterns are determined partly by the Earth’s rotation and the position of the continents and mountains. But overall, heat is transported northward and southward from the Equatorial regions, keeping the mid and high latitudes warmer than they would otherwise be. Close to the Equator, most heat is carried by the oceans, but north and south of about 30 degrees latitude the atmosphere is responsible for most of the poleward heat transport.

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• The tropical convection cells
• Ocean currents

The transfer of energy from the oceans to the atmosphere occurs primarily through the movement of moisture. Sunlight heats the oceans, causing water to evaporate from the surface. Molecules are more energetic in gas form than in liquid form, so the conversion of liquid water to water vapour requires energy, known as ‘latent heat’. Evaporation therefore transfers energy to the air above, cooling the ocean surface in the process. As the water vapour rises, it cools and eventually condenses into clouds. When the vapour turns back into liquid water droplets, it releases the latent heat into the surrounding air. These exchanges of energy between the oceans and atmosphere drive winds and atmospheric circulation.

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• Temperature, kinetic energy and condensation

Prof Jo Haigh’s work looks into how solar activity influences climate. ‘How variations in the Sun’s activity affect the Earth’s climate is important as it helps us to understand the role of natural factors in global climate change,’ says Jo. By using data taken from satellites observing the Sun and feeding them into computer models, Jo has focused her work on how solar ultraviolet radiation can influence the Earth’s atmosphere. Working at the cutting edge of climate science doesn’t come without its challenges, as Jo has had to overcome time and technical constraints as well as a grilling from Jeremy Paxman on Newsnight.

• esphere: Global Atmospheric Circulation
• NASA: The Earth’s Energy Budget
• World Meteorological Society