IGCSE Biology / Human influences on the environment

Greenhouse gases and global warming

Edexcel International GCSE Biology (4BI1), chapter 15: Human influences on the environment, specification point 4.13, 4.14, 4.15. Free lesson.

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The greenhouse effect keeps Earth warm because greenhouse gases absorb some of the infrared energy leaving its surface. Human activities add more greenhouse gases, strengthening this natural effect and contributing to global warming.

Which gases trap heat, and where do they come from?

Five gases are named as greenhouse gases. Water vapour is the most abundant and is almost entirely natural. Carbon dioxide and methane are especially important because human activities have increased them. Methane is present in much smaller amounts than carbon dioxide, but each molecule traps more heat. Water vapour comes from evaporation from oceans, lakes and rivers. Carbon dioxide comes from burning fossil fuels and deforestation. Deforestation releases carbon and removes trees that would have absorbed carbon dioxide. Methane comes from cattle and other livestock,…

How the natural greenhouse effect works

The key difference is the wavelength, or type, of energy coming in and going out. Sunlight reaches the surface as short-wavelength radiation. The warmed surface gives energy back as longer-wavelength infrared radiation. Greenhouse gases absorb some of this outgoing energy and send some back towards the surface, so less escapes to space. The natural greenhouse effect keeps Earth about 33 °C warmer than it would otherwise be. It is necessary; the problem is the enhanced effect caused by extra gases.

What the carbon dioxide graph shows

Atmospheric carbon dioxide rose from about 287 ppm in 1860 to about 414 ppm today, with the rise steepening from around 1980. Over the same period, mean global temperature rose by roughly 1 °C. These changes show a correlation, meaning they happened together. Correlation is consistent with one change causing another, but the graph alone does not prove cause. To describe a graph trend, include values and the time period. Saying only that carbon dioxide went up is not enough. Carbon dioxide and temperature need separate axes if shown together, because their values and units are…

Consequences of global warming

The enhanced greenhouse effect contributes to a warmer climate. This can affect sea levels, rainfall and the places where species can survive. Sea levels rise because ice melts and warmer water expands. Rainfall patterns change, affecting where and when rain falls. Species may shift to new areas or die out if they cannot adapt.

Keep ozone depletion separate

Ozone depletion and the enhanced greenhouse effect are different environmental problems. Greenhouse gases absorb infrared radiation emitted by Earth, so less energy escapes to space. Ozone depletion lets more ultraviolet radiation through, which can cause skin cancer. Mixing up their causes and effects loses the greenhouse-effect explanation.

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