How are
people changing
the climate?

Basics

1. Man-made climate change?

Where do the emissions come from?

Of all the greenhouse gases we emit, CO₂ is the greatest contributor to the man-made increase in the greenhouse effect. Not only do we emit huge amounts of CO₂, but it also takes a very long time to disappear from the atmosphere after it has been emitted. Since the Industrial Revolution, people have emitted large amounts of CO₂ through the consumption of fossil fuels such as coal, oil, gasoline, and natural gas.

Fossil fuels are the most important

Almost all of the man-made CO₂ emissions come from the consumption of fossil fuels – coal, oil, and natural gas. Fossil fuels are created from plants and animals that died millions of years ago, and lie far underground or under the ocean floor. When we burn these fuels, we release not only energy, but also CO₂.

Although most emissions of CO₂ come from natural sources, such as rotting vegetation, it is the emissions from human activity that tip the scales and create the imbalance that leads to global warming. This is because nature’s own emissions are part of a cycle whereby carbon rotates between the atmosphere, the oceans and land-based vegetation. For example, both rotting trees and the respiration of animals and people emit CO₂, while living plants and trees absorb equivalent amounts of CO₂ through photosynthesis.

When we burn fossil fuels, we add carbon to the atmosphere.  This carbon is in addition to the natural carbon cycle and comes from deposits far underground or under the ocean floor that otherwise would have remained stored for the foreseeable future. This creates a surplus of CO₂ that remains in the atmosphere for a long time.

Deforestation also contributes 

Burning wood or other types of biomass (dead organisms) also gives off considerable amounts of CO₂. If new trees or plants are allowed to grow in place of the forest or vegetation that was burned, then the new plants will absorb just as much CO₂ as was released from the burning. Thus the cycle can come into balance again.

On the other hand, permanent deforestation creates an imbalance and can increase the greenhouse effect in a number of ways. Deforestation means cutting down forests – by logging, using the wood for fuel, or converting the forested area to farmland or grazing areas – without replacing the lost trees with new ones. Regardless of whether the cut trees are burnt or allowed to decompose naturally, they will emit CO₂.  If new trees are not planted then there is nothing to absorb the carbon released from the trees that were cut down and this increases the amount of the CO₂ in the atmosphere.

Methane is also a culprit

While CO₂ is the most important greenhouse gas – mainly because there is so much of it – it is not the only greenhouse gas we need to worry about. Methane (CH₄) is also a very powerful greenhouse gas, although it has a much shorter lifetime in the atmosphere than CO₂. Like CO₂, methane comes from both natural and man-made sources. Natural sources include rotting wood and wetlands, while human activities that emit methane include agriculture and waste deposits.  Other greenhouse gases also contribute to man made climate change. They include nitrous oxide (N₂O) and several synthetic compounds from industry containing fluorine.

Rich countries emit the most

The biggest emitters of greenhouse gases are rich, industrialised, countries like the United States of America and those in the European Union. The emissions per person in a rich country are several times greater than the emissions per person in a poor country (see Figure 4). However, as the poorest countries develop and people's standard of living increases, emissions of greenhouse gases are likely to increase greatly.  China already has the second highest emissions per person and emissions from India have increased by over 50% since 1990. There is nevertheless far to go before the emissions from developing countries catch up with those from the rich countries.

This was the last page in this unit. 
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About this page:

Author: Camilla Schreiner - CICERO (Centre for International Climate and Environmental Research - Oslo) - Norway. 
Scientific reviewers: Andreas Tjernshaugen - CICERO, Norway - 2004-01-20 and Dr. Knut Alfsen - Statistics Norway, Norway - 2003-09-12. 
Educational reviewer: Nina Arnesen - Marienlyst School, Oslo, Norway - 2004-03-10. 
Last update: 2004-03-27.