The known facts
1. Carbon dioxide (CO2) is a greenhouse gas, as are methane, water vapour, nitrous oxide and ozone. The fundamental physics that links CO2 concentrations to temperature changes has been known since the late 19th century.
2. Earth receives energy from the Sun and re-emits it back to space. The atmosphere traps some of the outgoing energy and warms Earth. This is known as the greenhouse effect. Without this natural greenhouse effect, Earth would be some 30 °C colder, and life as we know it would not be possible.
3. CO2 concentrations in the atmosphere are rising. They have increased by about 38% since industrialisation began, from 280 ppm (parts per million) to 387 ppm. Two-thirds of that increase has occurred in the last 50 years. CO2 levels are now 30% higher than at any time over at least the last 800,000 years. (Click on image to see larger version)
4. Human activities from energy production, transport, industry and agriculture have led to an increase in greenhouse gases in the atmosphere, especially CO2. About half the carbon dioxide emitted in this way has remained in the atmosphere with much of the rest being absorbed into the land and ocean. The increase in CO2 concentrations traps more of the outgoing energy, causing an enhanced greenhouse effect, and increases the heating of Earth.
5. Earth is warming. Over the last 100 years Earth has warmed by about 0.75 °C. The last decade was the warmest on record despite a major La Niña event, which temporarily cools the global climate system. (Click on images to see larger versions)
6. The scientific consensus states that it is very likely that most of the warming over the last 50 years is a result of greenhouse gas emissions caused by human activity.
7. Increasing greenhouse gas concentrations will mean an upward trend in temperatures. However, natural variability in the climate means that not every year will necessarily be warmer than the last, with some years being cooler and others warmer. |
