Climate Change Knowledge in a Nutshell
1. Burning fossil fuels, deforestation and other human activities have released large amounts of greenhouse gases. The concentration of carbon dioxide in the air has risen from the pre-industrial level of 280 parts per million (0.028 percent, ppm means “parts per million”) to more than 380 ppm today. That is the highest level since at least 800,000 years and is forcing a massive anthropogenic, “human made”, greenhouse effect on Earth. Global mean temperature has already risen by about 0.8 degrees Celsius (1.5 degrees Fahrenheit) since 1900.
2. If business continues as usual, the world could heat up by about five degrees Celsius (nine degrees Fahrenheit) by 2100. This is roughly the temperature difference between an ice age and a planetary warm phase. Since Earth’s climate is currently in a warm phase humanity would thereby create a “fire age”.
3. Global warming leads to the thermal expansion of sea water and increases melt-water runoff from glaciers and ice sheets. Sea level has risen by about 15 to 20 centimetres during last century; another 50 to 150 centimetres are expected this century. Continued warming could destabilize the Greenland and Antarctic ice sheets. In Earth’s history sea level rose by 10 to 15 metres with each degree of global warming. It is not yet clear though, how much time that process took.
4. The anthropogenic greenhouse effect could push the Earth’s climate system past critical thresholds, so that important components may “tip” into qualitatively different modes of operation. This would affect climate on a sub-continental scale and could cause the complete disappearance of Arctic sea-ice and Himalayan glaciers or large-scale dieback of the Amazon rainforest. If the climate system started to release greenhouse gases, positive feedbacks could lead to runaway greenhouse dynamics.
5. According to current knowledge, the most dangerous impacts of climate change could be averted, if anthropogenic warming was limited to two degrees Celsius (3.6 degrees Fahrenheit). This requires confining the concentrations of all greenhouse gases and their effect to a level that does not exceed that of 450 ppm carbon dioxide. To reach the two degrees target, global emissions of greenhouse gases have to be reduced by 2050 to about half of the level of 1990.
6. If the potential for technological and institutional innovation were fully tapped, the measures implicated by the two degrees target would reduce global economic output by about one percent until 2100. Given a mean global economic growth of three percent, the delay would be offset in about four months.
7. Adaptation to climate change and the confinement of global warming to two degrees Celsius require a “Great Transformation” of the global economy and of urban and rural life.