What are the causes of climate change?
The Earth’s climate is a complex system of interactions among various components, including the atmosphere, biosphere, land masses, oceans, and ice masses. The individual components are continuously exchanging energy, while circulation patterns in the atmosphere and oceans are constantly changing. Due to the internal heat redistribution of this nature, short-term fluctuations on timescales of months to centuries occur naturally.
As a result of the greenhouse effect compounded by human activity, there is additional energy throughout the Earth’s entire climate system. This excess energy is distributed from the atmosphere to the other parts of the climate system. Only around one percent of the surplus energy remains in the Earth’s atmosphere, while some 93 percent enters the world’s oceans.¹ If, for example, the temperature of the atmosphere plateaus or falls (which happens from time to time, including over a period of several years) and the temperature of the oceans rises at the same time, the climate system as a whole still continues to warm up. Ocean heat content is therefore a better indicator of global warming than air temperature, which is subject to heavy, short-term fluctuations.
Due to interactions within the climate system, the global mean temperature fluctuations are usually only a few tenths of a degree. These short-term fluctuations as well as natural external forces that impact the climate overlap with the long-term warming trend resulting from greenhouse gases attributable to human activity. The curve of the global mean temperature is therefore a rising zigzag line.
The climate has changed many times over the millions of years of Earth’s history, and scientists have largely determined the main causes. Geological warm and cold periods were primarily triggered by changes in the Earth’s orbit around the sun and by continental drift. However, the resulting changes in global temperature are extremely slow compared to the warming we are currently seeing – Earth’s shortest orbital cycle is 23,000 years.
Scientists gain insight into what the climate was like in the past (a branch of research called paleoclimatology) by looking at natural climate archives, such as sediment deposits at the bottom of oceans and lakes. Drilling on Greenland and Antarctica has yielded ice samples containing atmospheric air bubbles up to 800,000 years old. These clues make it possible to reconstruct the concentrations of greenhouse gases in the atmosphere and the temperatures on Earth far into the past. One of the things that scientists have discovered is that the geological climate fluctuations can only be explained if the greenhouse effect is taken into account as well. Tree rings and corals, for example, provide information about the more recent past.²
Through a wide range of research and studies, scientists have been able to exclude natural causes as drivers of the very rapid and steep rise in temperature since the onset of industrialization. It can only be explained by an intensification of the greenhouse effect owing to human activity.³
² vgl. z.B. https://wiki.bildungsserver.de/klimawandel/index.php/Proxydaten; IPCC 2013, AR5, WG1, Kap. 5 – https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter05_FINAL…
³ IPCC 2014, AR5, SYR, SPM, SPM1.2 – https://www.de-ipcc.de/media/content/IPCC-AR5_SYR_barrierefrei.pdf