Carbon dioxide and the greenhouse effect The atmosphere is becoming more enriched in carbon dioxide (CO2), or to be more precise, carbon dioxide and other climate-relevant trace gases. Initially they allow the incoming short-wave radiation of the sun to pass through. This energy is transformed to heat at the Earth’s surface and is then emitted back as long-wave radiation. The gases in the atmosphere, like the glass panes of a greenhouse, prevent this long-wave radiation from escaping into space, and the Earth‘s surface warms up.
How humans are changing the climateThe human impact on climate has greatly increased over the past hundred years. We release vast amounts of climate-relevant trace gases into the atmosphere. This changes the radiation balance of the atmosphere and leads to global warming. In addition to carbon dioxide, these trace gases include methane, nitrous oxide (laughing gas), halogenated fluorocarbons, perfluorinated hydrocarbons, and sulphur hexafluoride. But carbon dioxide (CO2) is especially important for the Earth’s climate system because the worldwide output is so enormous. It is released primarily through the burning of fossil fuels (oil, natural gas, and coal) in power plants, vehicle engines or in household heating systems. Its atmospheric levels have risen to almost 390 parts per million (ppm) today as compared to the pre-industrial value of 280 ppm. With this increase the temperature has also risen during the twentieth century. The internally driven changes in the oceans such as changes in the Gulf Stream also occur within a time frame of decades or a few centuries. These have a decisive influence on climate and on the concentration of greenhouse gases in the atmosphere because they are strongly involved in global mass cycles such as the carbon cycle. For example, CO2 dissolves easily in water. However, the oceans have taken up about half of all the carbon dioxide produced by the burning of fossil fuels since the beginning of the industrial revolution, which has clearly dominated the natural variations. Whether the climate will change in the future, and by how much, can therefore be also deduced from the oceans.
- Climate will change very slowly in the future because the oceans with their immense volumes of water react very gradually to change. Therefore, many but not all of the consequences of climate change triggered by human activity will only gradually become noticeable. Some of these consequences could actually be irreversible when certain thresholds are crossed. At some point it will no longer be possible, for instance, to stop the complete melting of the Greenland ice sheet and the resulting seven-meter rise of sea level. The position of the threshold, however, is not precisely known. But one thing is certain: Even if the emission of carbon dioxide were stabilized at today’s levels it would not lead to a stabilization of the carbon dioxide concentration in the atmosphere, because carbon dioxide is extremely long-lived and the carbon dioxide sinks, mainly the oceans, do not absorb it as quickly as we produce it. The situation is different for short-lived trace gases like methane (CH4). If methane emissions were stabilized at the present level, the methane concentration in the atmosphere would also stabilize, because methane diminishes in the atmosphere at about the same rate as it is emitted. In order to maintain the carbon dioxide concentration at a given level, the emissions have to be reduced to a fraction of the present amounts.
- 1.4 > Even if it is possible to significantly reduce the emission of greenhouse gases, and CO2 in particular, by the end of this century, the impact will still be extensive. CO2 has a long life and remains in the atmosphere for many centuries. Because of this, the temperature on the Earth will continue to rise by a few tenths of a degree for a century or longer. Because heat penetrates very slowly into the ocean depths, the water also expands slowly and sea level will continue to rise gradually over a long period of time. Melting of the large continental ice sheets in the Antarctic and Greenland is also a very gradual process. Melt water from these will flow into the ocean for centuries or even millennia, causing sea level to continue to rise. The figure illustrates the principle of stabilization at arbitrary levels of CO2 between 450 and 1000 parts per million (ppm), and therefore does not show any units on the response axis
A looming catastropheLong after the stabilization of carbon dioxide levels, the climate will still further continue to change because of its inertia. Climate models indicate that the near-surface air temperature will rise for at least a hundred years. Sea level will continue to rise for several centuries because seawater expands slowly as a result of the gradual warming of the deep sea, and because the continental ice sheets in the Arctic and Antarctic will probably react very slowly to the warming of the atmosphere, and the glaciers will continue to melt for many millennia. It will therefore be a long time before sea level achieves a new equilibrium. But scientists also believe it is possible that, if the warming is strong, the Greenland ice sheet could completely melt within this millennium and disappear into the ocean. The ice sheet could actually break apart and giant pieces fall into the sea. The enormous amounts of fresh water could cause a critical change in ocean circulation, for example, in the Gulf Stream. In an extreme scenario, sea level could rise by more than a metre per century, regionally by even more The inertia of the climate system and the danger that the trend is irreversible should be sufficient reasons for forward-looking action. One should always keep in mind that the impacts of climate change that are measurable today do not yet reflect the total extent of climate change already caused by humans in the past. Humankind will only begin to feel them sharply in a few decades but has to take action right away.