What are the causes of temperature changes

Anthropogenic causes

For man-made reasons for the current warming there is not only the exclusionary argument that natural factors do not adequately explain the current climate change. Above all, the spatial and temporal patterns of warming, the 'fingerprint' of the cause of climate change, speak positively for a human influence on the climate. Propulsion by greenhouse gases is expected to warm the troposphere and cool the stratosphere, and near the ground, due to the larger land masses, a somewhat greater warming in the northern hemisphere than in the southern hemisphere. A solar drive, on the other hand, results in a general warming of the entire atmosphere, albeit with a similar warming pattern on the ground as with greenhouse gases.


Fig. 1: Model simulation of the temperature change 1890-1999 in OC per century through solar propulsion (above) and through the propulsion of long-lived greenhouse gases (carbon dioxide, methane, nitrous oxide, CFC)1

Computer model simulations confirm these assumptions. According to model simulations, the change in solar radiation from 1890 to 1999 leads to an almost uniform warming of the atmosphere right into the stratosphere (Fig. 1 above). A minimal cooling can only be seen at the altitude above the poles (because of the lack of radiation in winter), while in the higher latitudes of the northern hemisphere a stronger warming can be seen near the ground due to the ice-albedo feedback.

The warming caused by greenhouse gases 1890-1999 shows a similar pattern in the model near the ground as with the solar cause, but with a significantly higher temperature rise (Fig. 1 below). As with warming by solar influence, the troposphere is heated mainly in the higher latitudes of the northern hemisphere, but also in the central tropical troposphere, because of the ice-albedo feedback. At higher altitudes, however, the warming pattern deviates significantly from that of the solar pattern. In the stratosphere, unlike there, there is a cooling that increases with altitude. This stratospheric cooling has been confirmed by satellite measurements since 1958 (Fig. 2), whereby not only the increasing greenhouse gas concentration in the troposphere but also the decreasing stratospheric ozone is involved.


Fig. 2: Temperature changes in the lower stratosphere from 1958 to June 2007 as a deviation from the mean for the years 1979-19972

In addition to the spatial one, there is also a temporal 'fingerprint', which speaks in favor of warming due to greenhouse gases and not due to the sun. The difference between day and night temperatures (or maximum and minimum temperatures) has been 0.4 over the last 50 years OC decreased in global mean. The reason lies in the greater warming of the night compared to the daytime temperatures (Fig. 3). A higher greenhouse gas concentration reduces the radiation at night. In the case of warming due to higher solar radiation, the opposite result would rather be expected. In addition, there is an increase in cloud cover, which also slows down the radiation at night. The greater cloud cover can be explained both by greenhouse warming, which leads to higher evaporation, and by an increase in (anthropogenic) aerosols, which also increase cloud formation.


Fig. 3: Change in maximum and minimum temperatures 1950 to 2004 relative to 1961-19903

The temperature development in the 20th century can also be viewed as a temporal 'fingerprint', the trend of which corresponds more to the increase in greenhouse gases than to the change in solar radiation, especially with regard to the development of the last few decades. Since the beginning of industrialization, the atmospheric concentration of the greenhouse gases emitted by humans has demonstrably increased significantly. The CO2 concentration has increased by more than a third, the methane concentration by more than double and the nitrous oxide concentration by almost a fifth. This increase changes the radiation budget of the atmosphere. Since 1750, the propulsion from anthropogenic greenhouse gases, which also includes ground-level ozone and CFCs, has been 2.9 W / m2, whereas the solar propulsion at this time is only 0.12 W / m2. However, on the anthropogenic side, the cooling effect of the aerosols of -1.3 W / m2 must be taken into account, so that a net value of 1.6 W / m2 remains.4


Fig. 4: Temperature rise and CO2 concentration in the atmosphere since the middle of the 19th century

However, the increase in greenhouse gas concentration does not provide a sufficient explanation for the course of the temperature increase. The two distinctly different phases of warming in the 20th century are not reflected in the increase in the CO2 concentration in the atmosphere (or in the other anthropogenic greenhouse gases, which show a similar trend) (see Fig. 4). The strong fluctuations from year to year cannot be derived from the increase in greenhouse gases.

First, of course, the temperature development continues to depend on natural internal and external factors. An increase in solar radiation contributed to the rise in temperature between 1910 and 1945. The strong warming in 1998 is partly the result of the internal climatic fluctuation ENSO. And the cool years 1991/92 were caused by the eruption of the Philippine volcano Mt. Pinatubo.

Second, other anthropogenic factors also have an impact on the climate, such as changes in the albedo due to changes in land use and soot deposits and the emission of aerosols, e.g. when burning fossil fuels. The slight decrease in global temperature between the 1940s and 1970s is attributed to anthropogenic aerosols, which have a cooling effect.5 In the period after that, filter measures and the collapse of industry in the former socialist states led to a decrease in aerosol emissions. According to a current calculation, the global emission of sulphate aerosols fell from 73 to 54 TgS per year between 1980 and 2000. The warming caused by greenhouse gases has become more visible because it has been less masked by the effects of aerosols since the 1980s.

Last but not least, climate model simulations provide a weighty argument for humans as the main cause of current climate change. Climate models are used to recalculate the climate of the 20th century and its changes. This means that one starts from the state of the climate at the beginning of the 20th century, calculates its change when entering certain drives and compares the result with the actual climate change determined by observation.


Fig. 5: Model simulations of the global climate in the 20th century without (left) and with (right) consideration of anthropogenic driving forces6

In Fig. 5 the dark blue curve shows the observed temperature change compared to the mean 1901-1950. The green band in the left figure shows model simulations that only take into account natural climate forcing (solar activity and volcanic eruptions) in the 20th century. Especially in the last two to three decades, the increase in global temperature cannot be reproduced in this way. The natural driving factors would even have caused a slight cooling since 1980. If, in addition to natural, anthropogenic driving factors, i.e. man-made greenhouse gas emissions, are taken into account (right figure), the model simulations give a good representation of the observed climate change. This shows that global warming in the 20th century is largely human.

In the meantime, not only have a great number of such model calculations been carried out with always approximately the same results. They were also applied at the continental level and produced the same results there (Fig. 6): Climate models can only correctly calculate the actual temperature changes for the individual continents if they take into account both natural and anthropogenic driving factors.


Fig. 6: Changes in temperature relative to the mean for the years 1901-1950 between 1906 and 2000. Black line: observed; blue band: model calculations only with natural driving factors; red ribbon: model calculations with natural and anthropogenic driving factors7

A current study, which calculates the additional energy input into the climate system from the change in natural and anthropogenic driving factors and from this the temperature change, essentially confirms the previous results.8 The increase in energy between 1850 and 2010 then resulted in a temperature increase of approx. 0.8 ° C. The long-lived greenhouse gases alone caused a temperature increase of 1.31 ° C, the tropospheric ozone and the slightly increased solar radiation of 0.2 ° C each. This total warming of approx. 1.7 ° C was reduced by approx. 0.85 ° C due to anthropogenic aerosols. In addition, there are minor cooling effects due to the decrease in stratospheric ozone and volcanic eruptions. The largest share of the net warming of 0.8 ° C since the beginning of industrialization, namely 0.55 ° C, took place since the 1950s. While the sun still had a certain share of the warming in the first half of the century, the share of natural external forcing has been almost zero since 1950. The sun had a small warming effect of 0.07 ° C since the middle of the 20th century, but this was offset by volcanic eruptions. A maximum of a quarter of the warming since the middle of the 20th century could, however, also be caused by the internal variability of the climate system.

Remarks:
1. modified according to IPCC (2007): Climate Change 2007, Working Group I: The Science of Climate Change, Figure 9.11
2. changed from Met Office Hadley Center (http://hadobs.metoffice.com/hadat/images.html)
3. modified according to IPCC (2007): Climate Change 2007, Working Group I: The Science of Climate Change, Figure 3.2
4. IPCC (2007): Climate Change 2007, Working Group I: The Science of Climate Change, 9.2.1.1
5. IPCC (2007): Climate Change 2007, Working Group I: The Science of Climate Change, 9.4.1.2
6. Fig. Modified according to IPCC (2007): Climate Change 2007, Working Group I: The Science of Climate Change, Technical Summary, Figure TS-23
7. modified according to IPCC (2007): Climate Change 2007, Working Group I: The Science of Climate Change, FAQ 9.2, Figure 1
8. Huber, M., and R. Knutti (2012): Anthropogenic and natural warming inferred from changes in Earth’s energy balance, Nature Geoscience 5, 31-36