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The discovery of human influence on the climate

Full moon partially obscured by atmosphere © Nasa viaWikimedia CommonsIt was as early as 1824 that a researcher realised that the climate is determined by the trace gases in the atmosphere. Human influence was mentioned for the first time 80 years later. Admittedly they still saw no need to take action at that point. Quite the opposite, global warming was seen as a positive thing. It was only gradually that scientists came to understand the influence of people and the effects on the climate.

Full Moon partially obscured by the atmosphere of Earth © Nasa via Wikimedia Commons


The European Union has set itself the target of limiting the temperature increase resulting from the anthropogenic greenhouse effect to two degrees Celsius in comparison with pre-industrial figures. This target was first recognised in international climate negotiations in December 2010 in the Mexican city of Cancún. One year later in December 2011 in Durban, South Africa, the nations agreed on a timetable according to which a new climate protection treaty was to be adopted by 2015 at the latest. After controversial discussions the Kyoto Protocol was initially extended to the end of 2020 at the 2012 UN climate conference in Qatar. The new Kyoto period up to 2020 started at the turn of the year 2012/2013. In 2014 the intention is to reassess the defined emission targets and improve on them if possible.

As an increase in the global average surface temperature of approx. 0.8 degrees Celsius had already occurred by 2011, this continues to be a very challenging target that seems only achievable with extremely comprehensive measures: the increase in global greenhouse gas emissions should be stopped by 2020 and then emissions should be lowered with a trend reversal. This target, and – if it is taken seriously its implementation – illustrates that climate protection will be the dominant political theme of the 21st century.

The history of the discovery of climate change outlines how the understanding of the anthropogenic greenhouse effect has developed. The focus here is on scientific aspects, not on social and political perception. With the early theoretical predictions as a starting point, the observation of the initial indicators up to the point of the evidence that man has become the key climate factor in the industrial era are being considered in an exemplary approach.

Theoretical predictions

Svante Arrhenius, Swedish physicist and Nobel laureate for chemistry The processes in the atmosphere had already been described at a very early point. In this context, Jean Baptiste Fourier was already working on the natural greenhouse effect and the role of trace gases in 1824. In the 1860s John Tyndall was focusing on all aspects of the effect of natural greenhouse gases, especially water vapour. People were emitting carbon dioxide for the Earth’s climate. Svante Arrhenius, Swedish physicist and winner of the Nobel Prize for Chemistry, was the first to carry out detailed calculations for the natural greenhouse effect. In 1895 he was also the first to realise the significance of the carbon dioxide emitted by humans for the Earth’s climate, and calculated that in the event of the carbon dioxide content of the atmosphere doubling, there would be an increase in temperature of four to six degrees Celsius as a result. However his reasoning with regard to the effects was simplistic and optimistic: he derived more positive perspectives from the greenhouse effect and concluded: “The increase in CO2 will allow future generations to live under warmer skies.”

Then in 1941 German meteorologist and climatologist Hermann Flohn described “human activity as a climate factor” in the “Zeitschrift für Erdkunde” geography magazine. His reasoning was significantly more cautious: “But that means that human activity becomes a cause of a climate change that spans the Earth, the future significance of which no one can guess.” In the post-war era, Flohn became a pioneer of national and international climate research. As a focus of his scientific work he researched the dynamics of the climate system; in the 1970s he published diverse material on the influence of humans on the climate.

Signs that climate change is caused by human action

The first time human influence on the climate was shown clearly was the data from a monitoring station on Mauna Loa, Hawaii. Implemented by chemist Charles Keeling, carbon dioxide levels in the atmosphere were continuously recorded there from 1958 onwards using direct measurements. Based on the theoretical explanation of the influence of carbon dioxide on the climate by Arrhenius, the Swedish researcher, the idea was to find out how much carbon dioxide remains in the atmosphere. Even after a short time it was possible to obtain unexpected results. They show that carbon dioxide concentrations have been constantly on the increase since measurements began.

The annual increase in the vegetation cycles was superimposed on this: from May to October the figures drop because trees and other plants in the Northern hemisphere are in the growth phase and absorb carbon dioxide when the leaves shoot. From October levels rise again because one year-old plants die off and leaves fall. The rotting processes release carbon dioxide again. But this curve reaches a new peak in May each year. The first measurements for March 1958 showed an average level of 316 ppm. This means that for every one million air molecules there are 316 molecules of carbon dioxide. Until today – 50 years later – the figure has been increasing continually as high as 386 ppm. Indirect procedures, such as the analysis of air pockets trapped in ice cores, have been used to determine that the pre-industrial level was 280 ppm. That is equivalent to a 32 per cent increase over a good 250 years. A drastic change at a speed never before seen in the history of the Earth.

In addition to signs of human influence on the climate from measurements showing the composition of the atmosphere, there are also already visible indicators in the effects of the anthropogenic greenhouse effect. If you consider the development of the average global surface temperature (see illustration), then this shows a clear trend too: the average global temperatures have been increasing significantly since 1860 in comparison with the reference period from 1961 to 1990.

Within this period the most significant proportion has been observed since 1980. The feeling that the summers are becoming increasingly hot is backed up by these observations. For instance the average temperatures in large parts of Western and Central Europe in summer 2003 were more than three degrees above known maximum levels. Eleven of the twelve hottest summers since measurements began fell within the period of 1995 to 2006. The decade from 1990 to 1999 was the hottest of the last 1000 years. 2005 was the warmest year since the start of temperature measurement, with the most hurricanes so far and the lowest Arctic ice extent.

The fact that the global mean temperature has not continuously increased but has varied is not an argument against the anthropogenic influence. The extent of variation illustrates that there are many influential factors, including significant natural factors. It is also becoming equally apparent that there are geographical differences and the effects of climate change are not equally strong everywhere. For instance warming over the continents is significantly more noticeable than over the seas.

Nowadays we see many more indicators of the effects of the anthropogenic greenhouse effect. From the reduction in the snow and ice cover in the Northern hemisphere by ten per cent since 1960, the accelerated melting of glaciers, the rise in sea levels by 10 – 20 centimetres in the 20th century, to the increase in extreme weather events and the intensity of tropical hurricanes. It should be assumed that the effects on the environment, health, the economy and safety are still not completely foreseeable.

The scientific evidence

The fact that man influences the climate and that climate change is already occurring is undisputed in climate research today. However the precise extent of this influence – as well as the expected severity and regional distribution of the effects – is still a matter of discussion. Two factors have contributed to this consensus, on the one hand there have been continuous improvements to the climate model and outstanding methodical innovations, and also the intensive scientific debate conducted through the Intergovernmental Panel on Climate Change.

Fingerprint from 1995

There are plenty of indicators to suggest that human influence on climate exists, and that it is serious. However simply looking at indicators is often not enough to provide scientific substantiation for the necessary preventative action and legitimise it in a political sense from the basis of a precautionary idea. Since the chances of the effects of climate change becoming less severe are dependent on how early the trend is turned around, it is crucial to find clear evidence at an early stage. There was some success with this evidence in 1995 with the so-called fingerprint method. Using a combination of observation and model simulations the method showed a 95 per cent probability that the rise in ground-level air temperatures, as observed in 1964 to 1994, cannot be put down to the natural variability of the climate. Quite the opposite – it means that the influence of anthropogenic greenhouse gas emissions is dominant.

IPCC progress reports

In reports issued by the Intergovernmental Panel on Climate Change (IPCC), which was set up in 1988 as an advisory body for the WMO international climate talks and the United Nations Environmental Programme (UNEP), the state of knowledge of climate research is presented in detail every four or five years. Furthermore it is summarised and prepared for the benefit of political decision-makers. In the four progress reports that have been issued to date it was possible to increase the reliability of statements every time. Even in the first report a temperature increase between one and five degrees Celsius was predicted for the 21st century depending on scenario.

However a number of further uncertainties were detailed. By the point of the fourth report in 2007 it was possible to reduce the uncertainties significantly. The core statement of the fourth report is as follows: It is highly probable that the majority of the global warming observed since the mid 20th century was caused by man. “Highly probable” here means a probability of 90 to 99 per cent. In the worst-case scenario the warming is estimated at between 2.4 and 6.4 degrees Celsius by 2100. From the results, climate researchers deduce that warming needs to be limited to about two degrees Celsius. Even this would be associated with high risks and serious consequences in certain areas. But even limiting to two degrees Celsius can only be achieved by means of massive climate protection measures. To achieve this, a trend reversal for the emission of greenhouse gases has to be brought about within the next ten years. By 2050 a reduction in global emissions by 50 to 85 per cent is urgently required.


The influence of man on the climate was realised at an early point because of the increasing understanding of the natural greenhouse effect. Flohn’s fundamental message, already declared in 1941, that humans cause global climate changes, has today become reality and has been scientifically proven. We can already guess at the significance of this, and if we act quickly and responsibly it would be possible to mitigate the consequences even further.

Christiane Beuermann,
economics graduate; currently acting director in the "Energie, Verkehrs- und Klimapolitik" (Energy, Traffic and Climate Politics) research group at the Wuppertal Institute. The focuses of her work are: climate politics, economic instruments, sustainability politics.

Translation: Jo Beckett

Copyright: Bundeszentrale für politische Bildung
May 2013

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