As politicians debate how to halt the warming of the planet, some 200 years of climate science provide the basis for their deliberations. But this science hasn't always been a given.
Negotiators and political leaders about to meet in Paris are tasked with agreeing on a new global climate treaty to try to limit global temperature rise to a maximum 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels.
Meanwhile, the World Meteorological Organization (WMO) tells us that by the end of this year, the earth will be 1 degree warmer than it was before the industrial revolution. Yet the concentration of CO2 in the atmosphere has reached a record high, climbing steadily towards the400-parts-per-million (ppm)
Joining the dots
The need for action to stop the human-induced excessive warming of the planet through the emission of greenhouse gases caused especially by the burning of fossil fuels is based on the findings of climate science. But this science was not always taken for granted.
Climate science as a discipline in its own right is very recent. Previously, most climate researchers studied meteorology, oceanography or geophysics. The complex phenomenon of climate involves the study of many disciplines - and connecting the dots between them.
Not just the sun
The history of the scientific discovery of climate change dates back to the early 19th century. In the 1820s, French intellectual Joseph Fourier came to the conclusion that the earth should not be as warm as it is, given its distance from the sun.
Fourier worked out that there must be something other than solar radiation keeping the planet warmer - that the atmosphere must play a role. Fourier suspected that human activities such as changes in land use could be influencing the climate. This is often seen as the foundation for later research into what came to be known as the "greenhouse effect."
At a time when science was just beginning to find out about ice ages and search for causes of past changes in the earth's climate, Irish physicist John Tyndall (1820 to 1895) found out that different gases absorb and give off heat in different amounts. He suggested variations in the composition of the atmosphere could have been responsible for past changes in climate.
Tyndall discovered that water vapor was an important heat-trapping agent, and that carbon dioxide was also very good at trapping heat. Could the atmosphere work as some kind of warming blanket for the earth?
The coal-fired greenhouse
Swedish scientist Svante Arrhenius is credited with being the first to publish a theory on how human-induced changes in the carbon dioxide concentration in the atmosphere could have an impact on global temperature.
Arrhenius calculated that cutting CO2 in half would suffice to produce an ice age, whereas doubling atmospheric CO2 would give a total warming of 5 to 6 degrees Celsius.
In 1896, Arrhenius published the article "On the Influence of Carbonic Acid in the Air upon the Temperature of the Earth." The Swedish pioneer realized that emissions from burning coal, and from other industrial sources, would eventually lead to warming.
Arrhenius was also already suggesting relationships between temperature and clouds, and other processes still being researched today. He foresaw "feedback" effects, like extra warming caused by heat that would previously have been reflected back into space by white ice and snow being absorbed by the darker surfaces left when ice melts.
Arrhenius thought it would take thousands of years for emissions from coal burning to raise the temperature - and he presumed this would probably be beneficial to humanity.
No science without questioning
Science thrives on debate, on theories and counter-theories. Arrhenius's calculations were disputed. Many scientists dismissed the idea of this sort of planetary warming, arguing that oceans would quickly absorb any excess carbon dioxide. Many other theories were put forward to explain changes in the climate, from volcanism to solar variation.
In the 1920s and 1930s, attempts to connect the solar cycle with climate cycles were very much en vogue. In 1920, astronomer Milutin Milankovic worked out how varying distances between the earth and the sun, and varying angles of the sun's radiation, affect the earth's climate.
In recent years, some scientists have attributed global warming to these natural cycles, disputing the anthropogenic factor. But the scientific consensus today is that while natural cycles also play a role, it is impossible to explain the unprecedented rapid rise in temperature since the industrial revolution without human influence.
Greenhouse gas emissions will need to drop more than currently envisaged to avoid catastrophic climate impacts
Collecting the evidence
In 1957, Roger Revelle from the Scripps Institution of Oceanography and Hans Suess of the United States Geological Survey researched how the ocean has only a limited ability to absorb CO2, so that the concentration in the atmosphere was bound to increase.
Around the same time, a network of monitoring stations was set up around Scandinavia. But there were flaws in the methodology, and it was only when Charles David Keeling began to measure carbon dioxide in the atmosphere at a new observatory on top of Mauna Loa volcano in Hawaii in 1957 that a baseline could be established and later comparisons made.
Previously, it was thought that plants and oceans completely absorbed the CO2. Data shows CO2 concentration in the atmosphere changes with the seasons, and that the overall concentration had risen. Today's concentration, at almost 400 ppm, is unprecedented in the last 800,000 years.
Tree rings, ice and sediment cores and radiocarbon dating have all played a role in consolidating climate science. Satellite measurements provide data on current developments, and computer models help us predict what the consequences of our human-caused intervention with the world's climate are likely to be.
In 1979, the WMO held a "World Climate Conference," which came to this conclusion:
"It appears plausible that an increasing amount of carbon dioxide in the atmosphere can contribute to a gradual warming of the lower atmosphere, especially at higher latitudes. It is possible that some effects on a regional and global scale may be detectable before the end of this century, and become significant before the middle of the next century."
In 1988, prominent NASA scientist James Hansen testified to the US Congress that human-caused warming had already measurably affected global climate.
In that same year, theIntergovernmental Panel on Climate Change
was set up by the WMO with the support of the United Nations Environment Programme.
Scientific developments are summarized every five to six years in the IPCC assessment reports (1990, 1995, 2007 and 2014), which form the basis for negotiations within the UNFCCC (United Nations Framework Convention on Climate Change).
Nevertheless, a minority of scientists continued to cast doubt on the human role in changing the earth's climate. The findings of climate science indicate that we need to radically change our energy systems and lifestyles to reduce the amount of greenhouse gases we emit into the atmosphere. Given the huge economic and social implications of this, climate science has become emotionally charged in recent years.
Applied research has often been funded by companies with a large stake in the fossil fuels economy. Astudy
found that groups with funding from corporations like Exxon have been particularly effective at polarizing and misinforming the public on climate change.
Since 1998, Exxon has contributed more than $31 million to organizations and individuals blocking solutions to climate change, and spreading misinformation to the public. While Exxon's own scientists and research were completely in line with the expert consensus on human-caused global warming in past decades, and even funded its own ground-breaking climate science in the 1970s, the study finds the company later funded a campaign to manufacture doubt about that scientific consensus.
Science and advocacy
Respected German scientist Hans Joachim Schellnuber, who heads the Potsdam Institute for Climate Impact Research and has advised the German government on climate issues, recently published an impassioned call for action in a 700-page book with the provocative titleSelbstverbrennung,
or "Self-immolation." The cover shows a giant Atlas bearing a burning globe on his shoulders.
Schellnhuber says scientists were subjected to a "witchhunt" in the aftermath of the failed UN climate conference in Copenhagen in 2009.
He argues that it is time for scientists to abandon any stance of pseudo-neutrality, and back the urgent need for an effective world climate agreement.
Studies show a 97 percent consensus among scientists worldwide that human activity is causing global warming at an unprecedented rate.
With some of the most powerful leaders in the world coming to Paris for the UN climate summit in spite of terrorists attacks and amidst widespread conflict around the world, it appears climate science has finally come of age - and sets the basis for discussion.