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By Stefan Rahmstorf
BERLIN ― On June 10, 1859, six months before Charles Darwin published his ``Origin of Species," the physicist John Tyndall demonstrated a remarkable series of experiments at the Royal Institution in London.
The meeting was chaired by Prince Albert. But neither he, nor Tyndall, nor anyone in their distinguished audience could possibly have anticipated the extent to which the experiments' results would preoccupy the world 150 years later.
This month, thousands of people from all over the world, including many heads of state, will gather in Copenhagen to try to forge an agreement to drastically cut atmospheric emissions of an invisible, odorless gas: carbon dioxide.
Despite efforts by some leading countries to lower expectations ahead of the conference about what can and will be achieved, the meeting is still being called the most important conference since World War II. And at the conference's heart are the results of Tyndall's experiments.
But the story starts even before Tyndall, with the French genius Joseph Fourier. An orphan who was educated by monks, Fourier was a professor at the age of 18, and became Napoleon's governor in Egypt before returning to a career in science.
In 1824, Fourier discovered why our planet's climate is so warm ― tens of degrees warmer than a simple calculation of its energy balance would suggest. The sun brings heat, and earth radiates heat back into space ― but the numbers did not balance.
Fourier realized that gases in our atmosphere trap heat. He called his discovery l'effet de serre ― the greenhouse effect.
It was Tyndall who then put Fourier's ideas to the test in his laboratory. He proved that some gases absorb radiant heat (today we would say long-wave radiation).
One of these gases was CO2. In 1859, Tyndall described the greenhouse effect in beautifully concise words: ``The atmosphere admits of the entrance of solar heat, but checks its exit; and the result is a tendency to accumulate heat at the surface of the planet."
Then, in 1897, Svante Arrhenius, who earned a Nobel Prize for chemistry six years later, calculated how much global warming a doubling of CO2 in the atmosphere would cause. His answer was 4-6 degrees Celsius (a little more than the 2-4 degrees that modern studies consistently find).
Arrhenius was not in the least troubled by the prospect of global warming. Perhaps because he was Swedish, he proposed setting coal mines on fire to speed it up, since he thought a warmer climate was an excellent idea.
But it was all just theory in Arrhenius's time, since nobody had measurements to prove that CO2 levels in the atmosphere were in fact increasing.
That changed only in the late 1950s, when Charles Keeling started to measure CO2 with unprecedented accuracy in Antarctica and on Mauna Loa in Hawaii, far away from any sources. By 1960, he was able to prove that CO2 was indeed on the rise.
It then took only a few years until, in 1965, an expert report ― the first of many ― to U.S. President Lyndon B. Johnson warned of global warming: ``By the year 2000, the increase in carbon dioxide will be close to 25 percent. This may be sufficient to produce measurable and perhaps marked changes in climate."
In 1972, a more specific prediction was made in the leading science journal Nature, namely that temperatures would warm by half a degree Celsius by 2000. And, in 1979, the U.S. National Academy of Sciences issued a stark warning of impending global warming.
We did not have to wait until 2000 to find these predictions were correct: by the 1980s, global warming became apparent in temperature measurements from weather stations around the world.
In 1988, the Intergovernmental Panel on Climate Change (IPCC) was founded to analyze the issue in more detail, and in 1992 at the Earth Summit in Rio de Janeiro world leaders signed a historic treaty: the Framework Convention on Climate Change.
Its goal: ``stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system."
Unfortunately, little has been achieved in the 17 years since then. In fact, CO2 emissions from fossil fuels were almost 40 percent higher in 2008 relative to 1990. And even the rate at which emissions are increasing is now three times higher than in the 1990s.
Global temperatures had already risen by 0.5 degrees C above preindustrial levels by the early 1990s, and another 0.3 degrees C have been added to that since the Rio Earth Summit. And they keep rising.
Most countries now agree that global warming should be stopped at a maximum of two degrees centigrade. But this has become an extremely tough challenge, as growth in greenhouse-gas emissions and atmospheric stocks accelerated in the years since Rio.
That is why Copenhagen is so important: it may well be our last to address climate change before it addresses us.
Tyndall's measurements 150 years ago showed that carbon dioxide traps heat and causes warming. And, 50 years ago, Keeling's measurements showed that CO2 levels are increasing. In the meantime, earth's climate has been heating up, as predicted. How much more proof do we need before we act?
Stefan Rahmstorf is professor of physics of the oceans at Potsdam University and department head at the Potsdam Institute for Climate Impact Research. For more stories and information, visit Project Syndicate (www.project-syndicate.org).
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