It was an observation by the 19th century Irish scientist John Tyndall that was to become a cornerstone of what the world today knows as climate science: "Without water vapour," he wrote, "the Earth's surface would be held fast in the iron grip of frost."
In 1859, Tyndall started studying the ability of various gases to absorb and radiate heat and found that atmospheric water vapour, carbon dioxide and ozone absorbed larger amounts of heat radiation compared with the other gases in the atmosphere. It was this discovery that led to an understanding of the greenhouse effect - the ability of atmospheric water vapour, carbon dioxide and other substances to trap heat emitted from the Earth's surface. It is this process that makes life on Earth as we know it possible - and, if that balance is upset, leading to overheating, impossible.
More than a century after Tyndall made his discovery, humanity's understanding of the complexity of the climate system and our technical ability to track changes to it over time has evolved significantly. It is now known that concentrations of CO2 in the atmosphere are higher than they have been in the past 800,000 years. This increase has been attributed to human activities - chiefly, the burning of fossil fuels and the clearance of forests, natural sinks for CO2. It is also known that this increase has already made the Earth warmer and, if nothing is done, that the warming effect will intensify.
The big challenge confronting climate scientists now is to understand and explain exactly how warming will affect other natural processes on Earth. Some effects are already clear. "We [already] have global warming of almost one degree Celsius since pre-industrial time," says Dr Erich Roeckner, a former senior scientist from the Max Planck Institute for Meteorology in Hamburg, Germany. Dr Roeckner is now retired but is still doing research at the institute, where he led the development of Echam, the global climate model that has helped him and his colleagues investigate what life would be like in a warmer world (Echam stands for European Centre for Medium-Range Weather Forecasts and Hamburg). Echam simulations were among those used to inform the predictions and conclusions of AR4 - the most recent assessment report from the International Panel on Climate Change.
Graphic The evidence for global warming
Climate models calculate the interaction between the various parameters that constitute climate; the latest can take into account factors such as incoming sunlight, precipitation, concentrations of greenhouse gases, ice cover and land features. As such modelling increases in sophistication more complex questions are being answered. One line of research focuses on what scientists call "positive feedback mechanisms" - processes triggered by climate change that lead to more warming. One example is the Earth's projected loss of ice cover. As the heat-reflecting ice melts, darker land and water surfaces will be revealed and these in turn will absorb more heat, causing more warming and faster melting of ice.
"There is also methane in frozen state in the deep ocean, close to the continents, but if that is released through warming of the ocean, then this additional methane will be released into the atmosphere," says Dr Roeckner. Another predicted consequence is that climate change could tamper with the carbon cycle -the process by which carbon circulates through the land, ocean, atmosphere and the Earth's interior.
"It may be that in future land and ocean are less efficient in taking up CO2 because of warming," says Dr Roeckner. Not that scientists already know all there is to know. Expressing the carbon cycle as a series of mathematical equations that can be incorporated in a climate model is still a work in progress. "The process by which plants take up CO2 depends on many local factors such as temperature, precipitation, humidity," says Dr Roeckner. "This is the difficult part. In the past 10 years some progress has been achieved."
The latest version of ECHAM, which has just begun functioning, will for the first time be able to simulate in detail the complex interaction between climate and the carbon cycle. Faster computers have allowed for more accurate modelling. "In the past 10 to 20 years, the simulation time has increased and the grid refinement went down to around a hundred kilometres," says Dr Roeckner. While earlier models could run simulations for only a few years or decades, today's versions can simulate climate over the course of a century.
In the past, grid resolution - the distance between virtual points on the Earth for which climate data is calculated - was as large as 500 kilometres; in the new model, it is down to 80km. The higher the resolution, the greater the accuracy and the ability of the model to predict climate change on the regional level. High resolution also makes it possible for complex natural phenomena such as the carbon cycle to be simulated.
The new model will also be able to investigate the effects of cloud cover and atmospheric ozone on climate as well as the impact of aerosols - all new lines of inquiry. Besides computer modelling, another way to learn more about future climate change is to examine evidence that holds clues to the Earth's past, one of the many sources of data that inform climate models. While CO2 concentrations in the atmosphere were not measured extensively until 1958, the ice that covers the Earth's polar regions allows scientists to look back over millennia. By comparing the three isotopes, or forms, of oxygen found in ice cores in Greenland and Antarctica, which vary in proportion according to the prevailing temperature at the time they were trapped, scientists can deduce the contemporary air temperature. Chemical analysis of the ice can also reveal how wet or dry an area was, whether there was any volcanic activity and what kinds of pollen or chemical pollutants were floating in the air.
Furthermore, at depths beyond about 100 metres, scientists have discovered bubbles of "old" air, from which valuable information about the concentration of greenhouse gases have been obtained. Research through the European Project for Ice Coring in Antarctica (Epica), which began in 1996, provided 800,000 years of ice-core records. It was this project that demonstrated that at no time during this period were concentrations of carbon dioxide or methane in the atmosphere as high as in this century.
"CO2 concentrations were never above 300 parts per million [ppm]," says Dr Eric Wolff of the British Antarctic Survey, who chaired the science group of the Epica project. "Now they are at 385 ppm." Ice core research, he says, also confirmed the connection between the carbon cycle and climate; to deepen this understanding, scientists are now trying to find ice that is older than 800,000 years. Another line of research looks at the more recent past, focusing on ice that is only several hundred years old. This can reveal in detail the environmental changes that have taken place since the beginning of the Industrial Revolution.
Yet another line of inquiry is examining ice that provides records from the Eemian interglacial period, some 125,000 years ago, a time when sea levels were between four and six metres higher than today. "It is," says Dr Wolff, "a very good test of what could happen in future." At that time, Antarctica was an average 6°C warmer than it is today. The reasons for this are not well understood but work going on now in Greenland should provide a clearer picture; the North Greenland Eemian Ice Drilling project is expected to reach ice from the period in question next summer.
What all this work tells us, says Dr Roeckner, is that while there is a fine line between life as usual and disaster, stepping over that line is not an inevitability: "We have global warming of almost 1°C since pre-industrial time. If we stay below another one degree, most eco-systems and mankind will not suffer too much." A year ago, the Max Planck Institute for Meteorology produced a climate stabilisation scenario with CO2 concentrations at 450ppm, a model that predicts a global temperature increase of two degrees compared with pre-industrial times. This would involved "some loss of Arctic sea ice but not complete ice-free summer", says Dr Roeckner.
But, "A two degrees increase is the limit. If it goes up to three degrees, Greenland will melt away." And if that happens ... Current predictions say that, to prevent this happening, CO2 emissions need to start decreasing after 2015. In 2050, they need to be half the 1990 levels and, says Dr Roeckner, "At the end of the century, they need to be almost zero." @Email:firstname.lastname@example.org