Global Warming.

* Greenhouse gases such as carbon dioxide absorb some of the energy radiated by the earth instead of allowing it to escape into space. Thus an increase in greenhouse gas causes the earth to warm.

* The warmer the earth gets, the more energy it radiates back into space.

* The deep ocean absorbs some of the extra heat.

* Some greenhouse gas is given off naturally, for example, when plants die and decay. The right amount is needed to keep the Earth's surface at a life-sustaining temperature.

* Since the industrial revolution, human greenhouse gas emissions have been raising exponentially causing the temperature on earth to rise.

* Plants, soil organisms, and aquatic life consume some of the excess emissions and speed up this process as the concentration of emissions in the atmosphere rises. This lags behind the rate of growth in releases. Human emissions now exceed removal by about a factor of two.

In 2001, the Intergovernmental Panel on Climate Change (IPCC), a scientific panel organized by the United Nations, concluded that carbon dioxide (CO2) and other greenhouse gas emissions were contributing to global warming. The panel stated: "most of the warming observed over the last 50 years is attributable to human activities."

The amount of CO2 in the atmosphere is affected by natural processes and by human activity. CO2 emissions resulting from human activities, including combustion of fossil fuels and changes in land use, especially deforestation, have been growing since the start of the industrial revolution (Figure 1).

Figure 1. Global CO2 emissions resulting from human activity (billion tons of carbon per year)

Natural processes gradually remove CO2 from the atmosphere (for example, as it is used by plant life and dissolves in the ocean). Currently, the net removal of atmospheric CO2 by natural processes is about half of the human CO2 emissions. As a result, concentrations of CO2 in the atmosphere have increased from pre-industrial levels of about 280 parts per million (ppm) to about 370 ppm today (figure 2).

Figure 2. Atmospheric CO2 concentrations, parts per million.

Increases in the concentrations of greenhouse gases reduce the efficiency with which the Earth's surface radiates energy to space. This results in what is called a positive radiative forcing that tends to warm the lower atmosphere and surface. As shown in Figure 3, global average surface temperatures have increased since the start of the industrial revolution.

Figure 3. Average global surface temperatures, °C. The zero line is set to the average for the period 1961-1990.

However, we know from the information given to prepare us that rising greenhouse gas concentrations increase radiative forcing above current rates, causing warming to continue until the temperature of the surface has warmed enough for outgoing energy to once again balance insulation. Although we do not have enough information to determine whether the temperature would equilibrate by 2100 or later we know that immediate stabilization, rise and decline, and drops below current levels are all ruled out by the basic physics of the global heat balance.

In fact, the best scientific estimates suggest that warming would continue through 2100 due to the enormous inertia of the climate system, for example, the long time it takes for the oceans to adjust to higher carbon dioxide concentrations and higher temperatures. ? Ashleigh Cracknell

"Many believe global temperatures respond immediately to changes in carbon dioxide emissions or concentrations, so that a peak and decline in emissions causes an immediate peak and decline in temperature. Still more believe that stabilizing emissions near current rates, as called for in the Kyoto Protocol, would stabilize the climate"

Building new infrastructure takes a long time

The Intergovernmental Panel on Climate Change has made clear that stabilizing emissions at Kyoto rates ensures continued growth in greenhouse gas concentrations and continued warming. If you now understand the dynamics of climate change better you should be able to understand why there are calls for much more significant cuts in emissions than are proposed in the Kyoto protocols and why so many people think that this is so urgent. Emissions must fall below the rate at which greenhouse gases are removed from the atmosphere to reduce the concentrations that cause temperature to carry on rising. You may also better appreciate why there has been so much widespread condemnation of the Bush administration's Clear Skies Initiative, which actually calls for continued emissions growth with extra measures after 2012 only if 'sound science justifies further action'.

It looks as if the majority of the human race may be naïvely and complacently trusting political leaders with poor understanding of the climate system to make decisions on which our survival depends.

Extreme events last for a relatively short time and are usually a local experience, which makes them difficult for scientists to measure and predict. Climate models currently lack the spatial detail required to make confident projections of how these events might respond to climate change.

? Ashleigh Cracknell

? Ashleigh Cracknell