Chris de Freitas: Emotion clouding underlying science of global warming

Unlike most other hot button environmental issues, global warming is widely misunderstood. As a climate scientist thinking about this, it struck me that it was not surprising since accounts of the scientific basics of global warming almost never appear anywhere in the press.

There is not space here to include all the charts and numbers that might accompany such an account. In its place is a necessarily brief summary.

Most people are not shocked to learn that global warming discussions evoke polarised views, but many are surprised to discover that the scientific basics are not contentious. An awareness of these is helpful in building an understanding of the extent to which there is a problem and how it might be addressed.

On average, heat gained by the Earth from energy received from the Sun is equal to heat lost to space. Greenhouse gases in the atmosphere, mostly water vapour, carbon dioxide and methane, regulate this heat loss.

Global warming theory says that mankind's burning of fossil fuels such as oil and coal, adds to the carbon dioxide concentration in the atmosphere, which controls how fast the Earth looses energy to space.

More carbon dioxide causes warming in the lower atmosphere until Earth-to-space energy balance is once again restored.

The degree of warming directly caused by the extra carbon dioxide is, by itself, relatively small. This is not controversial. What is controversial is whether this initial change will trigger further climate changes that would be large or damaging.

Debate focuses on climate feedbacks that may or may not suppress, perpetuate or amplify an initial change caused by increasing concentrations of greenhouse gases. A doubling of carbon dioxide, by itself, adds only about one degree Celsius to greenhouse warming. Computer climate models project more warming because the modellers build in feedbacks from water vapour and clouds that amplify the initial change. These are the so called positive feedbacks. For example, higher temperature would mean more evaporation globally, which in turn means more heat-trapping water vapour is put into the atmosphere leading to even higher temperatures.

On the other hand, negative feedbacks might prevail. For example, more water vapour in the atmosphere could lead to greater cloud cover. Clouds reflect the heat from the Sun and cool the Earth, offsetting the initial rise in global temperature.

The role of negative feedback processes are played down by global warming alarmists, whereas sceptics point to the four-billion-year-old global climate record that shows runaway global cooling or warming has never occurred because negative feedbacks regulate the global climate system.

It is important to consider the above in the proper context. Change is a constant feature of climate, even through recent human history. During the Medieval Warm Period, from 900 to 1200AD, the Vikings sailed in Arctic waters that by 1700 had turned to permanent sea ice, and farmed in Greenland soil in a climate that soon became too cold for agriculture.

The Medieval Warm Period was followed by the Little Ice Age which ended around 1850. It in turn was followed by another warm period. The hottest year since 1850 was 1998. In the nine years since 2002 average annual global temperature has not risen.

Most people are surprised to hear that no one has uncovered any empirical real-world evidence that humans are causing dangerous global warming. Finding this evidence is crucial, since scientific issues are resolved by observations that support a theory or hypothesis. They are not resolved by ballot.

Scientific opinion is like climate models; neither is evidence of dangerous global warming.

None of this is to say we should simply walk away from considerations of a global warming threat, but prudent consideration of the scientific facts is essential.

No science should have to rely on one group or authority saying, "Just trust us," particularly when tens of millions of dollars of public policy decisions are on the line.

Chris de Freitas is an associate professor in the school of environment at the University of Auckland.