Viewpoint

Science and the Greenhouse Effect

Martin Manning, Brett Mullin, Jim Salinger, & Dr Blair Fitzharris

The debate over climate change and its effect on the way we live is more than hot air from scientists.

Most of the 160 nations attending the Berlin climate summit agreed that the risk of climate change means greenhouse gas emissions must be cut to below 1990 levels. As Environment Secretary Roger Blakeley, who was at Berlin, has said, there was no debate over the science of climate change. The Berlin summit also agreed that the Intergovernmental Panel on Climate Change (IPCC) is to continue as the major scientific advisory body to the United Nations on climate change science. Given recent media coverage of the subject in New Zealand, readers might wonder why.

In January, Professor Richard Lindzen, a US climate scientist, visited New Zealand and his sceptical views were widely reported. Greenhouse sceptics say there is no solid scientific basis for imposing controls on greenhouse gases.

These views contrast sharply with the assessments of the IPCC, in particular with a report now being compiled by more than 500 leading climate scientists.

Dr Chris de Freitas of Auckland University [Viewpoint, May] has raised questions on the uncertainties.

Recent news coverage appears to have focused on uncertainties in the details of predicting future climate while the certainties have been overlooked. Reviews by the IPCC and many other scientific bodies confirm the following:

• Atmospheric concentrations of greenhouse gases are increasing. Carbon dioxide is more than 25% higher than pre-industrial levels and methane has more than doubled.
• These increases are caused largely by human activities and for carbon dioxide are due mainly to burning of fossil fuels.
• Increasing greenhouse gases in the atmosphere alters the way in which radiation from the Sun drives the climate system. Changes in carbon dioxide play an important role.
• As a consequence the Earth's surface and lower atmosphere will become warmer.
• Global warming will cause an expansion of surface ocean water, leading to sea level rise.
• Over the last 100 years the world has warmed by about 0.5^oC on average.

Few scientists would dispute these major conclusions. The fundamentals of the greenhouse effect are "textbook" science, known for more than 100 years.

The warming of the Earth observed over the last 100 years will be due to a combination of many factors. Warming due to the increase in greenhouse gases alone cannot be determined so far. However, projections of greenhouse warming do not rest on recent observations, they come from scientific understanding of the climate system. A range of simple and complex computer models has been used over the last 30 years to put together knowledge of many climate processes.

Current models do a reasonable job of simulating today's climate. The difference between summer and winter temperatures, in response to change in heating from the Sun, is correctly simulated to within about 0.5^oC over most of the planet. Natural events provide additional tests of our understanding. For example the scale and duration of cooling due to the eruption of Mt Pinatubo were reproduced by climate models.

When greenhouse gases are increased in climate models the response is always an increase in average temperatures and water vapour. Models vary in their results but, in virtually all cases, doubling the amount of carbon dioxide increases global average temperatures by between 1.5^oC and 4.5^oC.

As a result of these findings, the IPCC states firmly that "we know enough to say with certainty that the risk of climate change is genuine and serious".

Why then are some dissenting voices heard on the science of climate change? In some cases it appears that critics are simply unaware of the depth of the science developed so far. Others are concerned because we do not understand everything about the climate system.

Some climate processes are too complex to be analysed in detail for the whole planet and others are not well understood. Models calculate changes only in those parts of the system that we understand. Perhaps the bits of the system we do not understand will neutralise the warming. However, the atmospheric parts of climate models appear to work quite well already.

More generally, we believe it is risky to assume the existence of some unknown "thermostat" which will hold temperatures at present levels. We know very clearly that the climate system has changed before and that average temperatures have risen and fallen with natural changes in greenhouse gases over geological time.

Some people question whether the estimated changes are important. Greenhouse warming adds a steady upward trend in temperatures on top of natural variability. The cumulative effect over the next 30 to 40 years would be to increase both high and low temperatures by more than present year to year variations.

The skeptics and others have suggested that the global warming issue has been contrived by self-interested scientists, that the "consensus" on global warming is weakening, and that the IPCC process is suspect. This "conspiracy theory" is astonishing to those of us who have been directly involved.

The IPCC's role is to provide assessments of climate change and its impacts (good and bad). It works by involving continually changing groups of hundreds of leading scientists in workshops and reviews. All active climate research groups have input to the process. Assessments are based on material published in the open scientific literature.

As a United Nations body, the IPCC is required to follow strict inter-governmental protocols for meetings and independent reviews. Key IPCC sessions involve industry groups and representatives of OPEC countries who would highlight any major inconsistencies. To imagine that some hidden "science" agenda is maintained throughout this is unrealistic.

The integrity of the IPCC is not at issue in international negotiations. It is accepted that climate change is likely but that uncertainty remains about its magnitude and timing and the associated regional impacts. The issue at Berlin was how to frame policy in this circumstance. The "precautionary" approach adopted so far does not depend on accurate predictions of climate. It merely accepts that the likelihood and seriousness of potential climate change require a risk averse approach. Developed countries are asked to freeze emissions at 1990 levels while longer term strategies are considered.

The alternative would be to continue increasing emissions until uncertainties were reduced. This may take some time and delaying action, implies accepting additional risks. It takes several decades for the climate system to fully respond to greenhouse gas increases and the longer we delay controls, the greater the climate change we become committed to.

Science can never afford to ignore dissenting arguments. But the immediate question is: can the global community afford to risk that the vast majority of climate scientists have got it all wrong?

Dr Blair Fitzharris is Associate Professor of Geography at Otago University.
Martin Manning is a senior scientist with NIWA in Auckland.
Brett Mullan is a senior scientist with NIWA in Auckland.
Jim Salinger is a senior scientist with NIWA in Auckland.