Monday, 8 August 2011

A degree of uncertainty about how much the planet is warming

How settled is the science of climate change? Is the connection between carbon dioxide emissions and any rise in global temperature a simple formula? Is there room for reasonable people to reach contrasting conclusions?

In 1896, the Swedish scientist Svante Arrhenius proposed a relationship between atmospheric carbon dioxide and global temperatures. He was researching whether ice ages were associated with periods of low CO2 levels.

It was a brilliant idea which conversely led to the notion that adding CO2 to the atmosphere would warm the earth, translated mathematically as about 5C temperature increase with any doubling of CO2 levels. Today at 390ppm (parts per million), CO2 levels are about 30 per cent above those a century ago and average global temperatures up about 0.7C.

The "hothouse" model as then described is simple, but the calculations are not. This is a task best left to professionals and not undertaken at home without supervision. The fact that Arrhenius produced numbers that still are in the ball park today is curious given the vast number of adjustments required to better represent heat transfer through earth's atmosphere and our climate in general.

A general concern about global warming including the possibility of runaway climate change did not emerge until the 1970s. With more elaborate modelling and bigger computers, calculations then suggested a 2C increase with any doubling of CO2. Since then, research has expanded greatly and spilled into public policy, beginning with the first Rio Earth Summit in 1992.

Given the scientific importance of this field of climatology, the public policy consequences, and availability of increased computing capability and research funding, there has been a large number of refinements to the early atmospheric heat transport and climate models.

 These include, but are not limited to, consideration of the influence of ocean currents, solar activity, atmospheric water vapour, clouds, many different feedback loops, wind patterns, convection currents, dust, aerosols, smog, pollution, reflection from bright surfaces such as deserts, ice and snow, urban hot spots, carbon sinks such as forests and oceans, methane emissions from coal mines, gas fields and animals.

Through each calculational iteration, the forecast temperature change moves between one and 10C and depends upon latitude and region. Today the central estimate is that an increase in atmospheric CO2 to 450ppm by mid century and holding thereafter will drive an average increase of global temperatures from 15 to 17C, , that is 2C. But there is a large uncertainty about this number reflected in the Inter-Governmental Panel on Climate Change reports.

Different and equally complex theoretical models then connect temperature changes to future shifts in our climate and environment. Among these are predictions of more severe droughts, intense bushfires, unreliable but heavy rainfall, flooding, hailstorms with larger hailstones, higher energy cyclones, rising sea levels, species destruction, melting icebergs, receding glaciers, bleaching coral reefs (but not the occurrence of earthquakes or tsunamis).

The severity and timescale for such shifts remain uncertain and so the implications for public policy are somewhat ambiguous. For example, mid point forecasts of sea level rises are for about another 30cm increase by the end of the century. Yet already some councils are changing coastal zoning conditions to allow for 1m increases.

This may be understandable but adds a very large margin of safety on to a somewhat elastic underlying prediction. And as with any model, if you torment the assumptions enough, you can generate any forecast you like. There is plenty of evidence for this in some of our policy making and on both sides of the climate change debate.

Some conclusions:
*The original greenhouse concept a century ago was an inspired analysis but it is now seen as more a metaphor for, rather than a model of, our warming climate
*The mathematics of greenhouse effects is unimpeachable classical physics, albeit not easily grasped by most of us but global climate is much more complex than this basic greenhouse framework. There is no simple equation that links real world temperature changes to increasing atmospheric CO2 levels, or that links human-produced emissions with GHG concentrations
*There are many additional factors that shape computations of the global climate and they require the biggest of today's supercomputers, which calculate algorithms beyond the comprehension of mere mortals or our intuition
*The science of global warming and climate change is settled in a general sense. However, many details continue to be debated by researchers. There are substantial regional differences in forecasts, and the severity of the consequences for our future climate remain quite uncertain. Here you need to be a much more serious scientist than I am to have an informed view.

While there is much room for debate, challenging the complex science or its conclusions from any one dimension is unacceptable, whether by focusing on one variable, a limited time window, or via anecdotes, or limited personal observation. I, for one, accept the peer-reviewed science but would not be surprised if forecasts shift a great deal in the years ahead. In view of all of the above, policy makers need to be careful.

Ziggy Switkowski is chancellor of RMIT University.

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