Evidence of climate change

We need evidence of climate change because our belief is that we are rational creatures. Show us evidence and we are persuaded.

The trouble is that the evidence we encounter tomorrow just as easily persuades us, even if it contradicts what we see today.

As for last week, well we have forgotten all about that.

We also differ in what we believe to be evidence. It can be a statement written or heard, a feeling, what we can see with our own eyes or all of the above.

Slippery stuff evidence.

August in Sydney, Australia is wintertime. At night it should be cold enough for the occasional frost, especially in the suburbs away from the heat island of the city. Daytime is often beautiful with clear skies, sun, light wind and temperatures in the low 20’s Celsius (in the 70’s for Fahrenheit followers).

On Sunday 23 August 2009 on the bank of the Nepean River in western Sydney it was 30 degrees C, a full 10 degrees warmer than the long term average. In the month there were 14 days where the daily maximum temperature was more than 2 degrees above the medium term average.

“Nice global warming” a passer by chirped as people basked in the sunshine and plants sprouted early blossom.

Evidence of climate change? Possibly.

Half way around the world in southern Africa the weather on the savannas of the inland is highly predictable. A long dry season covers autumn, winter and early spring (although the traditional north temperate four season description is not really apt in these parts) when there is no rain, save for an occasional drizzle on the odd August morning. Then in September it warms up, heralded by a sharp jump in daily minimum temperature, and beginning in October although they can be late to start, rains fall on and off for four months. It is an unfailing pattern of a long dry season and summer rain.

Although they know what is coming, people in southern Africa talk about the weather all the time. They discuss the rain. When it will start and how much there will be.

Average annual rainfall in Gaborone, the capital of Botswana, is 530mm – only it never rains on this average. Some years 900mm falls in great thunderstorm deluges and other years there may be only 200mm and drought is declared. Average years are actually uncommon as there are rainfall cycles with wet and dry decades superimposed on longer interval drier or wetter periods.

It creates endless debate as rain feeds the grass that feeds the crops and the all important cattle.

No change in the weather but pattern in the climate.

Pattern in climate from unusual events

We need evidence of climate change not weather change or natural cycles but evidence that there has been a shift in the extremes, the pattern, or both.

Usually when we require evidence we call for it. We ask witnesses to swear to tell the truth in a court of law and then a dozen citizens deliberate the likelihood of an event being true.

Or we ask the scientists to provide measurements. They measure things – temperature records, frequency of severe storms, frost free days and the like – then they look for trends in the numbers.

Trends are detected through the use of statistical analysis (number crunching tools that can tell us if a trend is unlikely to have happened by chance alone) and pronounced as significant or not.

Scientists searching for evidence of climate change make measurements of

• effects – temperature, rainfall, frost-free days

• events – severe storms, hurricanes, droughts, floods

• proposed causes – CO2 concentration in the atmosphere, sunspot activity, wobbles in the earth orbit

And there are numerous examples of the measurements, the data and their trends in the IPCC reports.

Lies, damn lies and statistics

The problem with statistics is that they are relative.

• to the data available

  It is easy enough to perform a test on a set of data and be able to say that the average temperature is trending upwards, more than by chance alone. We then conclude that something is pushing the temperature up, but the upward part is only relative to the data at hand. Use a longer data run and the statistical trend can be lost or even reversed.

• to what matters

  A statistically significant trend in values of a measurement does not mean that there is a effect or consequence that matters to us or the climate.

The evidence of climate change is actually more about what we mean by climate change than establishing facts.

A warm day by the Nepean River in Sydney was just that, a warm day for the time of year. There were other such warm days long before we started talking about climate change.

Similarly generations of African herdsmen have waited for the cold nights to end and the storm clouds to build, hoping for many showers of rain to nourish the grass.

We could provide them with evidence for climate change but it would not stop them wishing for good rains.

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