Climate change evidence

Real climate change evidence has to demonstrate that the pattern of the weather in a given region has altered consistently enough to represent a measurable change in climate.

An extra sunny day, a severe storm or a flood is not enough.

Any weather event on its own can easily be an anomaly, something that chance can bring with or without the climate changing.

It is also not enough to see a trend in the atmospheric concentrations of greenhouse gases. Reliable evidence should be a measurable difference in what we define as climate.

So first we need to be clear about what we mean by climate.

Climate change wisdom uses the definition provided by the IPCC, the Intergovernmental Panel on Climate Change, that says

Climate in a narrow sense is usually defined as the "average weather," or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization. These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.

So it is the average and the variation around this average for features such as temperature, rainfall (and/or sleet and snowfall), and wind over a long(ish) time period.

As a rough shorthand we could call it average weather.

Now we have a definition then we have a better idea of what to look for as climate change evidence.

We need to see a consistent change in the features sufficient to produce a new or measurably different average and/or new or measurably different variation around the new average.

Explaining the average and variance

Each week you go to the store and purchase groceries for the week.

Lets say it usually comes to around $100.

Only it is never exactly $100, even in round figures. Sometimes it's a cheap week, only $87; and then the week when you have to replace the 5 litre carton of olive oil it's $125.

But, when you add up the bills for the last 10 weeks and divide by 10, what you get is pretty close to $100.

Suppose that there is a run on fresh fruit and vegetables and for some reason there is a milk shortage. The cost of these items goes up and the total bill with it.

Then a few weeks later it's harvest time, the milk problem is over and the prices on those items drop.

Differences in the bill from week to week (what the statisticians call the variance) are generated by your choice of items and these are external factors that affect prices.

So one way to measure the "climate" of your weekly shop is the average price and the variability in bill from week to week.

Detecting change

Certainly five years ago it was cheaper. The average then was closer to $80.

When grandma went to the store in her youth, she always had change from a $10 bill.

Record the average price over many decades and we know there is a change.

The problem is how much information do we need to be sure of the change over shorter time periods.

We will save the gory details of statistical tests for difference between two averages for another time. All we need to know here is that we can take the difference between two averages and compare it with the variability in the numbers that we used to calculate the averages to find this out.

All we need are sets of numbers.

The longer the run of observations, the easier it will be to detect a statistical difference if one exists.

Detecting climate change evidence

The same principle applies to climate change evidence.

We need sets of data on the surface variables such as temperature, precipitation, and wind and then we can see if the average changes.

We can get these from

• direct measurements of the surface variables
• indirect measurements using indicators (consequences) of the surface variables

Our problem is that direct measurements have only been available for around 200 years.

Before this time we did not have the instruments or widespread expertise.

200 years is a short time in climate terms for hange takes longer than this.

Indirect measures can take us back in time and generate data for longer periods but these are surrogates for the real thing. There is less certainty in what the numbers reflect.

The IPCC now has AR4, the latest compilation of the evidence.It suggests climate change evidence is all but complete. We know that the equivalent of grandmas shopping bill is in the past and the current bill is rising.

Now all we need is to decide what to do about it.