Another round of tedious correction of contrarian disinformation in The Canberra Times. In response to an earlier letter, I wrote this piece which appeared in the print edition of 7 October:

Anthony Moore (Letters, 3 October) claims that satellite and land-based thermometer data show no warming over the past decade. In terms of climate temperature trends, this is not just incorrect; it ignores much more important parts of the picture.

To over-simplify, the word “climate” means the long-term average of the weather, typically over periods of 30 years or more. Annual averages in temperature are dominated by large short-term variations including cycles like El Niño. So-called “sceptics” (who seem to demonstrate a remarkable lack of genuine scepticism) like to talk about “the last decade” because they can start with a huge spike in the annual average (the 1998 El Niño) and end at a low point (the recent La Niña). This says nothing about the underlying climate trend; for that you need to look at long term averages. Look at a graph showing the 30-year average temperature: it’s been steadily increasing – in fact, accelerating – since the 1970s.

The big picture is very clear. The planet has been in energy imbalance since the 1970s: the amount of energy leaving the Earth is less than the incoming energy from the Sun. The planet as a whole is heating up, due mainly to the increase in CO2 and methane, and the energy imbalance is growing. Almost all of this heating has been in the oceans; land and atmosphere heating has been a tiny part of the picture so far.

Matt Andrews

This then attracted a response on 9 October from one of the regular peddlers of the usual ill-informed soundbites:

Matt Andrews’ response (Letters, October 7) to Anthony Moore (Letters, October 3) beggars belief.

The graph on which alarmists have consistently made their case for warming is the Global Temperature Land-Ocean Index which shows Annual Means and 5-Year Means from 1880 to 2000. And now Matt wants to move to 30-year averages to capture the bigger picture – from 1970.

That should give him just one data point!

Then he accuses sceptics of using the 1998 El Nino spike to demonstrate cooling.

Rubbish! Serious sceptics cull that spike from their data and demonstrate cooling from 2001 to the present.

And, like Barrie Smillie some time back, Matt completely misses the point.

This is not about the planet having warmed, which it has since coming out of the Little Ice Age in the mid-19th century, but whether that warming is driven by CO2.

And a planet that has had two prolonged cooling periods (1940-60 and 2001-09 and continuing) while CO2 has been rising, would suggest that CO2 is not the cause. Matt is doing the fast strokes.

Aert Driessen, McKellar

I then responded with this:

Aert Driessen (Letters, 9 October) is having trouble with 30-year moving averages of global temperature, so let me clarify: using the highest quality index, NASA’s GISS, if we use the average of the 30 years of temperatures that ended in 1975 as a starting point, we can see that for each and every year since, the 30-year average that ends at that year is a little bit warmer.

It shows a steady inexorable increase – the long-term trend we call “global warming”. The same pattern is clear in the 30-year averages of the other main index, from Hadley Climate Centre in the UK.

Taking a step back, satellite measurements show that the amount of energy received by the Earth is greater than the amount of energy that is leaving: the planet is in energy imbalance, and thus is heating up. It’s been in that state since the 1970s, and the imbalance is growing.

Matt Andrews