A State of the Climate — Global Analysis report last month declared January-October 2010 as the hottest period on record. The stats came from combining the global land and ocean surfaces temperatures and comparing them to historical time periods. But how is a global average temperature calculated? How is an “average” created in a planet where there is no such thing as “typical”? And has CO2 caused this increase in global temperature? Climate scientists explain.

Got a question about climate science? These next few weeks at Rooted we’re running the Ask a climate scientist series. Keep the questions coming as well, by either leaving one as a comment, or — preferably — emailing me directly.

These answers are coming from American Geophysical Union’s Climate Science Q&A service, where more than 700 volunteer scientists provide factual and peer-reviewed climate science information to journalists. The AGU only comments on science, not climate policy.

Crikey reader Kerry asks:

We hear a lot about the Average Global Temperature. How can anyone calculate the Average Global Temperature? I know they take measurements all over (or in some places) but what do these mean? As I understand it the real measurements are of the anomalies from some base number. And how do they measure this anomalous average ? — by putting together north and south hemispheres? And what about the oceans which comprise over 70% of the earth’s surface?

Professor James O’Donnell, from the University of Connecticut, responds:

This is a very deep question. The annual global mean temperature has become a very important statistic because it smooths out the year to year variations that are observed at any single station and reveals really long period trends. Since the air temperature varies with altitude, latitude and time of year, changes at any single station is obviously not representative of the whole earth.

We would like to measure the temperature everywhere in the world at the same altitude every second and then average that. That is obviously impractical. What we have is a network of instruments around the world that share their measurements. Unfortunately, there are more in some places than others.

There are few at high latitudes and altitudes and lots near big cities. And they come and go as funding levels vary and cities grow. Since the ’70s we also have satellite measurements of radiation and these can be used to estimate surface temperature over land and the ocean.

Over the past 20 years we have also developed a network of instruments in the ocean. Some float at the surface and are anchored, some drift around with the currents and some go up and down every day. But these records are still pretty short.

Figuring out how much of the globe is represented by a single station, and correcting for the errors in measurements etc, has been a very important area of research in climatology which I can’t explain in a few sentences. But the bottom line is that much of the available data put in a weighted average and the weights are carefully calculated to account for station density and measurement uncertainty.

There a few variants of the approach, but the results are generally consistent. This figure shows a few different versions. It is the difference from the average of measurements between 1960 and 1990.

It’s not perfect, but it is probably very close to the best we can do.

This answer was reviewed and approved by Kevin Schaefer from the University of Colorado.

Crikey reader Trevor asks:

CO2 have been given to explain the Little Ice Age and Mediaeval Warming, so what is the evidence that CO2, or some other human sourced greenhouse gas, has caused the increase in temperature since 1860? Is it just a positive correlation between CO2 and temperatures, or is there some other, more fundamental reason, keeping in mind, as I understand the argument, that the ability of CO2 to absorb long-range radiation increases only in a logarithmic fashion and that the amount of long-wave radiation absorbed by CO2 is miniscule compared to other greenhouses gases, such as water vapour, of which the presence of the latter doesn?t seem to me to be due directly to human activity?

Charlie Zender, Professor of Earth System Science at the University of California, responds:

It is not true that CO2 explains the Medieval Warming or the Little Ice Age. Natural causes of climate change were dominant until about 1950, and human causes have dominated after that.

Peter Fray

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