An election looms, and the temperature continues to rise…

In one of my subjects, I used data on the relationship between the global temperature and CO₂ concentrations to teach how variability can mask relationships, making inference uncertain. The example was based on an Australian political debate in 2009 about whether the temperature of the Earth had stopped increasing despite an increase in atmospheric CO₂.

With an Australian election looming in 2016, it seems timely to return to that debate, and evaluate the positions of the different protagonists now that we have some more data. It is nice to hold our politicians to account.

Let’s first look at the data. I will use the latest HadCRUT4 data for global surface temperature, and the average annual CO₂ concentration data measured at Mauna Loa (both downloaded on 20 April 2016). We have the CO₂ data from 1959 to 2015, so I will use that period. You can see that the CO₂ concentration has continued to increase since 2008, largely unabated it would seem.

Atmospheric CO₂ concentration as measured at Mauna Loa by NOAA. Data from 2009 onwards, available after the Senators’ debate, are shown in blue.

Temperature increases have been noticeably more variable, although that variation is consistent with annual variations of the past.

The HadCRUT4 measures of the global annual surface temperature anomaly, as measured by the Hadley Centre and the Climatic Research Unit.

Now, back to the debate.

Senator Wong believed that the Earth’s temperature would continue to increase in line with increases in CO2 concentrations. We can characterize that by a regression model that has a linear relationship with atmospheric CO₂ over the entire time period. Of course, Senator Wong’s position was not just influenced by the data, but also by the overwhelming majority of climate scientists who agreed with her position given their understanding the Earth’s climate system. Nevertheless, we will ignore that extra information here.

We can estimate the best fitting line for such a relationship, and predict the uncertainty around it – this uncertainty represents the range of predicted variation given that the recorded temperature at the Earth’s surface only approximates the heat content of the world. In the graphs I show here, the uncertainty bounds are 95% prediction intervals – we expect 95% of the observations to fall within the bound represented by the dashed lines.

Subsequently, we can determine how well that relationship fits the data that have been collected from 2009 onwards. With the CO2 concentration increasing, the Earth’s temperature has continued to increase. However, that increase has been somewhat below the average trend, with the notable exception of the data in 2015, which has exhibited a large spike.

Relationship between the global temperature anomaly and CO₂ concentration measured at Mauna Loa. The solid line is a linear regression, and the dashed lines are the 95% prediction interval. The regression was estimated on data up until 2008 (black dots), and the predictions are compared to data for 2009-2015 (blue dots).

However, we can say that Senator Wong’s position is largely consistent with the data collected after 2009; her predictions fall within the uncertainty bounds. Also, it is worth noting that the large spike in temperature in 2015 might well have been larger given the magnitude of increase in the CO₂ concentration.

Senator Fielding believed that the Earth’s temperature had stopped increasing after 1998. We can characterize that by a regression model that has a linear relationship with atmospheric CO₂ up until 1998, and then a flat line after that period. Again, we can estimate the best fitting model for such a relationship, characterize the uncertainty around the predictions, and compare those predictions to the data collected from 2009 onwards.

 

Relationship between the global temperature anomaly and CO₂ concentrations measured at Mauna Loa. The solid line is a piecewise linear regression that fits a sloped line to data up until 1998, and then a flat line from 1999 to 2008. Data for 1998 were removed to reduce the influence of cherry picking. The regression was estimated for data up until 2008 (black dots), and the predictions are compared to data for 2009-2015 (blue dots).

Senator Fielding’s position was also largely in line with the data up until 2014. While the model characterizing Senator Wong’s position was a touch on the high side of the data, the model characterizing Senator Fielding’s position notably under-predicted the temperature.

And Senator Fielding’s model severely under-predicts the temperature of 2015 – the temperature spike of 2015 is well outside the expected bounds, even when considering the variation in the data.

So, here we are 7 years and several governments after this political debate. The data  support Senator Wong’s position, and are now contradicting Senator Fielding’s position; the temperature continues to increase.

Note, the average temperature anomaly for 2016 so far is 0.979 C (based only on 2 months of HadCRUT4 data, so this is an uncertain estimate of the entirety of 2016). If that anomaly holds, we can conclude that Senator Fielding was sorely wrong, and even the model characterizing Senator Wong’s position might under-predict the temperate increase.

Seeing how atmospheric CO2 concentrations continue to increase, with concomitant increases in the heat content of the Earth, I can only conclude that the global community really should be acting much more rapidly. And I would hope that Australia would take the lead with this. I wonder the extent to which this will feature in the election campaign.