Don’t take my word—or anyone else’s—for anything. You don’t need to. The data is so clear that you can see, for yourself, the reality of human-induced climate change.

The graph above has two curves. The first, in blue, shows the cumulative amount of carbon dioxide that we’ve put into the atmosphere since 1880 whilst, in red, you can see how temperatures have risen since that same year. Temperatures fluctuate a bit, as you’d expect, but the two curves are remarkably similar. They rise together, in lockstep, and certainly seem to be strongly linked. However, this doesn’t necessarily mean that human carbon dioxide emissions are causing temperature change. There are four other ways the similarity could be explained:

Perhaps climate science has got things back to front so that, in reality, temperature rises are causing increased emissions. I hope you can see how ridiculous that is! It implies that the rapid improvements in standards of living (and hence emissions) since the industrial revolution were driven by warming rather than by human innovation. Another possibility is that emissions and temperature are both pushed by some third factor. Personally, I can’t think of anything that could do that. Can you?

A third alternative is that it’s just a coincidence. Perhaps the two curves will start to look different if we just wait a little longer. Well, this isn’t the place for a statistics lesson but it’s quite easy to work out that two curves will look this similar, by chance, less than 0.1% of the time. So it could be a coincidence but it’s not very likely. Would you bet the future of your grand-children on odds of a thousand to one against?

The final possibility is fraud. That would require conspiracy on an enormous scale as the data used in these graphs has been collected by thousands of different people over a period of 130 years. There would need to have been collusion, from the beginning, between collectors of economic data (mostly tax collectors) and collectors of temperature data (Victorian vicars, 20th century sea captains and NASA). Seems implausible to me.

So the only sensible conclusion is the scientifically conventional one—human greenhouse gas emissions are causing climate change.

The graphs also show that 1500 billion tonnes of carbon dioxide have elevated temperatures by about a degree. So, naively, we can only emit another 750 billion tonnes if we want to keep the temperature rise below 1.5 °C. This simple-minded calculation is backed up by the much more sophisticated calculations of the Intergovernmental Panel on Climate Change (IPCC) whose report last October estimated that we can emit another 770 billion tonnes.

That sounds a lot but, with seven and a half billion people on the planet, it’s only another 100 tonnes each. Unfortunately, a typical Brit produces about seven tonnes a year and so we’ll all use our remaining share within 15 years. We all need to go on a carbon diet and lose a few hundred kilograms a year each year. Two or three hundred kilograms a year is actually not that hard and I’ll talk about how in a future column. But, in my next one, I want to look at why we think a 1.5 °C rise is the maximum safe limit. It’s not a big temperature difference so why all the fuss?

First published in Marlborough News Online