Are we causing it?

More generally, scientists ask “What is causing it?”. If it is true that the world is warming, and that is indeed a bad thing, then it’s actually good news if we are causing it, as it means we can stop doing whatever it is that’s causing the problem!

So there are lots of things that do, or could, potentially cause temperature changes. Some examples, starting with those that have short cycles:

• Our periodic day/night and summer/winter cycles;
• Changes in ocean currents;
• Variations in the intensity of solar radiation coming from the sun;
• Variations in the amount of that solar radiation that gets reflected, e.g. due to sulphur dioxide in the atmosphere following volcanic eruptions;
• Variations in ‘greenhouse gases’ (like CO2) in the atmosphere, that cause heat to get trapped.

What does it mean for scientists to identify the cause? It means finding a model of the relative contributions of these different factors that best fits the data. If we found such a model that fits the data very well, it doesn’t prove that’s the cause - it is just the best thing until something better comes along. This is how science always works - Newton’s theory of gravity doesn’t prove that an object will fall to Earth. It just fits the data extremely well, and is great at making predictions, and you would be extremely unwise to step off the top of a 300ft building on the basis that there’s no proof that you will hit the ground at about 100 mph!

For climate change, the fit we are looking for is to the long term trend, as we do not yet have the sophisticated level of modelling to predict short term temperature changes (just as we can talk of the inexorable movement of the Pacific plate, but cannot predict when the next earthquake will be).

So what have been the conclusions from the various scientific studies? Considering the same sources of information as above, and starting again with the most recent US government report NCA4 (2017):

This assessment concludes, based on extensive evidence, that it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century.

This was in agreement with the earlier IPCC report AR5 (2014) [1] :

This evidence for human influence has grown since AR4. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.

Berkeley Earth provides even clearer arguments. In 2012 they completed a study [2] that concluded:

The best explanation for the warming seen over the past 250 years is human greenhouse gas emissions.

No contribution from solar variability was necessary to make a good match. This means that the conclusions are more worrying than those of an earlier 2007 IPCC [presumably AR4] report that concluded only that “most” of the warming of the past 50 years could be attributed to humans.

The graph below shows the average annual temperature (black) with the simple model based just on the amount of CO2 in the atmosphere, and volcanic activity (that has a cooling effect).

As can be seen, there is a very good fit between the prediction based on CO2/volcanic activity (red) and the actual measured results (black).

They also showed that it was the increasing CO2 that has led to recent temperature increases, i.e. it wasn’t that there was a temperature increase for some other reason, that then resulted in increased levels of CO2. They also demonstrated how, based on the amount of C-14 in the atmosphere, and the data of CO2 emissions from fossil fuels, that it is the burning of fossil fuels that is accounting for the increase in CO2, not some other natural cause.

In summary - any claim that increased CO2 from human activity is not the primary cause would need to put forward an alternative explanation, which fits the data just as well. No such alternative has yet come forward.