The world's biggest hoax? Part 2

This post follows on from last week’s post and aims to answer the question: does evidence support the idea that human activity has played a significant role in this change?

Most people have probably heard the story of CO₂ and the greenhouse effect, but here’s a diagram to recap.



Figure 4: How greenhouse gases like CO₂ cause the Earth to warm.

You may well have seen in the news recently that CO₂ reached a permanent concentration of 400ppm in theatmosphere, which probably means nothing to you until I tell you that’s the highest it’s been for over 800,000 years. The IPCC states that CO₂ is the largest single contributor to increases in temperature between 1750 and 2011.

As is commonly pointed out by sceptics though, CO₂ has many natural sources- so who’s to say us burning fossils fuels is to blame? Well, apart from the fact that we know we’ve burnt enough fossil fuels to contribute that much CO₂ to the atmosphere, CO₂ from fossil fuels has a chemical signal that means it can be identified.

Box 2: The Suess Effect (for those who like chemistry) Carbon has three naturally occurring isotopes: C12, C13 and C14. During photosynthesis, the different carbon isotopes are not absorbed with equal preference, leading to plants and algae being depleted in C13.  As fossils fuels were once live organisms, they too carry this isotopic signal. C14 is also depleted in fossil fuels as it is radioactive. As its half life is close to 5000 years, over the millions of years fossil fuels take to form most has decayed. By monitoring changes in the ratios of different carbon isotopes in the atmosphere we can calculate the contribution of CO₂ from fossil fuels. This was first noted by Hans Suess, and so is named the Suess effect.

It’s not just fossil fuel combustion either- greenhouse gas emissions (22% of the total according to the IPCC) also come from other human activities, such as methane from farming and CO₂ from deforestation.

The above evidence has contributed to the IPCC being able to state that ‘It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in GHG concentrations and other anthropogenic forcings together. The best estimate of the human induced contribution to warming is similar to the observed warming over this period.’

In other words, the very best interpretation of all the very best evidence by the very best collaboration of climate scientists suggests that we are causing the most extreme climate change seen on Earth for thousands, if not millions of years. I can’t argue with that.