Live stream: IPCC AR5: Three numbers that matter, and numbers that matter less than you think

A talk by Professor Myles Allen, Tuesday 14 October, 9 am BST

Watch it live. Tweet using #ecilive

What the IPCC 5th Assessment Report has to say to the negotiators in Paris 2015

“This lecture provides an overview of the climate change issue, highlighting what are, in my view, the most important findings of the latest IPCC report and their implications for climate negotiations under the United Nations Framework Convention on Climate Change (UNFCCC). We will focus on three numbers that matter a lot, and mention along the way some other numbers that matter rather less than you might think.

The first important number is 95%, the level of confidence the climate science community now has that human influence is the dominant cause of the warming observed over the past 60 years. I will explain where this number comes from, with a quick (and colourful) introduction to the methods used for “detection and attribution” in the IPCC Working Group I report, and explain why the apparent “pause” in ocean surface warming over the past decade or so doesn’t really change the big picture.

The second important number is 40.3 degrees C (105 degrees F), the national average temperature high over Australia on January 7th, 2013. While bad enough for Australia, the significance of that “Angry Summer” for the rest of the world is as an example of the kind of damaging weather event that, subsequent studies have shown, was made substantially more probable by human influence on climate. With the IPCC Working Group II finding that the impacts of climate change on human and natural systems are now evident on all continents and across the oceans, understanding the links between climate change and harmful weather events is becoming important for the UNFCCC’s new “Loss and Damage” agenda, and a key focus of Oxford’s climateprediction.net/weatherathome project.

The third number is the big one: one trillion tonnes. That is the total amount of fossil carbon that the IPCC estimates can be dumped into the atmosphere over the entire Anthropocene epoch while keeping the resulting warming to likely less than two degrees Celcius. Over half a trillion tonnes has already been emitted, and accounting for warming due to other forms of pollution cuts down the remaining “carbon budget” further still. This puts the mitigation challenge into perspective, and helps explain why the IPCC Working Group III report found such a pivotal role for carbon capture and storage in scenarios that have some chance of meeting the two degree goal.

What are the numbers that matter less than you might think? One of them, although much tweeted, is “97% of scientists agree…” I’ll explain how this kind of opinion poll isn’t really relevant to how science, or the IPCC, actually works. Another over-rated number is the Equilibrium Climate Sensitivity (the subject of earlier climateprediction.net experiments), which turns out to matter much less than people thought. Finally, if you are hoping for a purely scientific argument that two degrees is the threshold for dangerous anthropogenic interference in the climate system, you will be disappointed: the IPCC reports make it clear that the assessment of what is dangerous has an ethical and moral dimension and cannot be resolved by any purely technical assessment.”