COP21: UN report shows that the national pledges are inadequate

You may have heard that the UN has produced a "synthesis report" on climate change. The report is 66 pages and very technical. There's a short overview - but key parts are almost unintelligible.

Here's what it probably meant to say:

• If global temperature reaches two degrees above the pre-industrial level (say, 1850) we'll be in bad, indeed catastrophic, trouble.
• If we can keep total greenhouse gas emissions to no more than 1,000 Gigatons of CO2, ever, then we have a 66% chance of keeping the temperature increase below two degrees.
• 119 countries have made pledges to reduce or reduce the growth in their greenhouse gas emissions.
• If all these countries keep their pledges and others follow their current trends then greenhouse gas emissions will keep rising until at least 2030. But they won't rise quite as much as they might have without the pledges.
• It's likely that by 2030 we will have emitted about 750 Gtons of CO2 and will be emitting about 57 Gtons per year.

But history will not stop in 2030. What follows?

• If emissions stop rising in 2030 and continue at the same rate we'll have emitted the whole 1,000 Gtons by 2035.
• If we start to reduce emissions in 2030 and do so by equal steps of 6 Gtons we'll have emitted the whole 1,000 Gtons by 2040.

But these cases are unreal for a mixture of technical, humanitarian and political reasons. For instance, the world cannot go from emissions growth to 10% pa reduction in one year. So all the practical scenarios (based on known technology) in which we keep the risk of catastrophic climate change below 66% require emissions to peak well before 2030. The earlier the peak the easier the transition.

The bottom line: The pledges are inadequate. Even if met they create a serious risk of catastrophic climate change.