5.8 How do you compare different emission reduction targets and carbon budgets?
With difficulty! But we need to pause and think about this. I want to address the question of timescales and measurements. In both the scientific and policy literature you'll see all sorts of timescales and measures for carbon targets and budgets, making it almost impossible to compare like with like.
Common dates for targets for policies are 2012 (end of the first Kyoto period), 2020, 2050 and 2100. But you'll no doubt find others thrown in for good measure too (a recent paper looks at carbon budgets from 1750 to 2500). For any target, the baseline is important, and common ones are 1990 (baseline for the Kyoto Protocol), 2000, or ‘now’ (the current year) – but you'll also find 1750 (start of the Industrial Revolution).
The baseline matters. We may have a target for a 50% reduction in annual emissions by 2050. Against a 1990 baseline that's 50% of 36 GtCO2-eq, so 18 GtCO2-eq in 2050. But if we opt for a baseline from ‘now’ for which the latest data are for 2007, that's 50% of 48 GtCO2-eq, so 24 GtCO2-eq in 2050. So, by moving the baseline we've just increased the emissions allowed in 2050 by a third! Moving baselines matter a lot – watch out for them.
Did I mention CO2-eq? There's another one. Sometimes quantities will be in carbon, sometimes as CO2 (CO2 = 0.27 × carbon), and sometimes as CO2-eq. CO2-eq is a larger figure than CO2-only, but the relationship between them is complex (see Section 5.6 – ‘So how much warming is likely?’) – especially when CO2-eq might mean ‘the six gases governed by the Kyoto Protocol expressed as the equivalent emissions of CO2’ or ‘all of the enhanced greenhouse effect expressed as the equivalent emissions of CO2’ (see Section 2.7 – ‘What do all of these different measures and units mean?’)
It can be difficult for even seasoned professionals to compare a GtC budget for 1750–2500 with a CO2 budget for 2000–2050 versus a CO2-eq for 1990–2050!
But that's what we'll have to do for the next few questions on targets and budgets and temperature increase. I've tried to work the figures so that we can make direct comparisons – but you might not want to take my word for it!
Author's reflection on scientific data and policy
There's a wider question here about the social context in which scientific work occurs; but, as far as basic measurements goes, this is an area where the scientific community really does itself very few favours. Scientists often complain about a mismatch between policy interventions and what the science says is required. But if scientists want policy makers to see the relevance of the evidence, then they have to spell it out to them in language that policy makers can relate to. That's not to say that the questions, methods, analysis and conclusions have to be framed in policy-relevant terms – clearly, natural processes have no regard for the ever-changing political processes and administrative boundaries invented by people. But if scientists want their findings to be reflected in policies, then they must help policy developers by interpreting their findings in a policy-relevant context.
Policy developers can and do play on this – given the ambiguity and confusion in the scientific literature, it's all too easy to make just about any claim and defend it on the basis of one scientific output or another. Science progresses through vigorous disagreement and debate, so I'm certainly not suggesting we stifle that. But unnecessary obfuscation through differences in basic things such as the metrics used is unhelpful.
5.7 What is a ‘safe’ limit of climate change?