As the end of the second week of climate talks draws nigh in Warsaw, a group of high profile scientists have laid out what needs to happen to stay below two degrees of global warming. The answer? Deep greenhouse gas cuts, and no more excuses for delay.
A matter of urgency
In its latest report, the Intergovernmental Panel on Climate Change (IPCC) said limiting global warming requires “substantial and sustained” cuts in greenhouse gas emissions.
In a new letter in Nature Climate Change, co-chair of the IPCC Thomas Stocker and Myles Allen from Oxford University consider two arguments used to suggest emission cuts can be delayed.
The first is that scientists have slightly lowered their assessment of how big the warming effect of carbon dioxide is on the planet – known as the climate sensitivity.
The other argument is that reducing emissions of pollutants like black carbon and methane is a more achievable way to limit total warming, instead of tackling carbon dioxide emissions.
The authors examine both arguments, concluding neither “buys time” to delay efforts to reduce carbon dioxide. Delaying emissions cuts now will make it harder to reduce warming in the long run, they say.
Peak and decline
Two degrees is the most widely accepted political threshold beyond which the risks posed by climate change are considered unacceptably high.
The IPCC calculates that to stay below two degrees of warming, carbon dioxide concentrations in the atmosphere must peak by about 2050. That means carbon dioxide emissions will have to peak much sooner.
The question the letter deals with is: What’s the penalty for delaying cutting carbon dioxide emissions?
The cost of delay
Carbon dioxide has a predictable relationship with temperature. The warming we get is almost directly proportional to the total amount of carbon dioxide we release into the atmosphere.
Using this relationship, scientists can calculate a measure known as the transient climate response to carbon emissions ( TCRE). Technically-speaking, this is the change in average global surface temperature change for every trillion tonnes of carbon we emit.
In its latest report, the IPCC lowered its estimate of TCRE by about 25 per cent compared to the previous report released in 2007. But contrary to suggestions, this has little impact on the case for urgent mitigation, say the authors.
Why? A lower TCRE might mean the rate at which emissions need to be cut to stay under two degrees could be a little less drastic if we started now. But emissions are currently rising by 1.8 to 1.9 per cent per year, which is more than any reduction in warming that might come from a lower TCRE.
In other words, any possible benefit of a lower climate sensitivity is more than balanced out by the rate at which we’re already putting carbon dioxide into the atmosphere, the authors say.
And our current rate of emissions means each year we delay cuts significantly ups the chances that we’ll end up with more than two degrees warming. The authors say in the paper:
“If the same level of effort required in 2010 to limit carbon dioxide-induced warming to two degrees were applied starting in 2015, the resultant peak warming would be 10 per cent higher, at 2.2 degrees.”
In other words, the longer we delay emissions cuts, the faster those cuts will have to be to stay within the two degree budget. And the faster cuts have to be, the harder they are to achieve.
Limiting peak warming to two degrees above pre-industrial means reducing carbon dioxide emissions by 2.4 per cent per year from 2015 (green). Delaying cutting emissions until 2025 means the same rate of cuts results in 2.5 degrees warming (yellow). Staying below two degrees would require much faster emissions cuts. Source: Stocker & Allen ( 2013).
A second letter by Allen and colleagues, also published in Nature Climate Change, looks what the impact might be of cutting emissions of black carbon, methane and tropospheric ozone. These factors only have a short lifespan in the atmosphere, a few years to few decades.
The paper concludes that while cutting emissions of these factors might reduce the warming we see in the short-term by about 0.5 degrees Celsius, the impact on global temperature rise will be very small unless carbon dioxide is cut at the same time. The paper says:
“Proponents of climate mitigation through non-carbon dioxide measures rightly stress the difficulty of reducing carbon dioxide emissions: what this result illustrates is that the harder it turns out to be to mitigate carbon dioxide, the smaller the impact of non-carbon dioxide climate pollutants on peak warming, in both relative and absolute terms.”
If we are confident of reducing carbon dioxide emissions fast enough to limit warming to two degrees, the extra 0.5 degrees saving from cutting non-carbon dioxide factors means we could potentially delay mitigation by 12 years, the authors conclude.
But the higher the limit we allow for total warming, the shorter the opportunity to delay. For peak warming of three degrees, the extra 0.5 degree saving equates to a nine-year delay, dropping to four years for four degrees of warming.
Dr Chris Huntingford of the NERC Centre for Ecology and Hydrology, co-author on the second letter, tells us:
“The argument implicit in this paper is that over the next few decades, the emphasis really should remain on tackling the longer-lived gases, and predominantly carbon dioxide”.
The time is now
Both papers show there’s very little room for manoeuvre in climate change mitigation – a strong point to be making with the closing stages of the international climate policy negotiations in Warsaw underway. If no progress is made towards legally-binding emissions reductions targets, it seems won’t be for lack of supporting evidence about why we should get on with it.