The Paris Agreement means that signatories will be obliged to "to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels" (http://unfccc.int/files/meetings/paris_nov_2015/application/pdf/cop_auv_template_4b_new__1.pdf). To gain some understanding of the practical implications of this I turned first to the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), confining myself to the Summary for Policymakers of Working Group 1.
From this I deduced that the most optimistic scenario discussed there, referred to as "RCP2.6" would deliver a temperature increase in the order of 1.7°C, above pre-industrial levels. This was 0.2°C above the target in the Paris Agreement but that is near enough for our purposes.
This involved a bit of guesswork as the report doesn't give that figure as such. In Table SPM.2 (p 23) it gives the expected rise between 1986-2005 and 2081-2100 as 1.0°C (with wide margins of error) and on page 5 it gives the rise between 1850-1900 and 2003-2012 as 0.78°C.
Associated with this scenario, there is a "budget" of 990 gigatonnes of global CO2 emissions from 2012 to 2100 (Table SPM.3, p27).
Actual global CO2 emissions for 2012, 2013 and 2014 are estimated to total 106 gigatonnes, leaving a budget of 884 gigatonnes for 2015 onwards (source: EDGARv4.3, European Commission, Joint Research Centre (JRC)/PBL Netherlands Environmental Assessment Agency. Emission Database for Global Atmospheric Research). CO2 emissions in 2014 were 35.7 gigatonnes. If they continued at that rate, the budget would be exhausted in just under 25 years. With a steady, straight-line decline beginning in 2015, they would have to reach zero in twice that time, ie about 50 years - a rate of decline of 2% a year.
However, it is hardly realistic to assume that emissions will decline steadily over the next few years. Developing countries such as India claim a right to increase emissions until living standards have risen. We can probably expect the profile of emissions to be characterised by three phases - a short early phase of flat or rising emissions (reductions in the developed world equalled or outweighed by increases in the developing world); a middle phase of rapid reductions to a very low rate of emissions; and a final phase in which the emissions most difficult to eliminate are gradually phased out. Using a simple equation and reducing the problem to three straight line changes in the rate of emissions, and using different assumptions, it is easy to generate a range of emissions profiles consistent with remaining within the budget. One such profile is shown below:-
This profile assumes global CO2 emissions flatlining between 2015 and 2025 at a rate of 36 gigatonnes a year, slightly above the 2014 rate. They then decline rapidly down to 5% of their 2015 rate by 2050. This remaining 5% is slowly phased out over the remaining 50 years of this century.
Note that this appears to be a rather optimistic scenario with a 95% reduction in emissions over 25 years - an annual decline of 3.8% of the starting rate.
The immediate stimulus for this study was a discussion on airport expansion - Heathrow versus Gatwick. According to the UK's Committee on Climate Change, emissions from aviation amounted to about 6% of total UK emissions in 2013. If the UK's total emissions were to decline at the same rate as the whole world, UK aviation emissions would have to reduce by 2050 even if they were by then to represent 100% of total UK emissions. They would then have to be phased out completely over the following 50 years. Current policy is to hold aviation emissions in 2050 to their 2005 rate (see https://www.theccc.org.uk/archive/aws2/Aviation%20Report%2009/21667B%20CCC%20Aviation%20AW%20COMP%20v8.pdf)
Bear in mind that the developed countries will be expected to reduce their emissions sooner and faster than the developing countries. Hence the developed world may need to be completely carbon neutral by 2050 with aviation possible only if powered by non-fossil fuels. Without currently unforeseen developments in technology it looks inconceivable that aviation can continue on anything like its present scale if we are serious at aiming for the 1.5°C, bearing in mind that the scenario discussed above only delivers an expected 1.7°C limit on the temperature rise. Finally I have taken no account of non-CO2 emissions (eg methane emissions from oil and gas wells and leaky storage facilities).
A clear implication of all this is that aviation itself must decline substantially between now and 2050 and it is no longer sensible to contemplate airport expansion. Investment in additional airport capacity would soon become stranded assets.
If the government decides to allow expansion at either Heathrow or Gatwick, this will be a clear signal that it has no intention of honouring perhaps the most critical commitment in the Paris Agreement.
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