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Not all roadmaps follow viable roads

There is certainly concern in the academic community that the world is following an emissions pathway which will result in overshoot in comparison with the Paris goal to limit warming of the climate system to well below 2°C. Because of this, considerable effort is being expended on developing pathways and roadmaps that may help change that course; but not all are viable. One example emerged recently from a group of researchers representing the Stockholm Resilience Centre, the Royal Swedish Academy of Sciences, the International Institute for Applied Systems Analysis, the Potsdam Institute for Climate Impact Research and the Australian-German Climate and Energy College, School of Earth Sciences, University of Melbourne. This is a distinguished group, without a doubt.

They propose a global decadal roadmap based on a simple heuristic — a ‘carbon law’— of halving gross anthropogenic carbon-dioxide emissions every decade. In addition, they propose an expansion of the renewable energy base by keeping the historical doubling times of around 5.5 years constant in the next three decades. This would yield full decarbonisation in the entire energy sector by ~2040, with coal use ending around 2030–2035 and oil use, 2040–2045. The proposed transition also calls for rapidly scaling up carbon dioxide removal by technical means from zero to at least 0.5 GtCO2/year by 2030, 2.5 by 2040, and 5 by 2050.

While the pathway proposed and its component parts do illustrate the Herculean (their terminology for the period 2020-2030) efforts required to limit warming to 1.5°C, their proposal also stretches past the point of feasibility. This starts with the premise that carbon emissions must halve each decade, which means the biggest effort is required in the first decade. This is the opposite of an outcome that a technology pathway might deliver. For example, some months ago I looked at the theoretical pathway required in the passenger automobile sector to rapidly reduce emissions to near zero in under 40 years [Link]. If this is translated into emissions for that sector, on the basis of each internal combustion engine passenger vehicle doing 10,000 miles per annum and emitting some 4 tonnes of CO2, then the emissions profile would be approximately as given in the chart below (Blue line). The red dotted line represents a halving of emissions each decade, as the ‘carbon-law’ pathway proposes. As manufacturing of electric vehicles scales up in the early years, the impact on emissions is small, but after 2030 when the internal combustion engine vehicle stock begins to fall sharply, the emission reduction accelerates. The biggest drop in emissions is in the decade 2040-2050, not 2020 to 2030 as the ‘carbon law’ dictates.

The second major issue with the proposed roadmap is the notion of doubling of renewable energy capacity every five or so years. While this has been the case in the early years of renewable energy deployment, it will not continue through to full deployment. This is simple to test by considering the last doubling, when renewable energy capacity would need to shift from half its final level of deployment to full deployment in just five years. This would require an extraordinary scale up of manufacturing capacity for wind turbine blades and solar PV cells. But after five years that capacity would then be redundant as no further major deployment of renewable energy would be required. The investors in the manufacturing facilities would be left with stranded assets had the investments been made

While it is certainly the case that initial deployment of new energy technologies can take place at exponential rates, once the technology reaches material deployment (>1% of the energy system), the rate increase begins to decline, although the absolute level of deployment may well continue to rise. This is a typical pathway for growth rates and has been seen at national level, for example in China. The very high annual growth rates of a decade ago have passed, but absolute annual growth is higher today than it was in 2007.

Limiting warming to well below 2°C or even to 1.5°C will certainly require very rapid change in the energy system, but this may still follow historic patterns of deployment, even if the overall rate of change can be accelerated to levels not seen in prior transitions. Those transitions have followed hard to break deployment ‘laws’ that may yet guide the current transition.

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