A UN committee has released Pathways to Deep Decarbonization, their vision of a world that in coming decades reduces greenhouse gas emissions enough to prevent catastrophic climate change. Sound familiar? That's the kind of future we mapped out for New York City in 90 by 50. Appropriately, the UN study covers the whole planet, not just NYC, but the similarities and differences between the two studies are intriguing.
First, while they both analyze emissions in 2050, neither study uses forecasting, although that’s what’s used by most studies to estimate future growth in emissions based on current economic trends and energy use. Forecasting analyzes the effects of hypothetical strategies: for instance, what will happen if we were to implement all energy reduction measures with less than a five-year payback? Successful strategies make emissions grow more slowly or even shrink.
However, forecasting results are limited by what strategies are selected. For instance, if the analysis is restricted to approaches with less than a five-year payback, reductions by 2050 will be very small. In fact, if forecasts are based only on investments that would be considered acceptable in today’s business world, predicted reductions will fall far short of what is needed.
In contrast, both Urban Green and the UN studies use “backcasting.” This means the analysis starts with a target of emissions reductions needed to avoid a temperature increase above 2ºC, and then determines what it will take to hit that target. Climate science tells us that total global emissions by 2050 must be kept below 825 billion metric tons of CO2. This is a very challenging target. In 90 by 50, we estimated that a drop in emissions of 5.6% per year was needed, far faster than most forecasted estimates have thought possible. The UN study focused on the term “deep decarbonization,” which will require “a profound transformation of energy systems” and result in “global net emissions of greenhouse gases (GHG) (that) approach zero by the second half of the century.”
How can we do this? Both reports place an emphasis on super-efficient construction and retrofits, electrification of almost all loads, and then supplying those vastly reduced loads with carbon-free power. The similarities probably aren’t completely an accident: we had spoken to Sachs and his team about 90 by 50. More importantly, they would have come to the same place anyway, as deep decarbonization requires eliminating fossil fuels, and we must dramatically reduce loads first to make the conversion to a clean grid affordable.
There is one major difference between our analysis and the UN’s effort: they allow a substantial role for carbon capture and storage (CCS). With this technology we continue to burn some fossil fuels, capture the CO2 either before or after combustion, and pipe it somewhere safe, like into an old mine or deep under the ocean. CCS is getting considerable development support, but compared to improved energy efficiency in buildings or renewable electricity production, it’s not a good choice for two reasons: the CO2 left after burning coal takes up about three times the volume of the coal, even when condensed into a liquid or solid, so where will it go? And even if there is enough available volume, under the sea, for instance, how do we know it will stay there over geological time and not burst out? Major coal exporters like Australia made much greater use of CCS in their projections than the US team, but any CCS leaves the plan subject to complaints that they are using an unproven technology that would be best avoided.
That aside, the team under Jeffrey Sachs’ leadership has done a bang-up job. We congratulate them and hope the report gets the attention it deserves, especially at the upcoming UN conference on climate change. It may take a while for the concept to gain widespread respectability, but backcasting our way to deep decarbonization shows the only sure path to a sustainable future.
The UN SDSN report will be discussed at the UN Summit on climate change in New York this September. Join us at the People's Climate March on September 21 to speak out in support of working to avert a climate disaster.