The Intergovernmental Panel on Climate Change (IPCC) has presented various “Representative Concentration Pathways” (RCPs) to represent possible trajectories of atmospheric concentrations of green house gases (GHGs) over the next century. The scariest one is RCP8.5, which projects a mean temperature rise of 3.7°C, with a likely increase range of 2.6 to 4.8°C by 2100. RCP8.5 has been called the “business as usual” RCP, which means it’s considered a likely development path if current trends continue. According to the scientists who proposed RCP8.5 in 2011, its trajectory is plausible based on assumptions of “….low income, high population and high energy demand due to only modest improvements in energy intensity.”   Are these assumptions reasonable based on current trends - that is, do they represent a plausible  “business-as-usual” scenario? Let's look at income first.  Per RCP8.5, global GDP will reach around $250 trillion in 2100.  That does reflect a fairly low rate of economic growth, given a current (2014) global GDP of  about $78 trillion. How does that figure jive with current predictions of global economic growth?

The IMF expects a global GDP growth rate of 3.6% in 2017. A comparable growth rate (around 3.6 give or take) is predicted to continue for the next several decades for the G20 economies*, which represent about 84% of global GDP. Since the G20 economies are such a large part of the global GDP pie, I’m going to use projections about the G20 to anchor the discussion of whether the RCP8.5 projection of global GDP in 2100 is a reasonable approximation of “business as usual”.

The PWC World in 2050 report expects GDP for the G20 economies to double by 2037 and triple by 2050. The Carnegie International Economics Program estimates that GDP for the G20 economies will reach $161.5 trillion by 2050. Taking the Carnegie projection, if the G20 still represented 84% of the global economy in 2050, that means the global GDP in 2050 would be about $192 trillion in 2050 (161/.84) – an admittedly very rough estimate; for one thing, it is more likely that the G20 would represent a smaller percentage of the world economy by then, because smaller, less developed countries tend to grow at a faster rate than large developed countries. But I’m trying to be very modest in my projections here.

If we accept $192 trillion as a low-ball proxy for global GDP in 2050, what growth rate would be necessary for global GDP to reach $250 trillion by 2100, as per RCP8.5? Less than 1%. Given that current and expected annual global GDP growth is roughly 3.6%, RCP8.5 hardly represents a “business as usual” scenario.

Next, population growth.


*The G20 consists of: Argentina, Australia, Brazil, Canada, China, France, Germany, India, Indonesia, Italy, Japan, Mexico, Russia, Saudi Arabia, South Africa, South Korea, Turkey, the United Kingdom, the United States and the European Union.