Global Climate Change - How Warm is Warm?

While you may or may not believe in the concept of anthropogenic global climate change, a recent study by University of Washington's Adrian Raftery et al entitled "Less than 2 degrees C warming by 2100 unlikely" should at least give you pause to ponder the direction that our fragile world is heading.  This study is particularly pertinent given that a 2 degrees Celsius temperature increase is often seen as the "tipping point" that could lead to environmental calamity. 

The authors of the paper suggest that the Intergovernmental Panel on Climate Change (IPCC) projections for global temperatures out to 2100 are not based on a fully statistical approach.  The authors use a country-specific model to predict future greenhouse gas emissions based on each nations' population, per capita GDP and carbon intensity (carbon dioxide emissions per unit of GDP).  They note that carbon intensity for many nations has already peaked as shown in this graphic which expresses carbon dioxide emissions in tonnes of CO2 per USD100,000 in Purchasing Power Parity:

Based on 50 years of past data, the authors calculated the following parameters going out to the year 2100:

1.) Growth in per capita GDP - 1.8 percent annually with slower GDP growth in developing economies

2.) Decline in carbon intensity - 1.9 percent annually

3.) Global population - increases by 4 billion, from the current 7.2 billion to 11.2 billion with much of the increase in the population of Sub-Saharan Africa

When looking at the contribution of the three factors to uncertainty about carbon dioxide emissions in 2100, the authors measured that GDP per capita accounted for 50 percent of total uncertainty, carbon intensity accounted for 48 percent of total uncertainty and global population accounted for only 2 percent of total uncertainty.  

With this background in mind, here is a histogram showing the authors' predictive distribution of global mean temperature changes relative to the period from 1861 to 1880 in degrees Celsius:

Calculations show that, by 2100, there is a 90 percent chance that global temperatures will increase by 2.0 to 4.9 degrees Celsius with a median of 3.2 degrees Celsius.  There is a 5 percent chance of warming by less than 2 degrees Celsius and a 1 percent change of warming by less than 1.5 degrees Celsius. 

Let's close by looking at some quotes from the lead author, Adrian Raftery:

"Our analysis shows that the goal of 2 degrees is very much a best-case scenario. It is achievable, but only with major, sustained effort on all fronts over the next 80 years.  Our analysis is compatible with previous estimates, but it finds that the most optimistic projections are unlikely to happen.  We're closer to the margin than we think.  Overall, the goals expressed in the Paris Agreement are ambitious but realistic.  The bad news is they are unlikely to be enough to achieve the target of keeping warming at or below 1.5 degrees."

As we can see from this study, one of the biggest impacts on long-term greenhouse gas emissions is the change in carbon intensity.  If continuing innovations reduce global energy needs at the same time that economic output grows, we may not win the 2 degree Celsius battle but we may prevent the worst case scenario from occurring as is predicted in this rather sobering research.