Income groups
Two choices sit behind every income-group trend: which year's classification to use, and whether to average countries or divide totals. Made the other way, this chart told a different story.
How much CO₂ a dollar of each income group's output carries: the group's emissions divided by its GDP in constant 2015 dollars, with every country counted in the group the World Bank placed it in that year. That makes some of the movement membership rather than intensity: the low-income line steps down each time a large emitter leaves the group, China in 1997 (it was back for 1998), India in 2007, and Vietnam and Uzbekistan in 2009.
Both halves of that sentence are choices, and each one changes the chart. The classification moves: about half of the economies classified in 1990 are in a different group today. And an average of country ratios weights Bhutan like China, where a ratio of totals weights each economy by its size. This chart used to make both choices the other way, grouping every year by today's classification and averaging country by country, and that version told a different story.
In 1990 the old chart ranked Upper middle income the most carbon-intensive group, at 1.00 kg per dollar. Grouped as classified that year and weighted by output, the most carbon-intensive was Low income at 1.39 kg per dollar.
The top of the corrected ranking is mostly two countries. China was classified low income until 1998 and India until 2006, and weighted by output they dominate that group's figure through the 1990s. The old chart filed both under the groups they hold today and then averaged each group country by country, so China counted as one economy in fifty-odd, in a group it would not reach for another twenty years. Every other income-group rollup on this site cuts to a single recent year, where today's classification is the right one; over a trend it is not.