150 years ago, the global concentration of CO2 in the atmosphere was 280 ppm. Current CO2 levels have risen to over 400 ppm, leaving many to question what our world will look like if CO2 levels double to 560 ppm compared to the pre-industrial average. A recent study examined this question, taking into account previous studies with multiple kinds of evidence, including how clouds and water vapour will affect and be affected by global warming.
The study projected that an increase in global temperature, compared to the pre-industrial average, to 4.5 °C is unlikely to occur but that an increase of 1.5 °C will almost certainly be exceeded. The study estimates that there is a 66% chance that we can expect a temperature increase between 2.6-3.9 °C.