The impacts research community is a very large community. It's multidisciplinary and includes biologists, physical scientists, and people undertaking social science. So it's a community where consensus is not the easiest thing to obtain. But there is, by and large, a broad understanding that as temperatures rise, the impacts will become more widespread and more severe, and that these will then start to be felt increasingly as you go from 1.5°C to 2.5°C to 3.5°C. These impacts tend to become quite broad at about 2°C.
Take, for the example, species extinctions linked to climate change. It's estimated that 20% to 30% of species will become susceptible to extinction with an increase in temperature of 2°C above pre-industrial levels.
Also, if temperatures are sustained at that level for a very long period of time, then we risk substantially increasing sea levels. That's not just by centimetres or fractions of a metre, as had been projected by the IPCC for the year 2100, but by many metres—perhaps 10 metres—as a result of the melting of large parts of large ice sheets.