Because it is based on an aggregation of publications. The executive summary is approved line-by-line by all of the governments in the UN so it's much tamer than the report would suggest, but the synthesis of global data and studies is quite reliable.
It's all data-driven rather than opinion and while it paints a dark future, there is strong agreement in the scientific community that we have not yet triggered unavoidable positive feedback on a catastrophic scale. Even if we stopped emissions dead right now the future wouldn't be roses and perfume but it's not a nightmare. For the near and medium term the most important factor is still the amount of CO2 we emit, which far outweighs the current feedback loops and the ones that are likely to activate soon.
For instance siberian/canadian permafrost, clathrates and arctic albedo have all been triggered to some extent, but the releases have been outpaced by anthropogenic emissions. This will change, first with arctic sea ice, but at this point we are reasonably certain they won't pick up exponentially until we expect them to.
It's all data-driven rather than opinion and while it paints a dark future, there is strong agreement in the scientific community that we have not yet triggered unavoidable positive feedback on a catastrophic scale. Even if we stopped emissions dead right now the future wouldn't be roses and perfume but it's not a nightmare. For the near and medium term the most important factor is still the amount of CO2 we emit, which far outweighs the current feedback loops and the ones that are likely to activate soon.
For instance siberian/canadian permafrost, clathrates and arctic albedo have all been triggered to some extent, but the releases have been outpaced by anthropogenic emissions. This will change, first with arctic sea ice, but at this point we are reasonably certain they won't pick up exponentially until we expect them to.