I'm not in any way formally involved in either the IPCC or IPBES, although I've been involved indirectly in analysing many elements of both the language of the reports and the science underlying their predictions.
Today, The Guardian reported that a leaked copy of an IPCC report scheduled for release soon indicated that, well, the climate-change situation is in fact worse than has been previously reported in IPCC documents.
If you're a biologist, climatologist, or otherwise-informed person, this won't come as much of a surprise. Why? Well, the latest report finally recognises that the biosphere is not just some big balloon that slowly inflates or deflates with the whims of long-term climate variation. Instead, climate records over millions of years show that the global climate can and often does shift rapidly between different states.
This is the concept of 'tipping points'.
While the concept of tipping points has some scientific uncertainty at the global scale, it really embraces what ecologists call ecological hysteresis. This concept was beautiful described in one of my favourite papers of all time by Marten Scheffer and colleagues 20 years ago: Catastrophic shifts in ecosystems. It basically states that ecosystems have multiple stable states, and that the 'energy' (environmental forcings) that cause an ecosystem to shift from one state to another differs depending on the direction of transition.
As an example, let's say an ecosystem such as the Amazon rain forest biome (wet, tropical forest) is pushed via warming and increasing fire severity and frequency to a new stable state - a dry savanna (something not only predicted for the Amazon in the near future, but also something that has definitely happened in the past). If the system is cooled down by the same amount it was heated to shift from tropical forest to savanna, it is unlikely it will shift back into a tropical forest. To get the system back to its original state requires much more cooling (and wetting) before the tropical forest re-emerges.
i.e., A → B ≠ B → A
There are many physical reasons this happens (e.g., fire changes moisture regimes, which change weather patterns, etc.), but there are a lot of biological reasons as well. Biotic transitions and extinctions caused by the disturbing/changing process can also cause hysteresis - once certain species disappear, the system cannot go back to its original state, because some components (species) of its original state are gone forever.
Back to the IPCC. It seems that this report finally addresses a common critique of the IPCC reporting process that such tipping points have been largely ignored in the past. Why? You need look no further than the IPCC process itself.
For the same reasons that the language in the IPCC reports inflates measures of uncertainty due in part to the conservative political process that must ensue to generate consensus, much of the science has stuck to the conservative or even 'middle road' of predictions.
In other words, to make the predictions palatable to politicians and the business community, they've watered down the message.
So, it appears that we're getting closer to the truth - it's bad, and it's going to get a lot worse before it gets better. Thank you to the IPCC and the authors of this imminent report for being bolder and embracing a more truthful narrative.
CJA Bradshaw