Researchers say that extensive modelling shows a four degree temperature rise will be the threshold beyond which CO2 will start to increase more rapidly, as natural carbon 'sinks' of global vegetation become "saturated" and unable to sequester any more CO2 from the Earth's atmosphere.
Carbon sinks are natural systems that drain and store CO2 from the atmosphere, with vegetation providing many of the key sinks that help chemically balance the world - such as the Amazon rainforest and the vast, circumpolar Boreal forest.
Initially, higher atmospheric CO2 will encourage plant growth as more CO2 stimulates photosynthesis, say researchers. But the impact of a warmer world through drought will start to negate this natural balance until it reaches a saturation point.
The modelling shows that global warming of four degrees will result in Earth's vegetation becoming "dominated" by negative impacts - such as 'moisture stress', when plant cells have too little water - on a global scale.
Carbon-filled vegetation 'sinks' will likely become saturated at this point, they say, flat-lining further absorption of atmospheric CO2. Without such major natural CO2 drains, atmospheric carbon will start to increase more rapidly - driving further climate change.
The ISI-MIP team used seven global vegetation models, including Hybrid - the model that Friend has been honing for fifteen years - and the latest IPCC (Intergovernmental Panel on Climate Change) modelling.
The study has been published in a special issue of the journal Proceedings of the National Academy of Sciences.