They used two models that measure the levels of carbon dioxide (CO2) in the atmosphere as an indicator of rising temperature. One model found that CO2 levels could rise to 549 parts per million (ppm) by 2100, which would be a 50 percent increase from CO2 levels today. The other model found that CO2 levels could rise to 1,168 ppm, which would be three times the levels of CO2 today.
The rise in temperature depends on 1) when we reach the peak in carbon emissions, 2) how quickly the CO2 levels fall, and 3) whether global warming starts a chain of events in nature that increases and prolongs the warming, ocean heating and oxygen depletion.
Extreme events of ocean oxygen depletion leading to anoxia (dead zones) are thought to be prime candidates for explaining some of the large extinction events in Earth history including the largest such event at the end of the Permian 250 million years ago.
From Long-term ocean oxygen depletion in response to carbon dioxide emissions from fossil fuels : Abstract : Nature Geoscience:
Decreased oxygen solubility from surface-layer warming accounts for most of the enhanced oxygen depletion in the upper 500 m of the ocean. Possible weakening of ocean overturning and convection lead to further oxygen depletion, also in the deep ocean. We conclude that substantial reductions in fossil-fuel use over the next few generations are needed if extensive ocean oxygen depletion for thousands of years is to be avoided.
CO2 is just one global warming gas; there are others, like methane (CH4), nitrous oxide (N2O) and water vapor.
We can do nothing to change the warming gases released by volcanoes, wildfires, permafrost, decomposition, or as part of the water cycle.
We can, however, reduce warming gases released from fossil fuel production and use, animal husbandry, rice cultivation, biomass burning and waste management.
Plant trees, grow organic food, eat less meat, drive fuel efficient or electric vehicles, use recycled materials, buy alternative energy, reduce the amount of waste sent to landfills and do not buy products that are harvested in a nonsustainable way.
If you would like more information on oxygen depletion leading to dead zones, please read this post: Expanding Dead Zone in Gulf of Mexico.