Radiative forcing is a natural process whereby solar radiation is absorbed by the Earth and thermal radiation is re-emitted into the atmosphere. Some of this emitted energy is held in the atmosphere, which causes further release of CO2 and water vapour. These gases then prevent more thermal radiation escaping into space leading to a positive feedback cycle.
Figure 1 - Schematic of the runaway greenhouse effect
The runaway greenhouse effect will lead to the complete vaporisation of the oceans if radiative forcing becomes greater than 12 Wm-2. This phenomenon occurred on Venus four billion years ago, due to its proximity to the sun. It will also happen on Earth in approximately 1-2 billion years when the sun’s luminosity increases by 10%.
Four representative concentration pathways (RCP) have been predicted for CO2 emissions up until 2100 dependent upon anthropogenic factors (figure 2).
However there are certian limitations for these predictions due to permafrost thawing, which is a natural process that releases CO2 into the atmosphere. By 2100 the average annual emisssion rate will be ~10-16 billion tonnes.
Figure 2 - A graph showing the Representative Concentration Pathways
To model predicted temperature rises of the Earth by 2100, we started off modelling the RCP2.6 and RCP8.5 scenarios with(out) permafrost to get the predicted CO2 emissions.
The RCP2.6 scenario gave a value of 421ppm without permafrost and a value of 423ppm with, the RCP8.5 scenario gave a value of 937ppm without permafrost and a value of 950ppm with. Using this information the surface temperature for each scenario with(out) permafrost was calculated (Figure 3).
The RPC8.6 scenario results in a significant temperature change but it’s not enough to trigger the runaway greenhouse effect and neither is permafrost thawing.
Figure 3 - Resultant surface temperature by 2100
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