The previous estimates of the amount of CO2 that could be released before world temperature rose to 1.5C above pre-industrial levels was 70 billion tonnes of Carbon after 2015 but a reassessment allows for a “carbon budget” of another 240 billion tonnes of emissions before 1.5C above pre-industrial levels.
In a 2014 report, the UN’s Intergovernmental Panel on Climate Change (IPCC), said the carbon concentration in the atmosphere should not exceed 450 parts per million (ppm) of CO2 equivalent (CO2eq) by 2100 for a “likely”, 66-percent, chance of 2 C. According to the IPCC report, the concentration in 2011 was already 430 ppm CO2eq. On the basis of the IPCC figures, a budget of 400 billion tonnes was calculated as the maximum amount of CO2 humanity can emit into the atmosphere from 2011 and still keep the 1.5 C goal in sight.
For 2015, that number dropped to about 245 billion tonnes.
The new analysis, however, estimates the remaining budget from 2015 to be closer to 880 billion tonnes of CO2—nearly four times bigger than the UN estimate, the research team said.
The Paris Agreement has opened debate on whether limiting warming to 1.5◦C is compatible with current emission pledges and warming of about 0.9◦C from the mid-nineteenth century to the present decade. We show that limiting cumulative post-2015 CO2 emissions to about 200 GtC would limit post-2015 warming to less than 0.6◦C in 66% of Earth system model members of the CMIP5 ensemble with no mitigation of other climate drivers, increasing to 240 GtC with ambitious non-CO2mitigation.We combine a simple climate–carbon-cycle model with estimated ranges for key climate system properties from the IPCC Fifth Assessment Report. Assuming emissions peak and decline to below current levels by 2030, and continue there after on a much steeper decline, which would be historically unprecedented but consistent with a standard ambitious mitigation scenario (RCP2.6), results in a likely range of peak warming of 1.2–2.0◦C above the mid-nineteenth century. If CO2emissionsare continuously adjusted over time to limit 2100 warming to 1.5◦C, with ambitious non-CO2mitigation, net future cumulativeCO2emissions are unlikely to prove less than 250 GtC and unlikely greater than 540 GtC. Hence, limiting warming to 1.5◦C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation. Strengthening near-term emissions reductions would hedge against a high climate response or subsequent reduction rates proving economically, technically or politically unfeasible.