Totaling CO2 Emissions | Nature
The article, "Climate change: Too much of a bad thing," by Gavin Schmidt and David Archer reports on two papers in Nature that total [carbon and] CO2 emissions over time.
The European Union has adopted a goal of keeping temperatures below 2°C above pre-industrial levels. Others argue for a stabilization of atmospheric CO2 concentrations at 350 parts per million (p.p.m.), or 450 p.p.m., or higher. ... Papers elsewhere in this issue by Meinshausen et al. and Allen et al. explore the uncertain relationships between carbon emissions and climate response, with the aim of better estimating how much additional CO2 might indeed be too much. ...
Comparing the bottom-line results from the two studies is tricky because of the use of different units, different base periods and different experimental design. However, given that humans have already emitted roughly 520 GtC to the end of 2008, Allen and colleagues' best-estimate CWC — 2°C per 1,000 GtC emitted from 1750 to 2500 (compared with 2000–2050 in Meinshausen et al.) — implies that another 480 GtC would put us over 2°C with more than 50% likelihood. This is broadly consistent with the 310-GtC estimate from Meinshausen et al. over a much shorter time frame.
[Update on June 11: Edited excerpt above and added below, from the "Informal Background Q&A" distributed by Nature. - ed]
Q17: 1 Trillion tonnes of carbon emissions – what temperature does that correspond to?
A: This depends on which trillion tonne you look at.
First option (Allen et al.): One trillion tonne of carbon.
In this case, the trillion tonne refers to carbon emissions (1TtC) over all times. According to their study, this would induce a most likely warming of 2°C just because of CO2. Any warming from other human influences on the climate (net effect of greenhouse gases, aerosols etc.) would be on top of that.
Second option (Meinshausen et al.): One trillion tonne of carbon dioxide.
Here, the trillion tonnes (in the paper indicated as 1000 GtCO2) refers to carbon dioxide emissions (1TtCO2 = 0.27 TtC) refers to emissions over the time horizon 2000 to 2050. According to Meinshausen et al. this leads in a likely achievement of the 2°C target (25% probability of exceeding 2°C), taking already the non‐CO2 effects into account.
