By Nadir Jeevanjee
Nadir Jeevanjee is a climate scientist in Princeton University’s Program in Atmospheric and Oceanic Sciences. He has a background in theoretical physics and currently studies clouds and radiation in the climate system. He works for NOAA’S Geophysical Fluid Dynamics Laboratory, a major U.S. climate modeling center. Nadir is active in communicating climate science to non-specialists and to the public, both through his own talks as well as through the outreach group Climate Up Close.
Nadir has answered two related questions:
Q 1:
Net zero (where CO2 emissions = C02 removal) is predicted to stop progressive warming of the planet, (regardless what the atmospheric CO2 will be when that equilibrium is reached). My suspicion is that there is a threshold concentration of atmospheric CO2 above which warming will occur. In other words, if warming was negligible at 325 parts per million (ppm) (early 1970s) and warming was established at 350 ppm (1988), as long the atmospheric concentration is above 325-ish, global warming will progress. Is this not true and why?
Q 2:
How much will temperatures rise in the next 10 years and what are the reasons?
A:
There are a few questions here. The first is about how much warming one can expect on a short time scale (e.g. a decade or two). Global warming is driven mostly by carbon dioxide, which has been increasing at a rate of roughly 20 parts per million (ppm) per decade. We can then estimate (via, say, Eqn. 21 here) a corresponding warming rate of about 0.15 Celsius per decade. This is a gradual increase that accumulates over time, leading to the roughly 1.3 Celsius of total global warming we have now.
It is not the case that warming was “negligible” in the 1970s but then all of a sudden became significant in the 1980s. Rather, there are natural year-to-year fluctuations in the Earth’s mean temperature (mostly due to El Niño), which means that the warming of roughly 0.5°C in the 1970s was hard to discern relative to the natural noise in the Earth system. Once this warming got slightly larger, it started to “poke out” above the noise and some scientists (e.g. Hansen) became confident that this was the global warming signal emerging (see top left panel of Fig. 10.1 here).
Regarding the validity of the net zero paradigm: Earth system models show to a good approximation that zeroing out emissions is both necessary and sufficient to halt global warming (see e.g. Fig. 1 here). This is true no matter what the CO2 concentration is when net zero is reached. Note also that CO2 concentrations in the atmosphere won’t remain constant after net zero; they will instead decline for some time, as the ocean and land biosphere continue to take up the CO2 we’ve already emitted.
