Climate Letter #2138

An organized media effort is now underway to send out a new message about our human ability to hold the global temperature increase within the UN’s target limit of 1.5C.  I first wrote about this effort last Friday, in CL#2134, when I first learned about the way it was put together.  It was surprising to see that Michael Mann was deeply involved, and I wondered how quickly the promotion would unfold.  Yesterday’s Washington Post gave an answer with an article in its Perspective series, which you can read right here:  https://www.washingtonpost.com/outlook/2022/02/23/warming-timeline-carbon-budget-climate-science/.  

The key point being made deals with the broad question of what happens to the ongoing trend of temperature increases once we have put an end to the increase in CO2 emissions.  For a long time scientists have said they would keep going up by at least another half degree once complete stoppage happens, for reasons that are quite technical and not easily explained in everyday language.  I am not so sure the general public, all over the globe, ever fully understood the reasons or gave them much thought.  The public is almost certainly more concerned about the primary problem, which is the need to bring CO2 emissions that are under our control all the way down to zero.  At the moment, while emission growth has slowed, we are struggling to keep the yearly totals from reaching new annual highs.  With additional emissions from natural sources added on, both CO2 and methane, which is the second most powerful greenhouse gas, are increasing year by year by amounts that seemingly will never start making a downturn.  Those who realize the difficulty of simply turning this around, much less heading things all the way back to zero, have ample reason to feel despair.  A possible extra half degree beyond that point, which is far off in the future, is probably not weighing on many minds.

Whether or not my thoughts about how humans will react to this particular effort are accurate, the issues that have been brought forward are interesting from a scientific point of view and much worth discussing. Assuming that zero emissions of both CO2 and methane can be reached within a reasonable amount of time, what should we think will happen to temperatures? The combination of natural sources and sinks of various gases is a vastly complicated subject, and I don’t see how any scientist can stand still and say it has largely been resolved. New studies are sure to appear that may be favorable in some cases and not so favorable in others. It’s hard to keep a running score. Potential changes in solar albedo are just as problematic as greenhouse gases if not more so. James Hansen keeps insisting that cleaning up sulfate aerosols will have a major warming effect. Hansen also told us in one of his papers that rapid melting of the ice sheets could produce enough ice-cold meltwater to pull temperatures down by a substantial amount for a number of decades, before bouncing back up again when the melting slowed. That would at least buy us some time, and also serve as a handy way to neutralize a large amount of excess heat now being stored in the ocean depths. That’s only a small taste of a long list of separate issues affecting temperatures in a big way.

There is one more article on the subject, in the form of an explainer, that I just caught up with, published about a year ago on the Carbon Brief website: https://www.carbonbrief.org/explainer-will-global-warming-stop-as-soon-as-net-zero-emissions-are-reached.  It has additional information of pertinence that you should find interesting.

Carl