Climate Letter #1275

New research into the way a warmer climate affects tree growth in northern woodlands.  The work shows that soil moisture is the key, and that two-thirds of the time, due to warming-and-drying processes, soil moisture may be low enough to have a  negative effect on growth.  “These results show that low soil moisture will slow down or eliminate any potential benefits of climate warming on tree photosynthesis even in moist, cold climates like Minnesota, Canada and Siberia.”  That would of course reduce their uptake of CO2, as depended upon by many climate models.
https://phys.org/news/2018-10-warmer-climate-drier-negative-impacts.html

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There is a new report on the impact of global aridity change on human populations.  This is an important study which for some reason has not been picked up by any of the regular science news services.  It does have open access, and the message can be clearly gained by reading the Abstract and Significance statement at the beginning, plus the Conclusion for the principal details.  “Understanding how the water cycle will respond to climate change has important implications for human populations……The largest impacts are expected to occur in Africa, where current water scarcity levels are highest, and where the largest populations coincide with widespread drying.”

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Carbon Brief has a thorough and very readable review of the IPCC report.  It even offers considerable information on the uneven nature of atmospheric warming—a subject I have been harping on lately in these letters—supported by graphic imagery.  “Warming greater than the global annual average is being experienced in many land regions and seasons, including two to three times higher in the Arctic. Warming is generally higher over land than over the ocean……In fact, chapter one (pdf) of the report notes that 20-40% of the global population live in regions that have already experienced warming of more than 1.5C in at least one season.”  (An interpretation of the full meaning of the imbalance is not provided anywhere, as if it were not important.  I believe that oversight is a mistake.)

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A video discussion about estimates of climate sensitivity, calling it “The Most Important Number in Climate Change.”  Several scientists explain why the median conventional estimate of 3C for a doubling of CO2 is too low.  Michael Mann, for one, sees a range of three to four degrees applicable to current circumstances, as a way of explaining effects now being observed.  It is important to get the number right for the purpose of projecting what the actual impacts will be like.  Proper knowledge of those impacts would best instruct the kinds of action required to hold back the things we don’t want to see, as Bill McKibben explains.  The warnings issued by the IPCC in its report, dire as they may be, are based on the lesser impacts foreseen by sensitivity estimates in the 3 degree area, as compared with those of 3.5 or 4.  The implications are substantial.

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