finding 10.2 : key-finding-10-2

Climate change and induced changes in the frequency and magnitude of extreme events (e.g., droughts, floods, and heat waves) have led to large changes in plant community structure with subsequent effects on the biogeochemistry of terrestrial ecosystems. Uncertainties about how climate change will affect land cover change make it difficult to project the magnitude and sign of future climate feedbacks from land cover changes (high confidence)

This finding is from chapter 10 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The timing, frequency, magnitude, and extent of climate extremes strongly influence plant community structure and function, with subsequent effects on terrestrial biogeochemistry and feedbacks to the climate system. Future interactions between land cover and the climate system are uncertain and depend on human land-use decisions, the evolution of the climate system, and the timing, frequency, magnitude, and extent of climate extremes.

Description of evidence base: From the perspective of the land biosphere, drought has strong effects on ecosystem productivity and carbon storage by reducing microbial activity and photosynthesis and by increasing the risk of wildfire, pest infestation, and disease susceptibility. Thus, future droughts will affect carbon uptake and storage, leading to feedbacks to the climate system.440379ab-fe10-4c0b-b652-7107ba91f4c2 Reduced productivity as a result of extreme drought events can also extend for several years post-drought (i.e., drought legacy effects).95a21b96-f699-4b5c-8281-e1d6e2c8398f 04604a57-6638-4154-bc44-6e0997d727f4 abe49f4d-90c4-40e2-a4b9-a58158c00560 Under increased CO2 concentrations, plants have been observed to optimize water use due to reduced stomatal conductance, thereby increasing water-use efficiency.3b6c1b16-374c-4201-82e2-d7126fbb47de This change in water-use efficiency can affect plants’ tolerance to stress and specifically to drought.b673e746-6eca-4b77-8a62-e1ae9ed6380a

Recent severe droughts in the western United States (Texas and California) have led to significant mortality and carbon cycle dynamics ( e126059c-67f3-4522-8381-ae2499296312 Carbon redistribution through mortality in the Texas drought was around 36% of global carbon losses due to deforestation and land use change.7316c70f-6b67-4a68-a23a-c9d9f604c003

New information and remaining uncertainties: Major uncertainties include how future land-use/land-cover changes will occur as a result of policy and/or mitigation strategies in addition to climate change. Ecosystem responses to phenological changes are strongly dependent on the timing of climate extremes.e5b9d663-1638-464a-aa71-bfef43b6c4d1 Due to the complex interactions of the processes that govern terrestrial biogeochemical cycling, terrestrial ecosystem response to increasing CO2 levels remains one of the largest uncertainties in long-term climate feedbacks and therefore in predicting longer-term climate change effects on ecosystems (e.g., Swann et al. 2016b673e746-6eca-4b77-8a62-e1ae9ed6380a).

This finding was derived from figure -.2: Confidence / Likelihood

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