finding 2.3 : key-finding-2-3

The interconnected Earth–atmosphere–ocean climate system includes a number of positive and negative feedback processes that can either strengthen (positive feedback) or weaken (negative feedback) the system’s responses to human and natural influences. These feedbacks operate on a range of time scales from very short (essentially instantaneous) to very long (centuries). Global warming by net radiative forcing over the industrial era includes a substantial amplification from these feedbacks (approximately a factor of three) (high confidence). While there are large uncertainties associated with some of these feedbacks, the net feedback effect over the industrial era has been positive (amplifying warming) and will continue to be positive in coming decades. (Very high confidence)

This finding is from chapter 2 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The net effect of all identified feedbacks to forcing is positive based on the best current assessments and therefore amplifies climate warming. Feedback uncertainties, which are large for some processes, are included in these assessments. The various feedback processes operate on different time scales with carbon cycle and snow– and ice–albedo feedbacks operating on longer timelines than water vapor, lapse rate, cloud, and atmospheric composition feedbacks.

Description of evidence base: The variety of climate system feedbacks all depend on fundamental physical principles and are known with a range of uncertainties. The Planck feedback is based on well-known radiative transfer models. The largest positive feedback is the water vapor feedback, which derives from the dependence of vapor pressure on temperature. There is very high confidence that this feedback is positive, approximately doubling the direct forcing due to CO2 emissions alone. The lapse rate feedback derives from thermodynamic principles. There is very high confidence that this feedback is negative and partially offsets the water vapor feedback. The water vapor and lapse-rate feedbacks are linked by the fact that both are driven by increases in atmospheric water vapor with increasing temperature. Estimates of the magnitude of these two feedbacks have changed little across recent assessments.9e2542c2-865e-4863-98d1-242b11016592 404ac5a1-396d-4939-b065-54b5c143986d The snow- and ice-albedo feedback is positive in sign, with the magnitude of the feedback dependent in part on the time scale of interest.33e625ef-2b6b-4025-b666-7d892d86674c 2e0d8b4e-7fae-449d-8c54-2c99e42da22e The assessed strength of this feedback has also not changed significantly since IPCC 2007.c54b9473-cdc3-4f22-97a8-4df5253f9682 Cloud feedbacks modeled using microphysical principles are either positive or negative, depending on the sign of the change in clouds with warming (increase or decrease) and the type of cloud that changes (low or high clouds). Recent international assessments9e2542c2-865e-4863-98d1-242b11016592 404ac5a1-396d-4939-b065-54b5c143986d and a separate feedback assessment835f8401-b358-4078-aff6-e072f3d3178d all give best estimates of the cloud feedback as net positive. Feedback via changes in atmospheric composition is not well-quantified but is expected to be small relative to water-vapor-plus-lapse-rate, snow, and cloud feedbacks at the global scale.17b9846f-fc66-401a-b416-545ba3970853 Carbon cycle feedbacks through changes in the land biosphere are currently of uncertain sign and have asymmetric uncertainties: they might be small and negative but could also be large and positive.ba53cfd5-a373-4a9c-8e2b-0dacc8e9a135 Recent best estimates of the ocean carbon-cycle feedback are that it is positive with significant uncertainty that includes the possibility of a negative feedback for present-day CO2 levels.37519d28-5ca8-4f24-919d-759b277e749a 6c114a62-cf08-420d-8470-055289b6e180 The permafrost–carbon feedback is very likely positive, and as discussed in Chapter 15: Potential Surprises, could be a larger positive feedback in the longer term. Thus, in the balance of multiple negative and positive feedback processes, the preponderance of evidence is that positive feedback processes dominate the overall radiative forcing feedback from anthropogenic activities.

New information and remaining uncertainties: Uncertainties in cloud feedbacks are the largest source of uncertainty in the net climate feedback (and therefore climate sensitivity) on the decadal to century time scale.9e2542c2-865e-4863-98d1-242b11016592 835f8401-b358-4078-aff6-e072f3d3178d This results from the fact that cloud feedbacks can be either positive or negative, depending not only on the direction of change (more or less cloud) but also on the type of cloud affected and, to a lesser degree, the location of the cloud.835f8401-b358-4078-aff6-e072f3d3178d On decadal and longer time scales, the biological and physical responses of the ocean and land to climate change, and the subsequent changes in land and oceanic sinks of CO2, contribute significant uncertainty to the net climate feedback (Ch. 13: Ocean Changes). Changes in the Brewer–Dobson atmospheric circulation driven by climate change and subsequent effects on stratosphere–troposphere coupling also contribute to climate feedback uncertainty.b3bbc7b5-067e-4c23-8d9b-59faee21e58e f1b24246-385c-428c-8bdf-1a8409eb805b 3023fc1c-3084-48e0-8c97-ce9cdee3bd7d 08f77804-cb22-4f07-854c-6514e9b2c49f c7a8ad7e-9636-424b-b311-74468d87152a bd82df37-5937-4e92-b623-a1691cedfcfc

Assessment of confidence based on evidence: There is high confidence that the net effect of all feedback processes in the climate system is positive, thereby amplifying warming. This confidence is based on consistency across multiple assessments, including IPCC AR5 (IPCC 2013f03117be-ccfe-4f88-b70a-ffd4351b8190 and references therein), of the magnitude of, in particular, the largest feedbacks in the climate system, two of which (water vapor feedback and snow/ice albedo feedback) are definitively positive in sign. While significant increases in low cloud cover with climate warming would be a large negative feedback to warming, modeling and observational studies do not support the idea of increases, on average, in low clouds with climate warming.

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