There is very high confidence in projected changes in temperature extremes over the United States based upon the convergence of evidence from multiple model simulations, analyses, and assessments.

This finding is from chapter 5 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The tropics have expanded poleward in each hemisphere over the period 1979–2009 (medium to high confidence) as shown by a large number of studies using a variety of metrics, observations and reanalysis. Modeling studies and theoretical considerations illustrate that human activities, including increases in greenhouse gases, ozone depletion, and anthropogenic aerosols, cause a widening of the tropics. There is medium confidence that human activities have contributed to the observed poleward expansion, taking into account uncertainties in the magnitude of observed trends and a possible large contribution of natural climate variability.

Description of evidence base: The Key Finding is supported by statements of the Intergovernmental Panel on Climate Change’s Fifth Assessment Report47a5196b-4fba-4fdb-8647-8945627725bb and a large number of more recent studies that examined the magnitude of the observed tropical widening and various causes.37d85f6f-8d91-45e8-bf65-0ae8aee523a6 d377ad38-12f5-4186-998f-5643dfc6d141 025b4915-8b20-4d53-b097-c737d39f3d62 d5eb689b-306c-4092-92ab-80958283a00c a03de91a-43b9-4ca8-ba49-b9a55c28c493 798360ca-4177-462c-991a-c7a512d9287c 633af288-eb2c-4e4f-b5ba-9397ad897d7d a80ce47f-ac9a-43d2-9179-acad0e28e05a afc9d3fd-49bb-413c-bdec-75d9b75c2b8b c933c170-d353-4a2b-8855-d6b147d58d16 Additional evidence for an impact of greenhouse gas increases on the widening of the tropical belt and poleward shifts of the midlatitude jets is provided by the diagnosis of CMIP5 simulations.46b31794-a9c0-402d-a813-a65c4e87097b 938444d5-cf1e-43b2-93d8-d8e403a23344 There is emerging evidence for an impact of anthropogenic aerosols on the tropical expansion in the Northern Hemisphere.aaf41b2e-d066-40c4-8bbf-d14ed62e13d6 6e2d8723-4a29-4bea-97d2-b300a6c18cd6 Recent studies provide new evidence on the significance of internal variability on recent changes in the tropical width.d5eb689b-306c-4092-92ab-80958283a00c c18f45a8-0b79-49ed-b24f-98e994961b8a c84efce6-8036-4c20-bf98-d0d5b987cd2c

New information and remaining uncertainties: The rate of observed expansion of tropics depends on which metric is used. The linkages between different metrics are not fully explored. Uncertainties also result from the utilization of reanalysis to determine trends and from limited observational records of free atmosphere circulation, precipitation, and evaporation. The dynamical mechanisms behind changes in the width of the tropical belt (e.g., tropical–extratropical interactions and baroclinic eddies) are not fully understood. There is also a limited understanding of how various climate forcings, such as anthropogenic aerosols, affect the width of tropics. The coarse horizontal and vertical resolution of global climate models may limit the ability of these models to properly resolve latitudinal changes in the atmospheric circulation. Limited observational records affect the ability to accurately estimate the contribution of natural decadal to multi-decadal variability on observed expansion of the tropics.

Assessment of confidence based on evidence: Medium to high confidence that the tropics and related features of the global circulation have expanded poleward is based upon the results of a large number of observational studies, using a wide variety of metrics and data sets, which reach similar conclusions. A large number of studies utilizing modeling of different complexity and theoretical considerations provide compounding evidence that human activities, including increases in greenhouse gases, ozone depletion, and anthropogenic aerosols, contributed to the observed poleward expansion of the tropics. Climate models forced with these anthropogenic drivers cannot explain the observed magnitude of tropical expansion and some studies suggest a possibly large contribution of internal variability. These multiple lines of evidence lead to the conclusion of medium confidence that human activities contributed to observed expansion of the tropics.

Provenance

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