Toward Understanding and Predicting Regional Climate Variations and Change
Report from a NOAA Science Challenge Workshop
September 20-22, 2011
Boulder, CO
Workshop Organizing Committee
Harold BrooksNSSL
Randall Dole (chair)ESRL
Leo DonnerGFDL
David FaheyESRL
Mitch GoldbergNESDIS
Wayne HigginsNCEP
Martin HoerlingESRL
Arun KumarNCEP
Robin WebbESRL
Workshop Program Committee
Derek Arndt NCDC
Harold Brooks NSSL
Randall Dole (chair)ESRL
Leo Donner (co-chair)GFDL
Lisa Goddard IRI
James KinterCOLA
Siegfried Schubert NASA GMAO
Claudia Tebaldi Climate Central
Scott WeaverCPC
Robert Webb ESRL
Workshop Support
Ann KeaneESRL
Barbara HerrliESRL
Executive Summary
This report summarizes findings of a Science Challenge Workshop “Toward Understanding and Predicting Regional Climate Variations and Change.” Scientists from across NOAA, other agencies and institutions, universities and the private sector participated in this workshop. The focus of this workshop was on high priority regional science problems where significant scientific progress is possible over the next 5-6 years.Plenary presentations introduced examples of outstanding problems in understanding and predicting regional climate variations and change, including extreme events. These plenary presentations were followed by breakout sessions in which the scientists further refined and identified key challenges and discussed gaps, needs and opportunities to advance understanding and predictions of regional climate variability and change, with emphasis on high priority problems identified by the groups. This report summarizes comments and recommendations from scientists within and outside the agency on science needs and opportunities that will informdevelopment of NOAA’s science priorities andthe its new 5-year Research Plan.
The report illustratesthrough specific examples and case studies the nature and scope of regional climate challengesthat exist and their relevance to NOAA, the nation and the world. It identifies key requirements for scientific expertise in atmosphere and ocean dynamics, atmospheric composition, atmosphere-land surface interactions, and climate modeling within NOAA and with NOAA’s partnersnecessary to address regional climate science challenges. The report emphasizes the importance of integrating and coordinating these diverse scientific capabilities to achieve rapid scientific progress in scientific understanding and predictions of regional climate variability and change. It includes a discussion of challenges and needs for communicating scientific knowledge and uncertainties on problems in regional climate science. The report concludes with a summary of potentialnear-termpriorities in observations, process understanding and modeling neededto advance scientific understanding and predictions of climate variations and change at regional scales.
Table of Contents
Executive Summary
1. Introduction
2. Priority Topics and Key Science Questions: What is and isn’t known
3. Addressing Science Gaps
a) Observations
b)Process understanding
c) Modeling
d) Communication of present state of knowledge and uncertainties
4. Conclusions and Recommendations
Appendix 1: Participants
Appendix 2: Workshop Agenda/Minutes
Appendix 3: List of Acronyms
Appendix 4: Examples of Major Regional Climate Science Challenges
A1. Extreme heat waves of Europe and Russia
A2. Drought over Africa
A3. Extreme daily rainfall
A4. Arctic sea ice loss
1. Introduction
Regional climate events and trends during the last century havegreatlyinfluenced public perceptions of climate variability and change. In some cases, regional climatetrends or high impact eventswere unanticipated,leaving society unprepared or unable to adapt. The full spectrum of climate variability on regional scales remains to be determined, as well as the extent to which future changes and variability can be anticipated to help provide society with information to prepare forand adapt topotential impacts. Progress in anticipating future regional climate conditions will require significant advances in observations, process understanding and modeling capabilities, as well as the ability to clearly and accurately communicate scientific knowledge and remaining uncertainties on regional climate science.
Toward this end, enhanced global observations that describe regional climate conditions and trends and identify potential forcings and improved models that can more reliably test hypotheses of physical causes and system interactionswill be crucial. Physical process understanding must also advance substantially. Consider, for example, African drought. Understanding and predicting drought in Africa has enormoushuman and geopolitical implications and provides a major, ongoing test of our scientific understanding. The entire Sahel experienced prolonged drying during the latter half of the 20th Century, and often has often been cited as a poster child for climate change. Yet, it remains unclear to what extent natural climate variability or human influences, either via changes in land use, modification of atmospheric chemical composition and aerosols, or externally forced changes in ocean conditionsplayed a role in the sustained drying (Rotstayn and Lohmann 2002; Giannini et al 2003; Lu and Delworth 2005; Hoerling et al 2006; Giannini et al. 2008). Recent years have shown a modest recovery in summer rains over the western and central Sahel, butdrought has intensified over the tropical east African nations ofEthiopia, Kenya, and Somalia. A vital question is whether these regionalclimate anomalies are a manifestation of long-term climate change and thus aremore likely to persist (e.g. Williams and Funk 2011), or rather reflect large, multi-decadal variability that may return to previous conditions. A clearer physical understanding of the physical causes of these droughts, and especially an improvedability to distinguish transient natural variability from persistent changes due to increases in greenhouse gas forcing, will becrucial to addressing the questionwhether recent African climate conditions portend more drought in future decades.
Arctic warming and the sudden decline in the region’s sea ice pose another regional climate challengehaving major consequencesfor local inhabitants, natural ecosystems, and public perceptions of climate change. The rate of the decline in Arctic sea ice extent and thickness has been surprisingly rapid, exceeding the rate of sea ice loss simulated in most models used in the IPCC Fourth Assessment (e.g., Stroeve et al. 2007). Regional warming is strongly coupled with other climate system changes over the Arctic basin, with the Arctic amplification in surface warming closelyrelated to feedbacks produced by sea ice losses (e.g., Serreze and Francis 2006; Kumar et al. 2010). The physics of this relationship appears to be consistent with sea ice-Arctic climate relationships inferred from historical data (Bekryaev et al. 2010) . The relationship isalso similar to projections to the end of the 21st Century in response to scenarios of greenhouse gas forcing (Deser et al. 2010). However, a definitive explanation of the rapidity of the recent decline in Arctic sea icehasyet to be established. Unanswered questions includethe relative roles of human caused climate change and natural variability in both the atmosphere and ocean in explainingrapid sea ice losses, and more generally, rapid Arctic warming. Given the paucity of in situ observations over the Arctic Ocean, satellite observations will be an important source of information to ascertain the physics of recent Arctic climate trends (Wang and Key 2005),as well as validating the simulations of climate models. Improved understanding of the physical and dynamical processesinfluencing the Arctic Ocean, including an assessment of heat transports between various ocean basins, and advances in sea ice modelingwill also be important to better understandwhat factors may have caused the recent rapid declines and whether they should be expected to continue.
Another major scientific challenge of vital importance isto understand regional climate changes that are not consistent with changes anticipated from climate model simulations. These “dogs not barking in the night” are sources of great public confusion regarding climate change. They also provide major challenges to scientistsin understanding and modeling regional climate change. The absence of a warming trend over the central and southern United States during the past century is especially noteworthy (e.g., Knutson et al. 2006). Particularly striking is a strong cooling trend since the early 1900s in summertime maximum temperatures at virtually all meteorological observing stations located between the Rocky and Appalachian Mountains (Fig. 1). Expectations based on IPCC model simulations were that central North America would be a “climate change hot spot” insofar as that region’s rate of temperature rise would appreciably exceed increases in the global mean temperature. This expectation was due in part to the geographical location in the continental interior, as well as land surface feedbacks associated with anticipated soil moisture decreases in the region. Yet, the last two decades have witnessed coolsummertime conditions over the central US (Fig. 2, top) that have been quite favorable for crops. Rainfall has been much above normal during recent decades, consistent with the region’s coolness,contributing to numerous floodingevents along the Mississippi River and the Red River (of the north) drainage basins (Fig. 2, bottom). Figure 2 also shows time series of the summer season decadal averages from the 22 members of CMIP3 simulations. The ensemble mean (black line) reveals a signal of regional warming during 2000-2009 that is comparable in amplitude to the record warmest decade observed during the 1930s, the period of the Dust Bowl and prolonged severe drought (see Schubert et al. 2004, Cook et al. 2009 for discussions on the causes of the Dust Bowl drought). The absence of warming during the last two decades appears to be a very low probability outcome of the simulations (gray lines depict simulated extreme decadal anomalies), with no single model run generating decadal coolness during 2000-2009. The mean signal, under business as usual scenarios for anthropogenic greenhouse gas emissions for the upcoming decade, is expected to exceed the previous hottest summer over the central U.S. Canscientists reconcile such projections of record summertime warming over the nation’s heartland with the relative absence of warming in recent decades, and what are the implications for potential predictability?
This example offers opportunities for a holistic appraisal of a regional climate science challenge that considerall aspects of the system, including physical science of the coupled ocean-atmosphere system, land surface and biological processes, atmospheric chemistry and aerosols. This problem is clearly of high societal and economic relevance to the nation. It is worth noting that had the 2010 heat wave over western Russia occurred instead over the North American grain belt, the global economic consequences would have been much more dire. Estimates indicate that Russia suffered a 40% drop in its grain harvest due to this heat wave, which reduced world grain stocks from 79 days of consumption to 72 days. The Midwest U.S. grain yield is, by comparison to Russia, much larger. Estimates suggest that had such a 2010 heat wave occurred over Chicago, world grain stocks could have plummeted to a record low 52 days.
There are various proposed explanations for thisrelative lack of warming over parts of the continental U.S.. One involves the region’s sensitivity to decadal variations in the North Pacific and North Atlantic Oceans, and that cool conditions of recent decades over the US have been strongly affected by natural, slow variations in the adjacent oceans (e.g. Wang et al. 2009). Another holds that local hydrologic feedback occurring at scales not represented in global climate models can lead to a regional “warming hole” (Pan et al. 2004). Land use changes and surface vegetation modifications have also been shown to modify a region’s surface energy balance and alter its sensitivity to external forcing (e.g. Diffenbaugh 2005). An additional possibility is the effect of changes in atmospheric chemical composition and aerosols (IPCC 2007), though the impacts of aerosol forcing on regional climate responses are not well understood (National Research Council, 2005). The question remains open to what extent the response to changing aerosols is regional or global, and the effects of U.S. aerosols on U.S. regional climate have not been sufficiently studied (CCSP 2008; 2009). Recognizing that the coarse resolution models used in prior IPCC-related studies, and the uncertainties in local forcings and sensitivities to them, some regional climate change projections have low confidence. Whether the U.S. warming hole of recent decades represents strong natural variability that is temporary, or whether the cooler and moister climate state over much of central North America during the growing season is itself a “climate change” rather than natural variability is currently unresolved.
The science challenge related to this and the other examples mention of regional climate science problems represent majorintegrating themes that will require diverse expertiseacrossdisciplines within and outside NOAA and strong partnerships between NOAA and external partners in order to be addressed.
REFERENCES
Bekryaev, R. V., I. V. Polyakov, V. A. Alexeev, 2010: Role of polar amplification in long-term surface air temperature variations and modern arctic warming. J. Climate, in print, doi: 10.1175/2010JCLI3297.1
Cook BI, Miller RL, Seager R. Amplification of the North American ‘‘Dust Bowl’’ drought through human-induced land degradation. Proc Natl Acad Sci 2009, 106:4997–5001.
CCSP 2009: Atmospheric Aerosol Properties and Climate Impacts, A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. [Mian Chin, Ralph A. Kahn, and Stephen E. Schwartz (eds.)]. National Aeronautics and Space Administration, Washington, D.C., USA
CCSP, 2008: Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). Department of Commerce, NOAA's National Climatic Data Center, Washington, D.C., USA, 100 pp.Deser, C., R. Tomas, M. Alexander, D. Lawrence, 2010: The seasonal atmospheric response to projected arctic sea ice loss in the later twenty-first century. J. Climate, 23, DOI: 10.1175/2009JCLI3053.1
Diffenbaugh, N., 2005: Atmosphere-land cover feedbacks alter the response of surface temperature to CO2 forcing in the western United States Climate Dynamics (2005) 24: 237–251 DOI 10.1007/s00382-004-0503-0
Giannini A, Saravanan R, Chang P. Oceanic forcingof Sahel rainfall on interannual to interdecadal timescales. Science 2003, 302:1027–1030.
Giannini A, Biasutti M, Verstraete MM. A climate model-based review of drought in the Sahel: desertification, the re-greening and climate change. Glob Planetary Change 2008, 64:119–128.
Hoerling M, Hurrell J, Eischeid J, Phillips A. Detectionand attribution of twentieth-century northernand southern African rainfall change. J Clim 2006,19:3989–4008.
IPCC, Climate Change 2007: The Physical Science Basis. Contribution of Working
263 Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate
264 Change [Solomon, S.D. et al. (eds.)]. Cambridge University Press, Cambridge, 2007.
Knutson, T., and Coauthors, 2006: Assessment of twentieth-century regional surface temperature trends using the GFDL CM2 coupled models. J. Climate, 19, 1624–1651.
Kumar, A., J. Perlwitz, J. Eischeid, X. Quan, T. Xu, T. Zhang, M. Hoerling, B. Jha, and W. Wang (2010), Contribution of sea ice loss to Arctic amplification, Geophys. Res. Lett., 37, L21701, doi:10.1029/2010GL045022.
National Research Council, Radiative Forcing of Climate Change: Expanding the
313 Concept and Addressing Uncertainties, National Academies Press, Washington, DC,
314 2005.
Pan, Z., R W. Arritt, E.S. Takle, W J. Gutowski Jr., C J. Anderson, and M Segal, 2004: Altered hydrologic feedback in a warming climate introduces a‘‘warming hole’’. Geophy. Res. Lett. L17109, doi:10.1029/2004GL020528
Rotstayn LD, Lohmann U. Tropical rainfall trends and the indirect aerosol effect. J Clim 2002,15:2103–2116.
Schubert SD, Suarez MJ, Pegion PJ, Koster RD, Bacmeister JT. On the cause of the 1930s Dust Bowl. Science 2004, 303:1855–1859.
Serreze, M. C., and Francis, J. A., 2006: The Arctic amplification debate. Climatic Change, 76, 241–264.
Serreze, M. C., M. M. Holland, and J. Stroeve. 2007. Perspectives on the Arctic's shrinking sea ice cover. Science 315(5818): 1533-1536, doi:10.1126/science.1139426.
Screen, J. A., and I. Simmonds, 2010: The central role of diminishing sea ice in recent arctic temperature amplification. Nature, 464, doi:10.1038/nature09051.
.
Stroeve, J., M. M., Holland, W. Meier, T. Scambos, and M. Serreze, 2007: Arctic sea ice decline: faster than forecast. Geophys. Res. Lett. 24, L09501, doi:10.1029/2007GL029703.
Stroeve, J. M., and co-authors 2008. Arctic sea ice plummets in 2007. Eos, Trans. Am. Geophys. Un. 89, 13–20.
Wang, Xuanji, Jeffrey R. Key, 2005: Arctic Surface, Cloud, and Radiation Properties Based on the AVHRR Polar Pathfinder Dataset. Part II: Recent Trends. J. Climate, 18, 2575–2593.
Wang, H., S. Schubert, M. Suarez, J. Chen, M. Hoerling, A. Kumar, P. Pegion, 2009:
Attribution of the seasonality and regionality in climate trends over the United States during 1950-2000. J. Climate, 22, 2571-2590.
Williams AP, Funk C (2011) A westward extension of the warm pool leads to a
westward extension of the Walker circulation, drying eastern Africa. Clim Dyn DOI 10.1007/s00382-010-0984-y
Figure 1. The trend in seasonal summertime (June, July, August) daily maximum surface temperatures for the period 1910-2009. The trend is plotted as total change/100 yrs. Each circle denotes a station location based on the USHCN data set. Cooling (warming) trend plotted in blue (red), and the larger trends are denoted with larger circles.
Figure 2. The observed decadal anomalies (relative to 1901-2000 climatology) in average daily surface air temperature (top) and rainfall (bottom) over the central US for summer (June, July, August). The region of averaging is between 100°W-75°W, 37°N to 45°N, a region between the Rocky and Appalachian mountains. Color bars are observed as derived from the NCDC US Climate Division data set. Curves are based on the CMIP3 simulations using observed external radiative forcing for the 20th century, and then an A1B scenario for the 21st Century. The solid curve are decadal anomalies of the 22 member CMIP3 ensemble, and the gray curves denote the extreme decadal states occurring among the 22 members for each decade.