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Climate Variability and Change
The following are selected highlights of recent research supported by CCSP participating agencies (as reported in the fiscal year 2008 edition of the annual report, Our Changing Planet).
CVC research encompasses activities spanning the reconstruction of the climate before the era of modern observing systems (paleoclimate) through model-based projections of future climate change. This section begins with highlights from paleoclimate research published within the past year. Paleoclimate research has proven invaluable in identifying the rates and magnitudes of past climate changes, determining mechanisms that can produce such changes, and helping to constrain estimates of climate system sensitivity in response to changes in radiative forcing. Paleoclimate studies also enable evaluations of climate model performance over far longer time periods and ranges of forcing than would be possible from only modern observational records.
The Pliocene Paradox.2During the early Pliocene 3 to 5 million years ago, the intensity of sunlight incident on Earth, the global geography, and the atmospheric concentration of carbon dioxide were similar to today's values, but surface temperatures in polar regions were much higher than today. Continental glaciers were absent from the Northern Hemisphere, and sea level was 25 m higher than at present. This paradox–that the early Pliocene climate was much warmer than today despite similar external forcing–has potential implications for climate stability. It raises the possibility that future melting of glaciers, changes in the hydrological cycle, and a deepening of the upper mixed layer of the ocean could lead to a return toward much warmer conditions similar to the early Pliocene.
Paleoclimate Evidence for Future Ice-Sheet Instability.3Conditions during the last interglacial period (129,000-118,000 years ago) as deduced from coral records provide evidence that sea level during this epoch was from 4 to over 6 m above present levels. If current trends in polar warming continue over the next century, climate conditions similar to those of the last interglacial period may result in Arctic and Antarctic ice melt, with sea level rising well beyond current estimates.
High-Resolution Paleoclimate Records.4Research on marine fossils suggests that waters in the central Gulf of California were especially warm during the Medieval Warm Period from approximately AD 900 to 1160. A present-day relationship exists between warmer sea surface temperatures in the northern Gulf and more intense development of the North American monsoon in Arizona and New Mexico. Increased monsoonal rainfall during the Medieval Warm Period has also been found in Florida and the Indian Ocean. Increased solar radiance is considered as a possible forcing mechanism.
Reconstructions of Streamflow in the Upper Colorado and South Platte River Basins.5Measurements from moisture-sensitive trees in Colorado have been used to extend streamflow records in the Upper Colorado and South Platte River basins back 300 to 600 years, significantly augmenting river gage records. The results indicate that the 20th-century climate is not representative of the range of hydrological extremes due to natural climate variability. For example, multi-year drought events more severe than the 1950s drought occurred as recently as the 19th century. Water managers are now using the paleo-streamflow reconstructions to better estimate the potential range of natural hydroclimatic variability in the Upper Colorado and South Platte River basins.
Estimated Climate Sensitivity Constrained by Temperature Reconstructions over the Past Seven Centuries.6Climate sensitivity, defined as the equilibrium response of global mean surface temperature to a doubling of carbon dioxide, is an important indicator of the potential for future global warming. The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report gives a 5 to 95% range of climate sensitivity of 2 to 4.5°C, with a most probable value of 3.0°C. Some studies suggest, however, the possibility of much higher sensitivities, at or above 8°C. A recent study examines the plausibility of such high sensitivities by using an energy balance model forced by solar, volcanic, and greenhouse gases. This model is used to simulate paleoclimate reconstructions of Northern Hemisphere temperatures over the past 7 centuries. The method involves determining the climate sensitivities that yield simulations in best agreement with proxy reconstructions. After accounting for the uncertainties in temperature reconstructions and estimates of past external forcing, the analysis suggests that the most likely range of climate sensitivity is 1.5 to 6.2°C, and that higher climate sensitivities are inconsistent with paleoclimate evidence. Studies within the era of modern climate observations are helping to illuminate interactions among climate system components, as well as the connections between longer term climate changes and short-term weather and climate phenomena, including extreme events like hurricanes.
Pollution Darkened China's Skies.27Records from more than 500 weather stations across China for the years 1954 to 2001 indicate China has darkened over the past half-century. On the other hand, in the most comprehensive study to date of overcast versus cloud-free days in China, researchers have found that cloud cover has been decreasing for the past 50 years. Less cloud cover, regardless of its cause, should have resulted in more solar radiation reaching the surface. Surprisingly, though, the data show that both solar radiation and pan evaporation decreased in most parts of China by 1.9% (3.1 Wm-2) and 2.2% (39 mm) per decade, respectively. Combined with evidence from other studies of decreased sunshine duration, reduced visibility or clearness, and elevated aerosol optical depth, it appears that air pollution produced a fog-like haze, which reflected and absorbed radiation from the sun and resulted in less solar radiation reaching the surface, despite concurrent upward trends in cloud-free skies over China. CVC research also encompasses the development and application of climate models of varying complexity. These models are used to understand past climate variations, help explain causes of current climate conditions, and improve short-term climate predictions as well as projections of future climate change.
Mountain Snowpack Projected to Decline.28A global climate model with an embedded downscaling scheme predicts that regional mountain snowpack will decline by up to 50 to 80% in many regions of the globe over the next century in response to a scenario of increasing greenhouse gas concentrations in the atmosphere. Previous studies with regional climate models have suggested similar reductions for selected regions and decades in the 21st century. Now, for the first time, a global climate model provides global estimates of snowmelt with 5-km spatial resolution for the period 1980 to 2100. To achieve this resolution, a physically based downscaling scheme was added to the Community Climate System Model (CCSM) that is fully interactive with the atmosphere and land components of the CCSM. Snowpack is most sensitive to spatial resolution because of its dependence on both temperature and precipitation, both of which also depend on surface elevation.
Reducing Uncertainties in Projections of the Thermohaline Circulation.29The ocean thermohaline circulation (THC) plays an important role in Earth's climate by transporting heat from low latitudes. Changes in the THC and, in particular, a shutdown of this circulation due to large freshwater input at high latitudes, have been identified as a likely candidate for explaining some past rapid climate changes. A recent study applied a suite of climate models to examine how the THC may respond to additions of freshwater in the North Atlantic Ocean that could accompany future climate change. In response to expected levels of freshwater input, the models yield generally similar amounts of THC weakening over the next 100 years, on the order of 30% on average, and none of the models simulated a complete shutdown during this period.
Additional Past Accomplishments:
1) NRC, 2002: Abrupt Climate Change: Inevitable Surprises. National Academy Press, Washington, DC, USA, 230 pp.
2) Fedorov, A.V., P.S. Dekens, M. McCarthy, A.C. Ravelo, P.B. deMenocal, M. Barreiro, R.C. Pacanowski, and S.G. Philander, 2006: The Pliocene paradox (Mechanisms for a permanent El Niño). Science, 312(5779), 1485-1489, doi:10.1126/science.1122666.
3) Overpeck, J.T., B.L. Otto-Bliesner, G.H. Miller, D.R. Muhs, R.B. Alley, and J.T. Kiehl, 2006: Paleoclimatic evidence for future ice-sheet instability and rapid sea-level rise. Science, 311, 1747-1750, doi:10.1126/science.1115159.
4) Barron, J.A. and D. Bukry, 2007: Solar forcing of Gulf of California climate during the past 2000 years suggested by diatoms and silicoflagellates. Marine Micropaleontology, 62, 115-139.
5) Woodhouse, C. and J.J. Lukas, 2006: Multi-century tree-ring reconstructions of Colorado stream flow for water resource planning. Climatic Change, 78, 293-315, doi:10.1007/s10584-006-9055-0.
6) Hegerl, G.C., T.J. Crowley, W.T. Hyde, and D.J. Frame, 2006: Climate sensitivity constrained by temperature reconstructions over the past seven centuries. Nature, 440(7087), 1029-1032, doi:10.1038/nature04679.
7) Webster, P.J., G. Holland, J. Curry, and H.-R. Chang, 2005: Changes in tropical storm number, duration, and intensity in a warming environment. Science, 309(5742), doi:10.1126/science.1116448.
8) Emanuel, K., 2006: Climate and tropical cyclone activity: a new model downscaling approach. Journal of Climate, 19, 4797-4802.
9) Hoyos, C.D., P. Agudelo, P. Webster, and J. Curry, 2006: Deconvolution of the factors leading to the increase in global hurricane intensity. Science, 312(94), doi:10.1126/science.1123560.
10) Kossin, J.P., K.R. Knapp, D.J. Vimont, R.J. Murnane, and B.A. Harper, 2007: A globally consistent reanalysis of hurricane variability and trends. Geophysical Research Letters, 34(4), L04815, doi:10.1029/2006GL028836.
11) Santer, B.D., T.M.L. Wigley, P.J. Gleckler, C. Bonfils, M.F. Wehner, K. Achuta Rao, T.P. Barnett, J.S. Boyle, W. Brüggemann, M. Fiorino, N. Gillett, J.E. Hansen, P.D. Jones, S.A. Klein, G.A. Meehl, S.C.B. Raper, R.W. Reynolds, K.E. Taylor, and W.M. Washington, 2006: Forced and unforced ocean temperature changes in Atlantic and Pacific tropical cyclogenesis regions. Proceedings of the National Academies of Science, 103(38), 13905-13910, doi: 10.1073/pnas.0602861103.
12) Landsea, C.W., B.A. Harper, K. Hoarau, and J.A. Knaff, 2006: Can we detect trends in extreme tropical cyclones? Science, 313(5786), 452-454, doi:10.1126/science.1128448.
13) Wu, M.C., K.H. Yeung, and W.L. Chang, 2006: Trends in western North Pacific tropical cyclone intensity. EOS Transactions of the American Geophysical Union, 87(48), 537.
14) Velicogna, I. and J. Wahr, 2006: Measurements of time-variable gravity show mass loss in Antarctica. Science, 311, 1754-1756, doi:10.1126/science.1123785.
15) Rignot, E. and P. Kanagaratnam, 2006: Changes in the velocity structure of the Greenland Ice Sheet. Science, 311, 986-990, doi:10.1126/science.1121381.
16) Luthcke, S.B., H.J. Zwally, W. Abdalati, D.D. Rowlands, R.D. Ray, R.S. Nerem, F.G. Lemoine, J.J. McCarthy, and D.S. Chinn, 2006: Recent Greenland ice mass loss by drainage system from satellite gravity observations. Science, 314, 1286-1289, doi:10.1126/science.1130776.
17) Levitus, S.J., I. Antonov, and T.P. Boyer, 2005: Warming of the world ocean, 1955-2003. Geophysical Research Letters, 32, L02604, doi:10.1029/2004GL021592.
18) Willis, J., D. Roemmich, and B. Cornuelle, 2004: Interannual variability in upper-ocean heat content, temperature, and thermosteric expansion on global scales. Journal of Geophysical Research, 109, C12036, doi:10.1029/2003JC002260.
19) Roemmich, D. and 15 co-authors, 2006: Global warming and sea-level rise. WCRP Workshop on Understanding Sea Level Rise and Variability, Paris, June 6-9, 2006.
20) Thompson, R.S., K.H. Anderson, L.E. Strickland, S.L. Shafer, R.T. Pelltier, and P.J. Bartlein, 2006: Atlas of Relations between Climatic Parameters and Distributions of Important Trees and Shrubs in North America – Alaskan Species and Ecoregions. USGS Professional Paper 1650-D, 342 p.
21) Thompson, R.S., K.H. Anderson, R.T. Pelltier, S.L. Shafer, and P.J. Bartlein, 2007: Atlas of Relations between Climatic Parameters and Distributions of Important Trees and Shrubs in North America – Ecoregions of North America. USGS Professional Paper 1650-E, CD-ROM.
22) Mo, K.C., R.W. Higgins, E. Rogers, and J. Wollen, 2007: Influence of the North American Monsoon Experiment 2004 enhanced soundings on NCEP operational analyses. Journal of Climate, 20(9), 1821-1842, doi:10.1175/JCLI4083.1.
23) Gutzler, D.S., H.K. Kim, R.W. Higgins, H.M.H. Juang, M. Kanamitsu, K. Mitchell, K. Mo, P. Pegion, E. Richie, J. Schemm, S. Schubert, Y. Song, and R.Yang, 2005: The North American Monsoon Model Assessment Project: Integrating numerical modeling into a field-based process study. Bulletin of the American Meteorological Society, 86, 1423-1436.
24) Mo, K.C., J.E. Schemm, H. Kim, and W.R. Higgins 2006: Influence of initial conditions on summer precipitation simulations over the United States and Mexico. Journal of Climate, 19, 3640-3658.
25) Stolarski, R.S., A.R. Douglass, M. Gupta, P.A. Newman, S. Pawson, M.R. Schoeberl, and J.E. Nielsen, 2006: An ozone increase in the Antarctic summer stratosphere: A dynamical response to the ozone hole. Geophysical Research Letters, 33, L21805, doi:10.1029/2006GL026820.
26) Eichelberger, S.J. and D.L. Hartmann, 2005: Changes in the strength of the Brewer-Dobson Circulation in a simple AGCM. Geophysical Research Letters, 32, L15807, doi:10.1029/2005GL022924.
27) Qian, Y., D.P. Kaiser, L.R. Leung, and M. Xu, 2006: More frequent cloud-free sky and less surface solar radiation in China from 1955 to 2000. Geophysical Research Letters, 33, L01812, doi:10.1029/2005GL024586.
28) Ghan, S.J. and T. Shippert, 2006: Physically based global downscaling: Climate change projections for a full century. Journal of Climate, 19, 1589-1604.
29) Stouffer, R.J., J. Yin, J.M. Gregory, K.W. Dixon, M.J. Spelman, W. Hurlin, A.J. Weaver, M. Eby, G.M. Flato, H. Hasumi, A. Hu, J.H. Jungclaus, I.V. Kamenkovich, A. Levermann, M. Montoya, S. Murakami, S. Nawrath, A. Oka, W.R. Peltier, D.Y. Robitaille, A. Sokolov, G. Vettoretti, and S.L. Weber, 2006: Investigating the causes of the response of the thermohaline circulation to past and future climate changes. Journal of Climate, 19, 1365-1387.
30) Vecchi, G.A., B.J. Soden, A.T. Wittenberg, I.M. Held, A. Leetmaa, and M.J. Harrison, 2006: Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature, 441, 73-76, doi:10.1038/nature04744.
31) Held, I. and B. Soden, 2006: Robust responses of the hydrological cycle to global warming. Journal of Climate, 19, 5686-5699.
32) Kiehl, J.T., C.A. Shields, J.J. Hack, and W.D. Collins, 2006: The climate sensitivity of the Community Climate System Model Version 3 (CCSM3). Journal of Climate, 19, 2584-2596.
33) Schmidt, G.A., R. Ruedy, J.E. Hansen, I. Aleinov, N. Bell, M. Bauer, S. Bauer, B. Cairns, V. Canuto, Y. Cheng, A. DelGenio, G. Faluvegi, A.D. Friend, T.M. Hall, Y. Hu, M. Kelley, N.Y. Kiang, D. Koch, A.A. Lacis, J. Lerner, K.K. Lo, R.L. Miller, L. Nazarenko, V. Oinas, Jan Perlwitz, J. Perlwitz, D. Rind, A. Romanou, G.L. Russell, M. Sato, D.T. Shindell, P.H. Stone, S. Sun, N. Tausnev, D. Thresher, and M.S. Yao, 2005: Present-day atmospheric simulations using GISS ModelE: Comparison to in situ, satellite, and reanalysis data. Journal of Climate, 19, 153-192.
34) Knutson T.R., T.L. Delworth, K.W. Dixon, I.M. Held, J. Lu, V. Ramaswamy, M.D. Schwarzkopf, G. Stenchikov, and R.J. Stouffer, 2006: Assessment of twentieth-century regional surface temperature trends using the GFDL CM2 coupled models. Journal of Climate, 19, 1624-1651.
35) Cook, K.H. and E.K. Vizy, 2006: Coupled model simulations of the West African Monsoon System: Twentieth and twenty-first century simulations. Journal of Climate, 19, 3681-3703.
36) Annamalai, H., K. Hamilton, and K.R. Sperber, 2007: South Asian Summer Monsoon and its relationship with ENSO in the IPCC AR4 Simulations. Journal of Climate, 20(6), 1071-1092.
37) Joseph, R. and S. Nigam, 2006: ENSO evolution and teleconnections in IPCC's twentieth century climate simulations: Realistic representation? Journal of Climate, 19, 4360-4377.
38) Achuta Rao, K. and K.R. Sperber, 2006: ENSO simulation in coupled ocean-atmosphere models: Are the current models better? Climate Dynamics, 27, 1-15.