CGD's Dr. William Collins

Abstract

Over the past 20 years, evidence that humans are affecting the climate has accumulated inexorably, and with it has come ever greater certainty across the scientific community in the reality of recent climate change and the potential for much greater change in the future. This increased certainty is starkly reflected in the latest report of the Intergovernmental Panel on Climate Change (IPCC), the fourth in a series of assessments of the state of knowledge on the topic, written and reviewed by hundreds of scientists worldwide. The panel released a condensed version of the first part of the report, on the physical science basis of climate change, in February. Called the "Summary for Policymakers," it delivered to policymakers and ordinary people alike an unambiguous message: scientists are more confident than ever that humans have interfered with the climate and that further human-induced climate change is on the way. Although the report finds that some of these further changes are now inevitable, its analysis also confirms that the future, particularly in the longer term, remains largely in our hands-the magnitude of expected change depends on what humans choose to do about greenhouse gas emissions. The physical science assessment focuses on four topics: drivers of climate change, changes observed in the climate system, understanding cause-and-effect relationships, and projection of future changes. Important advances in research into all these areas have occurred since the IPCC assessment in 2001.

Figure caption: Projected temperature changes.

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Yoshioka, M., N.M. Mahowald, A.J. Conley, W.D. Collins, D.W. Fillmore, C.S. Zender, and D.B. Coleman, 2007: Impact of desert dust radiative forcing on Sahel precipitation: Relative importance of dust compared to sea surface temperature variations, vegetation changes and greenhouse gas warming. J. Climate, 20, 1445-1467.

Abstract

The role of direct radiative forcing of desert dust aerosol in the change from wet to dry climate observed in the African Sahel region in the last half of the twentieth century is investigated using simulations with an atmospheric general circulation model. The model simulations are conducted either forced by the observed sea surface temperature (SST) or coupled with the interactive SST using the Slab Ocean Model (SOM). The simulation model uses dust that is less absorbing in the solar wavelengths and has larger particle sizes than other simulation studies. As a result, simulations show less shortwave absorption within the atmosphere and larger longwave radiative forcing by dust. Simulations using SOM show reduced precipitation over the intertropical convergence zone (ITCZ) including the Sahel region and increased precipitation south of the ITCZ when dust radiative forcing is included. In SST-forced simulations, on the other hand, significant precipitation changes are restricted to over North Africa. These changes are considered to be due to the cooling of global tropical oceans as well as the cooling of the troposphere over North Africa in response to dust radiative forcing. The model simulation of dust cannot capture the magnitude of the observed increase of desert dust when allowing dust to respond to changes in simulated climate, even including changes in vegetation, similar to previous studies. If the model is forced to capture observed changes in desert dust, the direct radiative forcing by the increase of North African dust can explain up to 30% of the observed precipitation reduction in the Sahel between wet and dry periods. A large part of this effect comes through atmospheric forcing of dust, and dust forcing on the Atlantic Ocean SST appears to have a smaller impact. The changes in the North and South Atlantic SSTs may account for up to 50% of the Sahel precipitation reduction. Vegetation loss in the Sahel region may explain about 10% of the observed drying, but this effect is statistically insignificant because of the small number of years in the simulation. Greenhouse gas warming seems to have an impact to increase Sahel precipitation that is opposite to the observed change. Although the estimated values of impacts are likely to be model dependent, analyses suggest the importance of direct radiative forcing of dust and feedbacks in modulating Sahel precipitation.

Figure caption: Annual rainfall trend in the Sahel (10°-20°N, 18°W-20°E) in observations (black), AMIP.ND (blue solid), AMIP.NDV (blue dashed), AMIP.S (yellow), AMIP.SL (red solid), AMIP.SLV (red dashed), AMIP.SLVH (red dotted), AMIP.F (green solid), and AMIP.H (green dashed) simulations. (Precipitation values are 10-yr running means. For example, the abscissa value of 55 represents the period from 1951 to 1960). Both AMIP and SOM simulations are conducted with no dust radiative effect (ND), dust shortwave forcing and feedback (S), and dust shortwave and longwave forcing and feedback (SL). Simulations with shortwave and longwave forcing and feedback are also performed using the vegetation cover in the wet period described in the previous section (SLV). Additionally, AMIP simulations are also conducted with wet-period vegetation without radiative forcing (NDV) and with radiative forcing of a half of the simulated strength of dust. In addition, five member ensembles of the AMIP.F and AMIP.H simulations are also analyzed. AMIP.F simulations use the default model configurations and are forced by the historical SST field only while all other external forcings are interannually invariant. AMIP.H is similar to the AMIP.F but also includes time-varying forcing by volcanoes, greenhouse gases, aerosols, and solar variability.

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Meehl, G.A., J.M. Arblaster, and W.D. Collins, 2007: Effects of black carbon aerosols on the South Asian Monsoon. In press, J. Climate.

Abstract

A six-member ensemble of 20th century simulations with changes to only time-evolving global distributions of black carbon aerosols in a global coupled climate model is analyzed to study the effects of black carbon (BC) aerosols on the Indian monsoon. The BC aerosols act to increase lower tropospheric heating over south Asia and reduce the amount of solar radiation reaching the surface during the dry season as noted in previous studies. The increased meridional tropospheric temperature gradient in the pre-monsoon months of March-April-May, particularly between the elevated heat source of the Tibetan Plateau and areas to the south, contributes to enhanced precipitation over India in those months. With the onset of the monsoon, the reduced surface temperatures in the Bay of Bengal, Arabian Sea, and over India that extend to the Himalayas act to reduce monsoon rainfall over India itself, with some small increases over the Tibetan Plateau. Precipitation over China generally decreases due to the BC aerosol effects. There is a weakened latitudinal SST gradient due to BC aerosols as seen in observations, and is present in the multiple forcings experiments with CCSM3 that include natural and anthropogenic forcings (including BC aerosols). The BC aerosols and consequent weakened latitudinal SST gradient in those experiments are associated with increased precipitation during MAM in northern India and over the Tibetan Plateau, with some decreased precipitation over southwest India, the Bay of Bengal, Burma, Thailand, and Malaysia as seen in observations. In the summer monsoon season, the BC aerosols appear to have contributed to observed decreasing precipitation trends over parts of India, with increasing trends in the CCSM3 multiple-forcing experiments and observations over Pakistan and the Tibetan Plateau, and decreasing trends over Bangladesh, Burma, and Thailand.

Figure caption: Distributions of black carbon optical depth for the year 1999 by season, a) DJF, b) MAM, c) JJA, and d) SON. This pattern was scaled back in time based on globally averaged human population.

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Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, A. Gaye, J. Gregory, A. Kitoh, R. Knutti, J. Murphy, A. Noda, S. Raper, I. Watterson, A. Weaver, Z.-C. Zhao, J. Annan, J. Arblaster, C. Bitz, A. le Brocq, P. Brockmann, L. Buja, G. Clarke, M. Collins, E. Driesschaert, N.A. Diansky, K. Dixon, J. L. Dufresne, J. Kyurgerov, J. Eby, N. Edwards, S. Emori, P. Forster, R. Furrer, J. Hansen, G. Hegerl, M. Holland, A. Hu, P. Huybrechts, F. Joos, J. Kettleborough, M. Kimoto, M. Krynytzky, M.-F. Loutre, J. Lowe, M. Meinshausen, S. Muller, S. Nawrath, J. Oerlemans, T. Palmer, A. Payne, G.-K. Plattner, J. Raisanen, G.L. Russell, A. Rinke, D. Salas y Melia, G. Schmidt, B. Schneider, A. Shepherd, D. Stainforth, C. Tebaldi, H. Teng, L. Terray, A. Sokolov, P. Stott, E.M. Volodin, B. Wang, T.M.L. Wigley, Y. Yu, and S. Yukimoto, 2006: Chapter 10: Global Climate Projections, in the Intergovernmental Panel on Climate Change Working Group 1: The Physical Science Basis, S. Soloman et al, eds., Cambridge University Press, 1009 pp.

Abstract

The future climate change results assessed in this chapter are based on a hierarchy of models, ranging from Atmosphere- Ocean General Circulation Models (AOGCMs) and Earth System Models of Intermediate Complexity (EMICs) to Simple Climate Models (SCMs). These models are forced with concentrations of greenhouse gases and other constituents derived from various emissions scenarios ranging from nonmitigation scenarios to idealized long-term scenarios. In general, we assess non-mitigated projections of future climate change at scales from global to hundreds of kilometers. Further assessments of regional and local climate changes are provided in Chapter 11. Due to an unprecedented, joint effort by many modeling groups worldwide, climate change projections are now based on multi-model means, differences between models can be assessed quantitatively and in some instances, estimates of the probability of change of important climate system parameters complement expert judgment. New results corroborate those given in the Third Assessment Report (TAR). Continued greenhouse gas emissions at or above current rates will cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.

Figure caption: Comparison of shortwave and longwave instantaneous radiative forcings and flux changes computed from AOGCMs and line-by-line (LBL) radiative transfer codes (W.D. Collins et al., 2006). (a) Instantaneous forcing from doubling atmospheric CO2 from its concentration in 1860; b) changes in radiative fluxes caused by the 20% increase in water vapor expected in the climate produced from doubling atmospheric CO2. The forcings and flux changes are computed for clear-sky conditions in mid-latitude summer and do not include effects of stratospheric adjustment. No other well-mixed greenhouse gases are included. The minimum to maximum range and median are plotted for five representative LBL codes. The AOGCM results are plotted with box-and-whisker diagrams (see caption for Figure 10.2) representing percentiles of forcings from 20 models in the AR4 multi-model ensemble. The AOGCMs included are BCCR-BCM2.0, CCSM3, CGCM3.1 (T47 and T63), CNRM-CM3, ECHAM5/MPI-OM, ECHO-G, FGOALS-g1.0, GFDL-CM2.0, GFDL-CM2.1, GISS-EH, GISS-ER, INM-CM3.0, IPSL-CM4, MIROC3.2 (medium and high resolution), MRI-CGCM2.3.2, PCM, UKMO-HadCM3, and UKMO-HadGEM1 (see Table 8.1 for model details). The LBL codes are the Geophysical Fluid Dynamics Laboratory (GFDL) LBL, the Goddard Institute for Space Studies (GISS) LBL3, the National Center for Atmospheric Research (NCAR)/Imperial College of Science, Technology and Medicine (ICSTM) general LBL GENLN2, the National Aeronautics and Space Administration (NASA) Langley Research Center MRTA and the University of Reading Reference Forward Model (RFM).