CGD 2009 Profiles in Science: Dr. Kevin Trenberth
Summary of achievements
Kevin Trenberth continues to be prominent in all aspects of climate variability and climate change research and is a leader in the Intergovernmental Panel on Climate Change assessments and in the World Climate Research Programme. In 2009 his primary research has continued to focus on the global energy and water cycles and how they are changing, and his work mainly involves empirical studies and quantitative diagnostic calculations. Trenberth continues to be a primary advocate for the need to develop a climate information system that is an imperative for adaptation to climate change. In this vein, he has evaluated many datasets and been the primary promoter of the need to reanalyze global data into fields in ways that meet climate requirements for continuity and consistency. The climate information system framework developed by Trenberth is being used to help organize ocean observations and space-based observations, their processing, archival, and development into products. With John Fasullo, he has improved estimates of heat, energy and water transports within the atmosphere and ocean to a point where, when combined with top-of-atmosphere observed radiation, new estimates of ocean heat divergence and transports have become possible. This work is being used to validate coupled atmosphere-ocean climate models and understanding heat flows that are so important in climate change. He has continued to improve estimates of the global hydrological cycle. A particular focus is on changes in precipitation type, frequency, intensity and amount, and thus on how droughts and floods, and climate extremes change. In addition, with Aiguo Dai he has improved global estimates of runoff, streamflow, river discharge and the entire hydrological cycle, and how they change over time. He has also been to the fore in raising issues about how hurricanes change as the climate changes: in better determining the relation of hurricane to environmental variables, where the moisture that feeds the heavy rainfalls comes from, and the role of hurricanes in moving energy around.
Publications
Trenberth, K. E., and J. T. Fasullo, 2009: Global warming due to increasing absorbed solar radiation. Geophys. Res. Lttrs., 36, L07706, doi:10.1029/2009GL037527.
Figure 1: High resolution figure
Abstract: Global climate models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) are examined for the top-of-atmosphere radiation changes as carbon dioxide and other greenhouse gases build up from 1950 to 2100. There is an increase in net radiation absorbed, but not in ways commonly assumed. While there is a large increase in the greenhouse effect from increasing greenhouse gases and water vapor (as a feedback), this is offset to a large degree by a decreasing greenhouse effect from reducing cloud cover and increasing radiative emissions from higher temperatures. Instead the main warming from an energy budget standpoint comes from increases in absorbed solar radiation that stem directly from the decreasing cloud amounts. These findings underscore the need to ascertain the credibility of the model changes, especially insofar as changes in clouds are concerned.
Figure caption: Latitude-time series from 1960 to 2100 of zonal average (top) net radiation RT, (middle) -OLR, and (bottom) ASR in W m² where RT = ASR-OLR. (right) Average for 1950 to 2100.
The Issues and Events report on the viability of geoengineering to counter global warming did not address the ethical issue. I use the following fable to illustrate the point.
Trenberth, K., J.T. Fasullo and J. Kiehl. 2009: Earth's Global Energy Budget. Bulletin of the American Meteorological Society, 90, 311-324, doi:10.1175/2008BAMS2634.1.
Figure 3: High resolution figure
Weather and climate on Earth are determined by the amount and distribution of incoming radiation from the sun. For an equilibrium climate, Outgoing Longwave Radiation (OLR) necessarily balances the incoming Absorbed Solar Radiation (ASR), although there is a great deal of fascinating atmosphere, ocean, and land phenomena that couple the two. Incoming radiant energy may be scattered and reflected by clouds and aerosols or absorbed in the atmosphere. The transmitted radiation is then either absorbed or reflected at the Earth's surface. Radiant solar or shortwave energy is transformed into sensible heat, latent energy (involving different water states), potential energy, and kinetic energy before being emitted as longwave radiant energy. Energy may be stored for some time, transported in various forms, and converted among the different types, giving rise to a rich variety of weather or turbulent phenomena in the atmosphere and ocean. Moreover, the energy balance can be upset in various ways, changing the climate and associated weather.
Figure caption: The global annual mean Earth's energy budget for the Mar 2000 to May 2004 period (W m-2). The broad arrows indicate the schematic flow of energy in proportion to their importance.
Trenberth, K. E., L. Smith, 2008: Atmospheric energy budgets in the Japanese Reanalysis: Evaluation and variability. J. Meteor. Soc. Japan, Meteorological Society of Japan, 86, 579-592, doi: 10.2151/jmsj.86.579.
Figure 4: High resolution figure
Abstract: The vertically-integrated atmospheric energy and moisture budgets have been computed for all available months for the Japanese reanalysis (1979 to 2004), and results are described in detail for the month of January 1989 and compared with those of other reanalyses. Time series are also presented. The moistening, diabatic heating and total energy forcing of the atmosphere are computed as a residual from the analyses using the moisture, dry energy (dry static energy plus kinetic energy) and total atmospheric (moist static plus kinetic) energy budget equations. These fields are also computed from the model output based on the assimilating model parameterizations. Moreover, some component fields can also be computed from observations to evaluate the results. In particular, when the vertically-integrated forcings computed from the model parameterizations are compared with available observations and the budget-derived values, significant JRA model biases are revealed in radiation and precipitation. The energy and moisture budget-derived quantities are more realistic than the model output and better depict the real atmosphere. However, low frequency decadal variability is spurious and is mainly associated with changes in the observing system. Results also depend on the quality of the analyses which are not constructed to conserve mass, moisture or energy, owing to analysis increments. Although there has been considerable progress in depicting the diabatic components of the atmosphere, some problems remain, and suggestions are made on where research can make further improvements.
Figure caption: Correlations of monthly anomalies in total atmospheric energy divergence of JRA with NRA (top) and JRA with ERA-40 (bottom) at T31 resolution for 1979 to 2001. The contour interval is 20% and values exceeding 60% are coarse stippled and 80% fine stippled.
Hurrell, J.W., T. Delworth, G. Danabasoglu, H. Drange, S. Griffies, N. Holbrook, B. Kirtman, N. Keenlyside, M. Latif, J. Marotzke, G.A. Meehl, T. Palmer, H. Pohlmann, T. Rosati, R. Seager, D. Smith, R. Sutton, A. Timmermann, K.E. Trenberth, and J. Tribbia, 2009: Decadal Climate Prediction: Opportunities and Challenges. Community White Paper, OceanObs '09. [article]
Figure 5: High resolution figure
Introduction: The scientific understanding of Earth's climate system is now sufficiently developed to show that climate change from anthropogenic greenhouse gas forcing is already upon us, and the rate of change as projected exceeds anything seen in nature in the past 10,000 years. Uncertainties remain, however, especially regarding how climate will change at regional and local scales where the signal of natural variability is large. Decision makers in diverse arenas, from water managers in the U.S. Southwest to public health experts in Asia, need to know the extent to which the climate events they are seeing are the product of natural variability, and hence can be expected to reverse at some point, or are the result of potentially irreversible anthropogenic climate change.
Figure caption: The global number of temperature observations per month as a function of depth. The data sources are XBTs, fixed tropical moorings (TAO (Pacific), TRITON (Pacific), PIRATA (Atlantic), and the developing Indian Ocean array) and ARGO floats. The apparent horizontal strata reflect the successive influence of 450 m XBTs, 750 m XBTs, 500 m TAO-class moorings and 1000 m and 2000 m Argo floats.
Dai, A., T. Qian, K. E. Trenberth, and J. D Milliman, 2009: Changes in continental freshwater discharge from 1948-2004. J. Climate, 22, 2773-2791, doi:10.1175/2008JCLI2592.1.
Figure 6: High resolution figure
Abstract: A new data set of historical monthly streamflow at the farthest downstream stations for world's 925 largest ocean-reaching rivers has been created for community use. Available new gauge records are added to a network of gauges that covers ~80 × 106 km² or ~80% of global ocean-draining land areas and accounts for about 73% of global total runoff. For most of the large rivers, the record for 1948-2004 is fairly complete. Data gaps in the records are filled through linear regression using streamflow simulated by a land surface model (CLM3) forced with observed precipitation and other atmospheric forcings that is significantly (and often strongly) correlated with the observed streamflow for most rivers. Compared with previous studies, the new data set has improved homogeneity and enables more reliable assessments of decadal and long-term changes in continental freshwater discharge into the oceans. The model-simulated runoff ratio over drainage areas with and without gauge records is used to estimate the contribution from the areas not monitored by the gauges in deriving the total discharge into the global oceans.
Figure caption: Linear trends from 1948-2004 in annual (water-year) (a) discharge from each 4° lat × 5° lon coastal box estimated from available gauge records and reconstructed river flow, (b) the runoff trend inferred from the discharge trend shown in (a), observed surface air temperature (c) and precipitation (d) (from Qian et al. 2006), CLM3-simulated snow cover (e) and soil ice water. The bottom row shows the confidence level (%) for (g) the discharge and (h) runoff trends based on a ttest. A 95% confidence means the trend is statistically significant at the 5% level.
Trenberth, K.E. and L. Smith, 2009: The three dimensional structure of the atmospheric energy budget: methodology and evaluation. Clim. Dyn., 32, 1065-1079, doi:10.1007/s00382-008-0389-3.
Figure 7: High resolution figure
Abstract: Studies of the vertically-integrated energy and moisture budgets of the atmosphere are expanded to three dimensions. The vertical integrals of the moisture, energy and heat budget equations computed analytically act as a very strong constraint on any local computational results of the vertical structure. This paper focuses on the methodology and difficulties in closing the budgets and satisfying constraints, given the need to use a pressure coordinate because model coordinates all differ. Vertical interpolation destroys delicate mass balances and can lead to inconsistencies, such as from how geopotential or vertical motion is computed. Using the advective rather than flux form of the equations greatly reduces the contamination from these effects. Results are documented for January 1989 using European Centre for Medium Range Weather Forecasts reanalysis (ERA-40) data. The moistening, diabatic heating and total energy forcing of the atmosphere are computed as a residual from the analyses using the moisture, dry energy (dry static energy plus kinetic energy) and total atmospheric (moist static plus kinetic) energy equations. The components from the monthly averaged flow and transients, as a function of layer in the atmosphere, and as quasi-horizontal and vertical fluxes of dry static, latent and kinetic energy are examined. Results show the moistening of the atmosphere at the surface, its release as latent heat in precipitation and transformation into dry static energy, and thus net radiative cooling as a function of height and location. The vertically integrated forcings computed from the model parameterizations are compared with available observations and budget-derived values, and large ERA-40 model biases are revealed in radiation and precipitation. The energy and moisture budget-derived quantities are more realistic, although results depend on the quality of the analyses which are not constructed to conserve mass, moisture or energy, owing to analysis increments.
Figure caption: The latent heating term -Q 2 = L(E - P) (a) and total heating (including moistening) Q 1 - Q 2 (b) in W m-2.
Doherty, S.J., S. Bojinski, A. Henderson-Sellers, K. Noone, D. Goodrich, N.L. Bindoff, J.A. Church, K.A. Hibbard, T.R. Karl, L. Kajfez-Bogataj, A.H. Lynch, D.E. Parker, I.C. Prentice, V. Ramaswamy, R.W. Saunders, M. Stafford Smith, K. Steffen, T.F. Stocker, P.W. Thorne, K.E. Trenberth, M.M. Verstraete and F.W. Zwiers. 2009: Lessons learned from IPCC AR4: Scientific developments needed to understand, predict and respond to climate change. Bull. Amer. Meteor. Soc., 90, 497-513, doi:10.1175/2008BAMS2643.1.
Figure 8: High resolution figure
The periodic assessments of the Intergovernmental Panel on Climate Change (IPCC) of the causes, impacts, and possible response strategies to climate change are the most comprehensive and up-to-date reports available on the subject and form the standard reference for all concerned with climate change in academia, government, and industry worldwide. Hundreds of international experts contributed to the IPCC's Fourth Assessment Report (AR4), which has received unprecedented attention and acclaim by policy makers, scientists, industry, and the general public.
Figure caption: Vulnerability should be used to link pressing science questions with societal concerns. A starting point is to define and identify areas of greatest vulnerability to climate change, to use this information to guide which science questions to address, and to obtain better model predictions and process understanding, which in turn helps inform and mitigate societal concerns and assessments of vulnerability.
Trenberth, K.E. and L. Smith. 2009: Variations in the three dimensional structure of the atmospheric circulation with different flavors of El Niño. Journal of Climate, 22, 2978-2991, doi:10.1175/2008JCLI2691.1.
Figure 9: High resolution figure
Abstract: Two rather different flavors of El Niño are revealed when the full three-dimensional spatial structure of the temperature field and atmospheric circulation monthly mean anomalies is analyzed using the Japanese Reanalysis (JRA-25) temperatures from 1979 through 2004 for a core region of the tropics from 30°N to 30°S, with results projected globally onto various other fields. The first two empirical orthogonal functions (EOFs) both have primary relationships to El Niño Southern Oscillation (ENSO) but feature rather different vertical and spatial structures. By construction the two patterns are orthogonal, but their signatures in sea level pressure, precipitation, outgoing longwave radiation (OLR), and tropospheric diabatic heating are quite similar. Moreover, they are significantly related, with EOF-2 leading EOF-1 by about 4 to 6 months, indicating that they play complementary roles in the evolution of ENSO events, and with each mode playing greater or lesser roles in different events and seasons.
Figure caption: Patterns for EOF-1 of correlation (top) for all months and regression (lower 2 panels) for NDJFM, MJJAS with sea level pressure (left) and 300 hPa height (right). The units are hPa (left) or dam (right) per standard deviation of the time series.
Trenberth, K.E. 2009: Precipitation in a changing climate - More floods and droughts in the future. GEWEX News, 19, 8-10. [article]
Figure 10: High resolution figure
Evidence is building that human-induced climate change—or global warming—has a direct influence on changes in precipitation and the hydrological cycle. While precipitation amount is most commonly considered, even bigger changes occur in its intensity, frequency and type (rain vs. snow). A warmer climate increases risks of both drought and flood, but at different times and/or places. These aspects have enormous implications for agriculture, hydrology and water resources, yet they have not been adequately appreciated or addressed in many studies of climate change impacts. Because natural variability in weather provides resilience, the biggest impacts occur through changes in extremes.
Figure caption: Annual water year (October-September) continental freshwater discharge (solid line; shading indicates ± one standard error, 1 Sv=106 m3 s-1) into the global oceans during 1950-2004 estimated using streamflow records from the world's largest 925 rivers supplemented with simulated streamflow using a land surface model forced with observed precipitation and other atmospheric forcing. Dashed line is observed precipitation integrated over global land areas. The correlation between the two curves is 0.65. The timing of the Mount Pinatubo eruption is given by the black arrow at the bottom. From Trenberth and Dai (2007).
Trenberth, K.E. 2009: An imperative for climate change planning: tracking Earth’s global energy. Current Opinion in Environmental Sustainability, 1, 19-27. [doi:10.1016/j.cosust.2009.06.001.]
Figure 11: High resolution figure
Planned adaptation to climate change requires information about what is happening and why. While a long-term trend is for global warming, short-term periods of cooling can occur and have physical causes associated with natural variability. However, such natural variability means that energy is rearranged or changed within the climate system, and should be traceable. An assessment is given of our ability to track changes in reservoirs and flows of energy within the climate system. Arguments are given that developing the ability to do this is important, as it affects interpretations of global and especially regional climate change, and prospects for the future.
Figure caption: Global sea level since August 1992. The TOPEX/Poseidon satellite mission provided observations of sea level change from 1992 until 2005. Jason-1, launched in late 2001 continues this record by providing an estimate of global mean sea level every 10 days with an uncertainty of 3-4 mm. The seasonal cycle has been removed and an atmospheric pressure correction has been applied. http://sealevel.colorado.edu/ Courtesy Steve Nerem (reproduced with permission).
Trenberth, K.E. 2009: Changes in the Flow of Energy through the Earth’s Climate System. APS Physics. [article]
Figure 12: High resolution figure
Introduction: Weather and climate on Earth are determined by the amount and distribution of incoming radiation from the sun. For an equilibrium climate, outgoing longwave (infrared) radiation (OLR) necessarily balances the incoming absorbed solar radiation (ASR), so that the Net =ASR-OLR =0. There is a great deal of fascinating atmosphere, ocean and land phenomena that couple the ASR and OLR and the balance is only for the annual mean, not individual months or seasons. Incoming radiant energy may be scattered and reflected by clouds and aerosols, or absorbed in the atmosphere. The transmitted radiation is then either absorbed or reflected at the Earth’s surface. Radiant solar (shortwave) energy is transformed into sensible heat, latent energy (involving different water states), potential energy (involving gravity and height above the surface (or in the oceans, depth below)) and kinetic energy (involving motions) before being emitted back to space as longwave radiant energy. Energy may be stored for some time, transported in various forms, and converted among the different types, giving rise to a rich variety of weather or turbulent phenomena in the atmosphere and ocean. Moreover, the energy balance can be upset in various ways (so the Net ? 0), changing the climate and associated weather.
Figure caption: Zonal mean meridional energy transport by total (solid), the atmosphere (dashed), and by the ocean (dotted) accompanied with the associated ±2α range (shaded). Adapted from Fasullo and Trenberth (2008b)
Stott, P., and K. Trenberth, 2009: Linking Extreme Weather to Climate Variability and Change. EOS Trans. AGU, 90(21), doi: 10.1029/2009EO210004.
Abstract: International Group on Attribution of Climate-Related Events (ACE); Boulder, Colorado, 26 January 2009; Climate change is likely to be manifested on societies around the world mainly through changes in extremes. As a result, the scientific community faces an increasing demand for regularly updated appraisals of evolving climate conditions and extreme weather. Such information would be immensely beneficial for adaptation planning. A group of climate scientists representing the United Kingdom, the United States, Australia, Canada, and South Africa assembled on 26 January 2009 at the National Center for Atmospheric Research (NCAR), in Colorado, to discuss how to meet this challenge. This first meeting of the International Group on Attribution of Climate-Related Events (ACE) was sponsored by the Science and Innovation Network of the U.K. Foreign and Commonwealth Office (FCO) and NCAR and was organized in collaboration with the U.S. National Oceanic and Atmospheric Administration (NOAA), the Met Office Hadley Centre, and the University of Oxford.