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CGD 2008 Profiles in Science: Dr. Kevin Trenberth

Summary of achievements

Kevin Trenberth

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 2008 his primary research has focused 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

Fasullo, J.T., and K.E. Trenberth, 2008: The Annual Cycle of the Energy Budget. Part I: Global Mean and Land-Ocean Exchanges. J. Climate, 21, 2297-2312.

Abstract: The mean and annual cycle of energy flowing into the climate system and its storage, release, and transport in the atmosphere, ocean, and land surface are estimated with recent observations. An emphasis is placed on establishing internally consistent quantitative estimates with discussion and assessment of uncertainty. At the top of the atmosphere (TOA), adjusted radiances from the Earth Radiation Budget Experiment (ERBE) and Clouds and the Earth's Radiant Energy System (CERES) are used, while in the atmosphere the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis and 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) estimates are used. The net upward surface flux (FS) over ocean is derived as the residual of the TOA and atmospheric energy budgets, and is compared with direct calculations of ocean heat content (OE) and its tendency (dOE/dt) from several ocean temperature datasets. Over land, FS from a stand-alone simulation of the Community Land Model forced by observed fields is used. A depiction of the full energy budget based on ERBE fluxes from 1985 to 1989 and CERES fluxes from 2000 to 2004 is constructed that matches estimates of the global, global ocean, and global land imbalances. In addition, the annual cycle of the energy budget during both periods is examined and compared with ocean heat content changes.

The near balance between the net TOA radiation (RT) and FS over ocean and thus with OE, and between RT and atmospheric total energy divergence over land, are documented both in the mean and for the annual cycle. However, there is an annual mean transport of energy by the atmosphere from ocean to land regions of 2.2 ± 0.1 PW (1 PW = 1015 W) primarily in the northern winter when the transport exceeds 5 PW. The global albedo is dominated by a semiannual cycle over the oceans, but combines with the large annual cycle in solar insolation to produce a peak in absorbed solar and net radiation in February, somewhat after the perihelion, and with the net radiation 4.3 PW higher than the annual mean, as it is enhanced by the annual cycle of outgoing longwave radiation that is dominated by land regions. In situ estimates of the annual variation of OE are found to be unrealistically large. Challenges in diagnosing the interannual variability in the energy budget and its relationship to climate change are identified in the context of the episodic and inconsistent nature of the observations.

Figure caption: CERES-period-mean best-estimate FM1 TOA fluxes (PW) globally and for the (right) global land and (left) global ocean regions.


Fasullo, J.T., and K.E. Trenberth, 2008: The Annual Cycle of the Energy Budget. Part II: Meridional Structures and Poleward Transports. J. Climate, 21, 2313-2325.

Abstract: Meridional structure and transports of energy in the atmosphere, ocean, and land are evaluated holistically for the mean and annual cycle zonal averages over the ocean, land, and global domains, with discussion and assessment of uncertainty. At the top of the atmosphere (TOA), adjusted radiances from the Earth Radiation Budget Experiment (ERBE) and Clouds and Earth's Radiant Energy System (CERES) are used along with estimates of energy storage and transport from two global reanalysis datasets for the atmosphere. Three ocean temperature datasets are used to assess changes in the ocean heat content (OE) and their relationship to the net upward surface energy flux over ocean (FoS), which is derived from the residual of the TOA and atmospheric energy budgets. The surface flux over land is from a stand-alone simulation of the Community Land Model forced by observed fields.

In the extratropics, absorbed solar radiation (ASR) achieves a maximum in summer with peak values near the solstices. Outgoing longwave radiation (OLR) maxima also occur in summer but lag ASR by 1-2 months, consistent with temperature maxima over land. In the tropics, however, OLR relates to high cloud variations and peaks late in the dry monsoon season, while the OLR minima in summer coincide with deep convection in the monsoon trough at the height of the rainy season. Most of the difference between the TOA radiation and atmospheric energy storage tendency is made up by a large heat flux into the ocean in summer and out of the ocean in winter. In the Northern Hemisphere, the transport of energy from ocean to land regions is substantial in winter, and modest in summer. In the Southern Hemisphere extratropics, land - ocean differences play only a small role and the main energy transport by the atmosphere and ocean is poleward. There is reasonably good agreement between FoS and observed changes in OE, except for south of 40°S, where differences among several ocean datasets point to that region as the main source of errors in achieving an overall energy balance. The winter hemisphere atmospheric circulation is the dominant contributor to poleward energy transports outside of the tropics [6-7 PW (1 petawatt = 1015 W)], with summer transports being relatively weak (~3 PW)—slightly more in the Southern Hemisphere and slightly less in the Northern Hemisphere. Ocean transports outside of the tropics are found to be small (<2 PW) for all months. Strong cross-equatorial heat transports in the ocean of up to 5 PW exhibit a large annual cycle in phase with poleward atmospheric transports of the winter hemisphere.

Figure caption: Zonal mean departures from the annual mean of (a) albedo (fraction), (b) ASR, and (c) OLR are shown based on CERES retrievals with positive (negative) differences from ERBE fields stippled (hatched) where they exceed ±2σI and ± W m-2 (or ±0.02 in the case of albedo). (d) The zonal annual mean terms as a function of latitude. The CERES values are based on averages from March 2000 to May 2004, the CERES period, while ERBE values are based on averages from February 1985 through April 1989, the ERBE period. For ASR and OLR the units are 0.01 PW (°)-1.


Trenberth, K.E., and J.T. Fasullo, 2008: An Observational Estimate of Inferred Ocean Energy Divergence. J. Phys. Oceanogr., 38, 984-999.

Abstract: Monthly net surface energy fluxes (FS) over the oceans are computed as residuals of the atmospheric energy budget using top-of-atmosphere (TOA) net radiation (RT) and the complete atmospheric energy (AE) budget tendency (δAE/δt) and divergence (∇ ⋅ FA). The focus is on TOA radiation from the Earth Radiation Budget Experiment (ERBE) (February 1985-April 1989) and the Clouds and Earth's Radiant Energy System (CERES) (March 2000-May 2004) satellite observations combined with results from two atmospheric reanalyses and three ocean datasets that enable a comprehensive estimate of uncertainties. Surface energy flux departures from the annual mean and the implied annual cycle in "equivalent ocean energy content" are compared with the directly observed ocean energy content (OE) and tendency (δOE/δt) to reveal the inferred annual cycle of divergence (∇ ⋅ FO). In the extratropics, the surface flux dominates the ocean energy tendency, although it is supplemented by ocean Ekman transports that enhance the annual cycle in ocean heat content. In contrast, in the tropics, ocean dynamics dominate OE variations throughout the year in association with the annual cycle in surface wind stress and the North Equatorial Current. An analysis of the regional characteristics of the first joint empirical orthogonal function (EOF) of FS, δOE/δt, and ∇ ⋅ FO is presented, and the largest sources of uncertainty are attributed to variations in OE. The mean and annual cycle of zonal mean global ocean meridional heat transports are estimated. The annual cycle reveals the strongest poleward heat transports in each hemisphere in the cold season, from November to April in the north and from May to October in the south, with a substantial across-equatorial transport, exceeding 4 PW in some months. Annual mean results do not differ greatly from some earlier estimates, but the sources of uncertainty are exposed. Comparison of annual means with direct ocean observations gives reasonable agreement, except in the North Atlantic, where transports from the ocean transects are slightly greater than the estimates presented here.

Figure caption: (a) Annual mean FS as computed by the residual from RT and NRA atmospheric energy budgets for the ERBE period (W m-2 ). Departure from the annual mean of the (b) JJA and (c) DJF surface flux out of the ocean (W m-2). Stippling (hatching) denotes areas where NRA-based estimates exceed (fall below) those of ERA-40 by more than 10 W m-2 in (a) and by more than 30 W m-2 in (b) and (c).


Anthes, R. A., P. A. Bernhardt, Y. Chen, et al., 2008: The COSMOC/FORMOSAT-3 - Mission early results. Bulletin of the American Meteorological Society, 89, 313-+.

Abstract: The global positioning system (GPS) radio-occultation (RO) limb-sounding technique for sounding Earth's atmosphere was demonstrated by the proof-of-concept GPS Meteorology (GPS/MET) experiment in 1995-97 (Ware et al. 1996). Following GPS/MET, additional missions, that is, Challenging Minisatellite Payload (CHAMP); Wickert et al. 2001) and the Satellite de Aplicaciones Cientificas-C (SAC-C; Hajj et al. 2004), have confirmed the potential of RO sounding of the ionosphere, stratosphere, and troposphere.

At 0140 UTC 15 April 2006, six microsatellites were launched into a circular, 72° inclination orbit at an altitude of 512 km from Vandenberg Air Force Base, California (Cheng et al. 2006). The mission is a collaborative project of the National Space Organization (NSPO) in Taiwan and the University Corporation for Atmospheric Research (UCAR) in the United States. This mission is called the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) in the United States and the Formosa Satellite Mission 3 (FORMOSAT-3) in Taiwan. All satellites began delivering useful data within days after the launch (Anthes 2006). This paper summarizes the mission and the early scientific results, with emphasis on the radio-occultation part of the mission.

Figure caption: Schematic diagram illustrating radio occultation of GPS signals. [Figure courtesy of NSPO.]


Trenberth, K. E., 2008: Observational needs for climate prediction and adaptation. WMO Bulletin, 57 (1), 17-21.

Introduction: The climate is changing. In general, temperatures are increasing (Figure 1), owing to human-induced changes in the composition of the atmosphere, notably increased carbon dioxide from the burning of fossil fuels (IPCC, 2007). Land is mostly warming faster than the ocean. A close examination of Figure 1, however, shows that the temperatures actually declined from 1901 to 2005 in the south-eastern USA and the North Atlantic, changes in ocean currents clearly contribute. Over the south-eastern USA, changes in the atmospheric circulation that brought cloudier and much wetter conditions played a major role (Trenberth et al., 2007). This non-uniformity of change highlights the challenges of regional climate change that has considerable spatial structure and temporal variability.

A foundation of climate research and future projections comes from the observations. These come from many and varied sources. Many are taken for weather forecasting purposes. Changes are common in instrumentation and siting, thereby disrupting the climate record, for which continuity and homogeneity are vitally important for assessing climate variations and change. Increasing volumes of observations come from space-based platforms, but satellites have a finite life time (typically five years or so), the orbit drifts and decays over time, the instruments degrade and, hence, the apparent climate record can become corrupted by spurious changes. An ongoing challenge is to create climate data records from the observations to serve many purposes.

Loss of Earth-observing satellites is also of concern, as documented in the recent National Research Council decadal survey (2007). Ground-based observations are not being adequately kept up in many countries. Calibration of climate records is critical. Small changes over a long time are characteristic of climate change but they occur in the midst of large variations associated with weather and natural climate variations, such as El Niño. Yet the climate is changing and it is imperative to track the changes and causes as they occur and identify what the prospects are for the future - to the extent that they are predictable. We need to build a system based on these observations to inform decision-makers about what is happening and why and what the predictions are for the future on several time horizons.

In this article, an outline is given a subset of activities related to the needs of decision-makers for climate information for adaptation purposes. It builds on some discussions held at a workshop on learning form the Fourth Assessment of the Intergovernmental Panel on Climate Change (IPCC) (Sydney, Australia, 4-6 October 2007). The Workshop was sponsored by the Global Climate Observing System, the World Climate Research Programme of the International Council for Science. Within WCRP, the WCRP Observations and Assimilation Panel (WOAP), which the author chairs, attempts to highlight outstanding issues and ways forward in addressing them.

Figure caption: Linear trend of annual temperatures for 1901-2005 (° C century -1. Areas in grey have insufficient data to produce reliable trends. Trends significant at the 5% level are indicated by white + marks. (From Trenberth et al., Climate Change 2007: The Physical Science Basis, Intergovernmental Panel on Climate Change).


Trenberth, K. E., and J. Fasullo, 2008: The energy budgets of Atlantic hurricanes and changes from 1970. Geochemistry, Geophysics, Geosystems., in press.

Abstract: Based on the current observational record of tropical cyclones and sea surface temperatures (SSTs) in the Atlantic, estimates are made of changes in surface sensible and latent heat fluxes and hurricane precipitation form 1970 to 2006. The best track dataset of observed tropical cyclones is used to estimate the frequency that storms of a given strength occur after 1970. Empirical expressions for the surface fluxes and precipitation are based on simulations of hurricane Katrina in August 2005 with the advanced Weather and Research Forecasting (WRF) model at 4 km resolution without parameterized convection. The empirical relationships are computed for the surface fluxes and precipitation within 400 km of the eye of the storm for all categories of hurricanes based upon the maximum simulated wind and the observed sea surface temperature and saturation specific humidity. Strong trends are not linear but are better depicted as a step function increase from 1994 to 1995, and the large variability reflects changes in SSTs and precipitable water, modulated by El Niño events. The environmental variables of SST and water vapor are nonetheless accompanied by clear changes in tropical cyclone activity using several metrics.

Figure caption: The record of numbers of named storms and hurricanes for the Atlantic from 1944 to 2006 based on the best track data. The smoothed curves show decadal variability using a 13 point filter with end values computed using reflected values.


Trenberth, K. E., and L. Smith, 2008: The three dimensional structure of the atmospheric energy budget: methodology and evaluation. Climate Dynamics, in press. doi:10.1007/s00382-008-0389-3.

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: For Jan 1989 based on ERA-40 analyses, a) vertically integrated Q1-Qf truth versus b) results from the advective method, and c)differences between the two. The right hand side panels show the zonal averages. The plots are smoothed to T42 resolution and the units are Watts per square meter.


Trenberth, K. E., and L. Smith, 2008: Atmospheric energy budgets in the Japanese Reanalysis: Evaluation and variability. J. Meteor. Soc. Japan, in press.

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. By emphasizing the differences and the errors, there is a tendency to overlook the considerable progress in depicting diabatic components of the atmosphere, while also pointing to where research can make further improvements.

Figure caption: For Jan 1989 based on JRA analyses, vertically integrated Q1, -Q2 and Q1 - Q2. The right hand side panels show the zonal averages. The plots are smoothed to T42 resolution and the units are W m-2. Contour interval is 80 W m -2, and stipple and hatching begin at ±120 W m -2 and more densely at ±200 W m -2.


Trenberth, K. E., T. Koike, and K. Onogi, 2008: Progress and prospects in reanalysis. EOS, 89, 26, 24 June 2008, 234-235.

Abstract: Analyses of global atmospheric observations in real time for numerical weather prediction (NWP) lack continuity over time as the operational system evolves. Reanalysis of the observations - with more complete data, improved quality control, and a constatnt state-of-the-art assimilating model and analysis system - greatly improves the homogeneity of the record and makes it useful for examining climate variations. This whole endeavor is now referred to as "reanalysis".

However, even as atmospheric reanalysis of past observations has greatly improved our ability to determine climate variability, challenges still exist in depicting multidecadal changes. Moreover, although several reanalyses - from the U.S. National Oceanic and Atmospheric Administration's National Centers for Environmental Prediction (NCEP), NASA Goddard Space Flight Center (GSFC), the European Centre for Medium-Range Weather Forecasts (ECMWF), and the Japan Meteorological Agency (JMA) - now exist, the task is far from done. Further improvements to reanalysis - including expansion to encompass key trace constituents and the ocean, land, and sea ice domains - hold promise for extending their use in climate change studies, research, and the practical applications (such as how extremes of climate and their impacts on agriculture have changed).

Global gridded analyses of observations taken for many purposes - such as weather forecasting in the atmosphere or core oceanographic research - become part of the climate record but often display biases that mask long-term variations. Many climate data sets are inhomogeneous: The record length either is too short to provide decadal-scal information or is inconsistent owing to operational changes in instruments, their siting, and data transmission and processing and to the absence of adequate metadata. Hence, major effors have been required to homogenize the observed data for them to be useful for climate purposes. Reanalysis of atmospheric observations using a constant state-of-the-art assimilation model has helped enormously in making the historical record more homogeneous and useful for many studies. Indeed, in the 20 years since reanalysis was first proposed by Trenberth and Olson [1988] and Bengtsson and Shukla [1988], there have been great advances in our ability to generate high-quality temporally homogeneous estimates of the past climate.


Trenberth, K. E., J. T. Fasullo, and J. Kiehl, 2008: Earth's global energy budget. Bull. Amer. Meteor. Soc., in press.

Abstract: An update is provided on the Earth's global annual mean energy budget in the light of new observations and analyses. In 1997 Kiehl and Trenberth provided a review of past such estimates and performed a number of radiative computations to better establish the role of clouds and various greenhouse gases in the overall radiative energy flows, with top-of-atmosphere (TOA) values constrained by Earth Radiation Budget Experiment values form 1985 to 1989, when the TOA values were approximately in balance. The Clouds and the Earth's Radiant Energy System (CERES) measurements from March 2000 to May 2004 are used to TOA but adjusted to an estimated imbalance from the enhanced greenhouse effect of 0.9 W m-2. Revised estimates of surface turbulent fluxes are made based on various sources. The partitioning of solar radiation in the atmosphere is based in part on the International Satellite Cloud Climatology Project (ISCCP) ISCCP-FD computations that utilize the global ISCCP cloud data every 3 hours, and also accounts for increased atmospheric absorpton by water vapor and aerosols. Surface upwards longwave radiation is adjusted to account for spatial and temporal variability. A lack of closure in the energy balance at the surface is accommodated by making modest changes to surface fluxes, with the downward longwave radiation as the main residual to ensure a balance. Values are also presented for the land and ocean domains that include a net transport of energy from ocean to land of 2.2 Petawatts (PW) of which 3.2 PW is from moisture (latent energy) transport, while net dry static energy transport is from land to ocean. Evaluations of atmospheric reanalyses reveal substantial biases.

Figure caption: The global annual mean Earth's energy budget for the March 2000 to May 2004 period in W m-2. The broad arrows indicate the schematic flow of energy in proportion to their importance.


Trenberth, K. E., C. A. Davis, and J. Fasullo, 2007: Water and energy budgets of hurricanes: Case studies of Ivan and Katrina. J. Geophys. Res., 112, D23106, doi:10.1029/2006JD008303.

Abstract: To explore the role of hurricanes in the climate system, a detailed analysis is made of the bulk atmospheric moisture budget of Ivan in September 2004 and Katrina in August 2005 from simulations with the Weather and Research Forecasting (WRF) model at 4 km resolution without parameterized convection. Heavy precipitation exceeding 20 mm h-1 in the storms greatly exceeds the surface flux of moisture through evaporation, and vertically integrated convergence of moisture in the lowest 1 km of the atmosphere from distances up to 1600 km is the dominant term in the moisture budget, highlighting the importance of the larger-scale environment. Simulations are also run for the Katrina case with sea surface temperatures (SSTs) increased by +1°C and decreased by -1°C as sensitivity studies. For hours 42 to 54 after the start of the simulation, maximum surface winds increased about 4.5 m s-1 (9%), and sea level pressure fell 11.5 hPa per 1°C increase in tropical SSTs. Overall, the hurricane expands in size as SSTs increase, the environmental atmospheric moisture increases at close to the Clausius-Clapeyron equation value of about 6% K-1 and the surface moisture flux also increases mainly from Clausius-Clapeyron effects and the changes in intensity of the storm. The environmental changes related to human influences on climate since 1970 have increased SSTs and water vapor, and the results suggest how this may have altered hurricanes and increased associated storm rainfalls, with the latter quantified to date to be of order 6 to 8%.

Figure caption: Precipitation (mm h-1) fields for simulated hurricanes (left) Ivan at 1200 UTC 14 September 2004 and (right) Katrina at 0000 UTC 28 August 2005.


Trenberth, K. E., and J. Fasullo, 2007: Water and energy budgets of hurricanes and implications for climate change. J. Geophys. Res., 112, D23107, doi:10.1029/2006JD008304.

Abstract: On the basis of simulations of hurricane Katrina in August 2005 with the advanced Weather and Research Forecasting (WRF) model at 4 km resolution without parameterized convection, empirical relationships are computed between the maximum simulated wind and the surface fluxes and precipitation and provide a reasonable fit to the data. The best track data set of global observed tropical cyclones is used to estimate the frequency that storms of a given strength occur over the globe after 1970. For 1990-2005 the total surface heat loss by the tropical ocean in hurricanes category 1 to 5 within 400 km of the center of the storms is estimated to be about 0.53 x 1022 J a-1 (where a is year) (0.17 PW). The enthalpy loss due to hurricanes computed on the basis of precipitation is about a factor of 3.4 greater (0.58 PW), owing to the addition of the surface fluxes from outside 400 km radius and moisture convergence into the storms typically from as far from the eye as 1600 km. Globally these values correspond to 0.33 W m-2 for evaporation, or 1.13 W m-2 for precipitation. Changes over time reflect basin differences and a prominent role for El Niño, and the most active period globally was 1989 to 1997. Strong positive trends from 1970 to 2005 occur in these inferred surface fluxes and precipitation arising from increases in intensity of storms and also higher sea surface temperatures. Confidence in this result is limited by uncertainties in the best track tropical cyclone data. Nonetheless, the results highlight the importance of surface energy exchanges in global energetics of the climate system and are suggestive of the deficiencies in climate models owing to their inadequate representation of hurricanes.

Figure caption: Estimate of observed precipitation based on surface gauges, adapted from a compilation by Climate Prediction Center, NOAA (printed with permission, courtesy Rich Tinker and Jay Lawrimore). These may be underestimates as many data were missing in the vicinity of New Orleans.


Trenberth, K. E., 2008: The Impact of Climate Change and Variability on Heavy Precipitation, Floods, and Droughts, The Encyclopedia of Hydrological Sciences. John Wiley & Sons, Ltd., Chichester, UK. DOI 10.1002/0470848944.hsa211.

Summary: There is a direct influence of global warming on changes in precipitation and heavy rains. Increased heating leads to greater evaporation and thus surface drying, thereby increasing intensity and duration of drought. However, the water-holding capacity of air increases by about 7% per 1 °C warming, which leads to increased water vapor in the atmosphere, and this probably provides the biggest influence on precipitation. Storms, whether individual thunderstorms, extratropical rain or snow storms, or tropical cyclones and hurricanes, supplied by increased moisture, produce more intense precipitation events that are widely observed to be occurring, even in places where total precipitation is decreasing. In turn, this increases the risk of flooding. Patterns of where it rains also have been observed to change, with dry areas becoming drier (generally throughout the subtropics) and wet areas becoming wetter, especially in mid to high latitudes. This pattern is simulated by climate models and is projected to continue into the future. Since more precipitation occurs as rain instead of snow with warming, and snow melts earlier, there is increased runoff and risk of flooding in early spring, but increased risk of drought in deep summer, especially over continental areas.

Figure caption: Latitude-time section of zonal average annual anomalies for precipitation (%) over land from 1900 to 2005, relative to their 1961-1990 means. The values are smoothed with a 1/12(1-3-4-3-1) filter to remove fluctuations less than about six years. The color scale is nonlinear and gray areas indicate missing data (From Trenberth et al. 2007) and reproduced by permission of IPCC).


Blogs, testimonies, other publications

Models can be useful tools for planning ahead: A response to Thomas Chase: 'A caution to policymakers: climate models fail key tests for accuracy', Ogmius, 22, summer 2008, pp 2-3.

An update on human-induced climate change. The U. S. Senate Committee on Environment and Public Works, The United States Senate, Room 406 of the Dirksen Senate Building, 10:00 a.m., July 22, 2008. Senate Testimony July 2008.

Are we good stewards of the planet Earth? Graduation address to Bridge School, Boulder, Friday May 30, 2008. Boulder Daily Camera.

Nations should act now to reduce carbon emissions, Denver Post, 16 March 2008, www.denverpost.com.

"Experts Debate Global Warming", Fort Collins Forum, February 19, 2008. Trenberth responds to Gray.

Atlantic hurricanes and global warming. Google News, 23 January, 2008.