Monthly Archives: April 2006

Important New Climate Satellite to be Launched

As announced in a Colorado State University press release, an important new satellite mission;

“CloudSat, a NASA Earth System Science Pathfinder Mission, is expected to launch into orbit on April 21 at Vandenberg Air Force Base, Calif. The world’s most sensitive cloud radar is designed to measure properties of clouds essential to understanding Earth’s weather and climate processes.

‘CloudSat observations will allow us to peer inside clouds, like a CT scan, to observe for the first time the important processes that affect our weather and climate on the large, global scale,’ said Graeme Stephens of Colorado State’s Department of Atmospheric Science who has worked on CloudSat for 12 years.”

Stephens is CloudSat’s principal investigator.

“‘Clouds are a very important part of Earth’s weather and climate, and the lack of understanding of cloud feedback is widely acknowledged in the scientific community to be a major obstacle confronting credible prediction of climate change,’ said co-principal investigator Deborah Vane of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. ‘Despite the fundamental role of clouds in climate and weather, there is much that we do not know. The CloudSat mission aims to provide observations necessary to greatly advance understanding of these climate issues.'”

In a related news release, Professor Stephens states,

“CloudSat could also provide new insight into climate change and global warming. ‘Clouds grossly affect the greenhouse effect on the planet,’ said Stephens. ‘So they play a very profound role in weather and climate. Yet taken altogether they are one of the most poorly understood aspects of the climate change problem.'”

The successful launch of this new atmospheric monitoring system and its detailed diagnosis of clouds will be a very important achievement. The need for this data shows that the understanding and skillful prediction of the climate system has more uncertainties than has been recognized in multi-decadal climate projections, such as reported by the U.S. National Assessment of the Potential Consequences of Climate Variability and Change and the Intergovernmental Panel on Climate Change assessments.

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Tropical Deforestation as a First-Order Climate Forcing

The Economist, in its March 25, 2006 issue, had an excellent article on “the Logging Trade” (subscription required). While the article focused on the issue of illegal logging, it very much has a climate forcing perspective which should have also been included in the article. An extract from the article states,

“Indonesia is losing almost 2m hectares of forest a year—an area about the size of Wales or Massachusetts. Over the past 15 years, that amounts to one-quarter of its total forest cover. For virgin forest, the most commercially and ecologically valuable sort, the statistics are even worse: one-third of Indonesia’s stock has vanished over the past 15 years, and it continues to disappear at a rate of 3% a year. In some countries, such as Nigeria and Honduras, the destruction of tropical forests is proceeding even faster, although globally, the rate is rather slower, at 0.62% a year.

As we discuss in our paper “Pielke Sr., R.A., J.O. Adegoke, T.N. Chase, C.H. Marshall, T. Matsui, and D. Niyogi, 2006: A new paradigm for assessing the role of agriculture in the climate system and in climate change. Agric. Forest Meteor., Special Issue, in press.”, we discuss the role of landscape change, which, of course, includes deforestation, as a first order climate forcing. In the tropical regions, such deforestation can result in major alterations of monsoon circulations as thunderstorm patterns are altered in response to the land conversion (see and see).

As the Australian Conservation Foundation reported in 2001, landscape conversion continued at a rapid pace in the 1990s at many places around the world.

While it has been recognized that tropical deforestation alters climate in terms of its effect on atmospheric concentrations of CO2, its influence as a heterogeneous regional climate forcing in changing thunderstorm patterns appears to be of a greater consequence in terms of alterations of weather patterns regionally and globally.

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New Paper on the Importance of Regional Climate Forcing

On the Climate Science weblog, the importance of heterogeneous regional climate forcing has been emphasized (e.g. see and see), since weather pattern variability and change result from regional tropospheric temperature variability and change. A global average surface temperature trend or globally averaged top-of-the-atmosphere radiative forcing , as the icons to describe weather pattern trends, are inadequate choices.

The 2005 National Research Council Report listed a priority recommendation to

“Use climate records to investigate relationships between regional radiative forcing (e.g., land-use or aerosol changes) and climate response in the same region, other regions, and globally.”

The Report also stated that,

“‘regional variations in radiative forcing may have important regional and global climate implications that are not resolved by the concept of global mean radiative forcing.”

and that,

“This regional diabatic heating produces temperature increases or decreases in the layer-averaged regional troposphere. This necessarily alters the regional pressure fields and thus the wind pattern. This pressure and wind pattern then affects the pressure and wind patterns at large distances from the region of the forcing which we refer to as teleconnections.â€?

A new paper has been accepted by Geophysical Research Letters entitled “Measurement-Based Estimation of the Spatial Gradient of Aerosol Radiative Forcing” by Toshihisa Matsui and Roger A. Pielke Sr. which provides quantitative demonstration of the importance of the heterogeneous regional climate forcings, and confirms the 2005 National Research Council findings. The paper diagnoses aerosol radiative heating for the period March 1 2000 and February 28 2001 for the latitude band between 37N and 37S.

The abstract of the paper reports that,

“This paper diagnoses the spatial mean and the spatial gradient of the aerosol radiative forcing in comparison with those of well-mixed green-house gases (GHG). Unlike GHG, aerosols have much greater spatial heterogeneity in their radiative forcing. The heterogeneous diabatic heating can modulate the gradient in horizontal pressure field and atmospheric circulations, thus altering the regional climate. For this, we diagnose the Normalized Gradient of Radiative Forcing (NGoRF), as a fraction of the present global heterogeneous insolation attributed to human activity. Although the GHG has a larger forcing (+1.7 Wm-2) as measured than those of aerosol direct (-1.59 Wm-2) and possible indirect effect (-1.38 Wm-2) in terms of a spatially averaged top-of-atmosphere (TOA) value, the aerosol direct and indirect effects have far greater NGoRF values (~0.18) than that of GHG (~0.003).”

Among the conclusions that are in the paper,

“This paper evaluates the mean and spatial gradient of aerosol radiative forcing in comparison with that of the well-mixed GHG. The appropriate metric to assess the importance of the gradient of diabatic heating is the resulting gradients in the horizontal pressure field that fundamentally drives the atmospheric circulation…… ”

The message from this paper is that neglecting the role of human- and natural regional climate forcings in climate change and variability, but instead focusing on global average metrics, necessarily results in an erroneous understanding of the climate system, including the human influence on it. Communicating to policymakers a global average surface temperature trend or global averaged top-of-the-atmosphere radiative forcing as the icons of climate change are seriously misleading in terms of how weather is actually influenced by human- and natural- climate forcings.

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Comments on the Jim Hansen “super-El Niño Prediction”

There has already been effective discussion on the forecast of Jim Hansen on the “super- El Niño Prediction” (see and see).

This weblog provides background on this issue, as well as raises questions based on observations as to the science behind his pronouncement.

The conclusion that more frequent and intense El Niño should be expected was made by Kevin Trenberth in the 1997 and 1999, as reported in National Center for Atmospheric Research press releases. In 1997 he was quoted as saying,

“Trenberth believes that global warming and El Niño reinforce each other in their impact on the environment and society, primarily through their combined effects on the hydrological cycle and the repercussions for water supplies.”

In 1999, the news release stated,

“Trenberth theorizes that much of the additional heat trapped by increasing amounts of greenhouse gases may be going into the oceans. It is later released through El Ninos that are larger, more frequent, or less efficient in releasing the ocean-stored heat. The atmospheric warming induced by El Nino also helps to further dry out regions already prone to drought during
El Nino, such as Indonesia, Australia, and parts of Africa and Brazil.

“Naturally occurring droughts, such as from ENSO, are likely to set in quicker, plants will wilt sooner, and the droughts may become more extensive and last longer with global warming.”

The NCAR press releases are based on peer reviewed papers by Trenberth; e.g. the 1996 Trenberth and Hoar Geophysical Research Letters paper entitled “The 1990–1995 El Niño-Southern Oscillation event: Longest on record”. An extract from their abstract states,

“Both the recent trend for more ENSO events since 1976 and the prolonged 1990-1995 ENSO event are unexpected given the previous record, with a probability of occurrence about once in 2,000 years. This opens up the possibility that the ENSO changes may be partly caused by the observed increases in greenhouse gases. ”

In the conclusion of the paper, they stated with respect to the trend in more ENSO events.

“Is this pattern of change a manifestation of the global warming and related climate change associated with increases in greenhouse gases in the atmosphere? Or is this pattern a natural decadal-timescale variation? We have shown that the later is highly unlikely.”

The current observed ocean heat content anomalies (e.g. see the European Centre for Medium Range Forecasting (ECMWF) analyses at 5 meter depth and in the equator crossection) do not show evidence of anomalous precursors to an extreme El Niño.

Indeed the accumulation of heat at depth in the oceans that was reported in 2004 by J. Willis, D. Roemmich, and B. Cornuelle in the Journal of Geophysical Research ” Interannual variability in upper ocean heat content, temperature, and thermosteric expansion on global scales suggests that a significant portion of the observed recent global warming is unavailable for short-term feedback into the atmospheric portion of the climate system.

An extract from their abstract states that,

“Maps of yearly heat content anomaly show patterns of warming commensurate with ENSO variability in the tropics, but also show that a large part of the trend in global, oceanic heat content is caused by regional warming at midlatitudes in the Southern Hemisphere.”

In their paper, they report that,

“……a strong, fairly linear warming trend is visible in the Southern Hemisphere, centered on 40°S. This region accounts for a large portion of the warming in the global average.”

Also,

“……..the warming around 40°S appears to be much steadier over the course of the time series, as seen in Figure 7. In addition, this warming extends deeper and is more uniform over the water column than the signal in the tropics. ”

Finally,

“…..the warming rate in the early 1970s is comparable to the present rate. This suggests that the present rate is not outside the range of recent decadal variations. With the present time series, it is therefore not possible to identify whether the recent increase in ocean warming is due to an acceleration of heat uptake by the ocean or is simply decadal variability. An additional 5 to 10 years of data will be necessary before such a distinction is likely to be possible.”

(I discussed this issue also in other Climate Science weblogs; e.g. see and see)

I have invited Jim Hansen to write a guest weblog on Climate Science, but he has not replied.

Clearly, in reading the literature, an obvious question to Jim Hansen is how the accumulation of ocean heat content in the Southern Hemisphere midlatitudes through depth provides energy for a super-El Niño? There seems to be a disconnect between the observations and the prediction, which Dr. Hansen needs to clarify.

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Interesting Quotes on the State of Our Understanding of Climate Science

On today’s weblog, several examples of quotes are reproduced below that demonstrate the limitations of our current understanding of human- and natural- climate variability and change. The quotes are from individuals (Mike MacCracken, Joel Smith and Anthony Janetos, and Jim Hansen, each of whom is a well respected scholar) and from a CCSP Committee.

1. From page 145 of the 2006 Response to the Public Comment of the CCSP Report “Synthesis and Assessment Product 1.1 Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences?”

“Owing to natural internal variability, models cannot be expected to reproduce regional patterns of trend over a period as short as 20 years from changes of radiative forcings alone.”

2. From MacCracken, Michael, Joel Smith and Anthony C. Janetos, 2004. Reliable regional climate model not yet on horizon. Nature Vol. 429, No 6993, p. 699,
June 17, 2004.

“The US National Assessment of the Potential Consequences of Climate Variability and Change (USNA) – in which we were involved-did not attempt to provide regional or even national predictions of climate change……”

Later in the letter in Nature, they conclude with,

” We strongly agree that much more reliable regional climate simulations and analyses are needed. However, at present,…….such simulations are more aspiration than reality.”

3. From the February 2004 issue of Scientific American Magazine
Jim Hansen has an interesting quote on page 24 in “Defusing the Global Warming Time Bomb”

“It will not be possible to optimize strategies for dealing with global
warming until all important climate forcings, including carbonaceous
aerosols, have been well quantified.”

Clearly, if we accept that

“The needed focus for the study of climate change and variability is on the regional and local scales. ”

the conclusion on the Climate Science weblog that

“In terms of climate change and variability on the regional and local scale, the IPCC Reports, the CCSP Report on surface and tropospheric temperature trends, and the U.S. National Assessment have overstated the role of the radiative effect of the anthropogenic increase of CO2 relative to the role of the diversity of other human climate climate forcing on global warming, and more generally, on climate variability and change”;

“Global and regional climate models have not demonstrated skill at predicting climate change and variability on multi-decadal time scales “;

and

“Attempts to significantly influence regional and local-scale climate based on controlling CO2 emissions alone is an inadequate policy for this purpose. ”

have published supportive quotes from what might be expected to be unlikely individuals. The quotes that are reproduced above should be discussed widely by the climate science community, including the individuals who are quoted to determine if they continue to agree with what they published.

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Another paper on the Important Role of Land Surface Processes Within the Climate System

The papers that document scientifically the role of land surface processes as a first order climate forcing continue. A new article is in the May 2006 issue of Climate Dynamics by V. Brovkin , M. Claussen, E. Driesschaert, T. Fichefet, D. Kicklighter, M. F. Loutre, H. D. Matthews, N. Ramankutty, M. Schaeffer, 1 and A. Sokolov and is entitled “Biogeophysical effects of historical land cover changes simulated by six Earth system models of intermediate complexity”. What is particularly appealing regarding the models used in this study is that climate is treated as a coupled system involving the oceans, land, atmosphere and cryosphere, rather than relying on atmosphere-ocean coupled models to be the dominate driver of land surface processes. This coupled approach was recommended in the 2005 NRC Report (see Figure 1-1).

The abstract reads

“Six Earth system models of intermediate complexity that are able to simulate interaction between atmosphere, ocean, and land surface, were forced with a scenario of land cover changes during the last millennium. In response to historical deforestation of about 18 million sq km, the models simulate a decrease in global mean annual temperature in the range of 0.13–0.25°C. The rate of this cooling accelerated during the 19th century, reached a maximum in the first half of the 20th century, and declined at the end of the 20th century. This trend is explained by temporal and spatial dynamics of land cover changes, as the effect of deforestation on temperature is less pronounced for tropical than for temperate regions, and reforestation in the northern temperate areas during the second part of the 20th century partly offset the cooling trend. In most of the models, land cover changes lead to a decline in annual land evapotranspiration, while seasonal changes are rather equivocal because of spatial shifts in convergence zones. In the future, reforestation might be chosen as an option for the enhancement of terrestrial carbon sequestration. Our study indicates that biogeophysical mechanisms need to be accounted for in the assessment of land management options for climate change mitigation.”

This paper illustrates further why we need to focus on landscape change as a major influence on climate variability and change. These types of models, however, need to move beyond focusing on a global average metric (the “global mean annual temperature”) and CO2 concentrations. There is the clear societal need to explore regional variability and changes in climate that result due to advertant and inadvertant land management practices, as well as regional variability and change due to the other first order climate forcings.

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A Global/Mesoscale Modeling Breakthrough

A new GRL paper on applying fine scale spatial resolution in a global model has appeared. This is a major engineering breakthrough and provides a means to avoid some of the pitfalls of dynamic downscaling of a gobal model using a different mesoscale/regional model (see).

The article (subscription required) is by

Shen, B.-W.; Atlas, R.; Chern, J.-D.; Reale, O.; Lin, S.-J.; Lee, T.; Chang, J. is entitled “The 0.125 degree finite-volume general circulation model on the NASA Columbia supercomputer: Preliminary simulations of mesoscale vortices”
Geophys. Res. Lett., Vol. 33, No. 5, L05801

The abstract reads,

” The NASA Columbia supercomputer was ranked second on the TOP500 List in November, 2004. Such a quantum jump in computing power provides unprecedented opportunities to conduct ultra-high resolution simulations with the finite-volume General Circulation Model (fvGCM). During 2004, the model was run in realtime experimentally at 0.25 degree resolution producing remarkable hurricane forecasts (Atlas et al., 2005). In 2005, the horizontal resolution was further doubled, which makes the fvGCM comparable to the first mesoscale resolving General Circulation Model at the Earth Simulator Center (Ohfuchi et al., 2004). Nine 5-day 0.125 degree simulations of three hurricanes in 2004 are presented first for model validation. Then it is shown how the model can simulate the formation of the Catalina eddies and Hawaiian lee vortices, which are generated by the interaction of the synoptic-scale flow with surface forcing, and have never been reproduced in a GCM before.”

An extract from the conclusion states (“GCMS’ refer to Global Circulation Models and “MMs” refer to Mesoscale Models),

“Traditionally, GCMs and MMs have been developed by different research groups with different goals. Because of a breakthrough in computing power provided by the Columbia supercomputer, the 0.125° fvGCM becomes one of a few mesoscale resolving GCMs, and has a resolution comparable to the first mesoscale GCM at the Earth Simulator Center, and to the finest resolution of the NASA QuikSCAT 12.5 km seawinds data. Numerical simulations of mesoscale vortices, which include three major hurricanes, the Catalina Eddy and Hawaiian vortices in 2004, have been discussed to demonstrate the model’s capability of simulating scale interactions between convection and large-scale flow, between coastal surface forcing and synoptic-scale flow, and between high mountains and nonlinear flow. To our knowledge, the fvGCM is the first to simulate the formation of these mesoscale vortices in a global environment”

The model has already demonstrated important potential predictive skill with hurricane track (see Shen et al., 2005: Hurricane forecasts with a global mesoscale resolving model on the NASA Columbia Supercomputer preliminary simulations of Hurricane Katrina (2005) ).

This new pioneering model will permit improved understanding of climate processes. It also is a challenge to other global/regional/mesoscale modeling groups.

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Climate Science Weblog Summary

The text below was posted today as a preamble dated April 4 2006 on the Climate Science Weblog, where it will remain indefinitely. Constructive scientific comments that support or refute the conclusions listed are welcome.

The Climate Science Weblog has clearly documented the following conclusions since July 2005:

1. The needed focus for the study of climate change and variability is on the regional and local scales. Global and zonally-averaged climate metrics would only be important to the extent that they provide useful information on these space scales.

2. Global and zonally-averaged surface temperature trend assessments, besides having major difficulties in terms of how this metric is diagnosed and analyzed, do not provide significant information on climate change and variability on the regional and local scales.

3. Global warming is not equivalent to climate change. Significant, societally important climate change, due to both natural- and human- climate forcings, can occur without any global warming or cooling.

4. The spatial pattern of ocean heat content change is the appropriate metric to assess climate system heat changes including global warming.

5. In terms of climate change and variability on the regional and local scale, the IPCC Reports, the CCSP Report on surface and tropospheric temperature trends, and the U.S. National Assessment have overstated the role of the radiative effect of the anthropogenic increase of CO2 relative to the role of the diversity of other human climate climate forcing on global warming, and more generally, on climate variability and change.

6. Global and regional climate models have not demonstrated skill at predicting climate change and variability on multi-decadal time scales.

7. Attempts to significantly influence regional and local-scale climate based on controlling CO2 emissions alone is an inadequate policy for this purpose.

8. A vulnerability paradigm, focused on regional and local societal and environmental resources of importance, is a more inclusive, useful, and scientifically robust framework to interact with policymakers, than is the focus on global multi-decadal climate predictions which are downscaled to the regional and local scales. The vulnerability paradigm permits the evaluation of the entire spectrum of risks associated with different social and environmental threats, including climate variability and change.

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Important New Meeting on Climate Change is Scheduled

A climate meeting is scheduled which promises to include a diversity of perspectives on climate forcings and feedbacks. The meeting is the 2006 SORCE Science Meeting, motivated by the NASA/EOS Solar Radiation and Climate Experiment (SORCE). The agenda consists of invited and contributed oral and poster presentations. Participation is encouraged by the Conference organizers. The abstract deadline is July 14, 2006.

As described on the meeting website,

“SCIENCE PROGRAM OVERVIEW

Solar radiation is the primary energy source for many processes in Earth’s environment and is responsible for driving the atmospheric and oceanic circulations. Since its launch in 2003, the SOlar Radiation and Climate Experiment (SORCE) has measured solar irradiance at the top of the Earth’s atmosphere with unprecedented accuracy, precision, and spectral coverage across the ultraviolet, visible, and near-infrared regions of the spectrum. The magnitude and spectral distribution of solar radiation is modified from the SORCE-measured values via scattering and absorption within the atmosphere and at the surface. Identifying and understanding those processes which perturb the distribution of solar and terrestrial radiative energy is essential in determining the climate response to changes in concentrations of various gases and aerosol particles from natural and anthropogenic sources, as is discerning their associated feedback mechanisms.

The theme of the 2006 SORCE Science Team Meeting is The Earth’s Radiative Energy Budget Related to SORCE. Several of the key questions and issues to be addressed include:

What is the present state of knowledge of the Earth’s radiation budget from space, from within that atmosphere, and at the surface?

What are the key processes that control Earth’s albedo?

What are the key radiative forcing agents, of natural and anthropogenic origin, and how have their relative influences changed over the past three centuries?

What are the important feedback mechanisms for regulating Earth’s climate?

What is the sensitivity of climate to induced radiative forcing and over what time scales does climate respond?

What is the role of the biosphere? ”

This meeting will permit an update of the National Research Council Reports:

National Research Council, 2005: Radiative forcing of climate change: Expanding the concept and addressing uncertainties. Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C.,

and

National Research Council, 2003: Understanding Climate Change Feedbacks (2003)
Board on Atmospheric Sciences and Climate, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C.

Invited confirmed speakers and topics include:

Al Arking, Johns Hopkins University, Baltimore, Maryland
Short- and long-wave surface radiation budgets: implications for climate

Roni Avissar, Duke University, Durham, North Carolina
Land use/land change

Robert Cahalan, GSFC, NASA, Greenbelt, Maryland
Clouds and radiation

Jim Coakley, Oregon State University, Corvallis
The aerosol indirect effect

Bill Collins, National Center for Atmospheric Research (NCAR), Boulder, Colorado
Radiative forcing by greenhouse gases

Judy Curry, Georgia Institute of Technology, Atlanta
Hurricane response in the climate system

Ellsworth Dutton, NOAA Earth System Research Laboratory, Boulder, Colorado
The surface radiative energy budgets

Jerry Harder, LASP, University of Colorado, Boulder
The role of VIS-IR / SIM in climate science

Jay Herman, GSFC, NASA, Greenbelt, Maryland
Ozone variability and the biosphere

Ken Jezek, Byrd Polar Research Center at Ohio State University, Columbus
The ice feedback

Greg Kopp, LASP, University of Colorado, Boulder
The role of TSI / TIM in climate science

Judith Lean, Naval Research Laboratory, Washington, DC
Solar radiative forcing

Norm Loeb, NASA Langley Research Center, Hampton, Virginia
The accuracy of TSI / SSI in climate models

Bill McClintock, LASP, University of Colorado, Boulder
The role of UV / SOLSTICE in climate science

Roger Pielke Sr., University of Colorado, Boulder
Regional and global climate forcings

Peter Pilewskie, LASP, University of Colorado, Boulder
Overview of the radiation budget in the lower atmosphere

V. Ramanathan, Scripps Inst. of Oceanography, Univ. of California, San Diego
The regulation of Earth’s albedo

Graeme Stephens, Colorado State University, Fort Collins
The cloud-climate feedback

Ka-Kit (KK) Tung, University of Washington, Seattle
Climate responses to forcing

Bruce Wielicki, NASA Langley Research Center, Hampton, Virginia
Earth’s radiation budget from space

Tom Woods, LASP, University of Colorado, Boulder
SORCE mission update, The role of EUV/XUV / XPS in climate science

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Global Precipitation Trends

The excellent “No Se Nada” weblog of Kevin Vranes alerted us to an important new paper on precipitation trends.

The new GEOPHYSICAL RESEARCH LETTERS paper by Thomas M. Smith, Xungang Yin and Arnold Gruber is entitled “Variations in annual global precipitation (1979–2004), based on the Global Precipitation Climatology Project 2.5° analysis”,

with the abstract,

“The Global Precipitation Climatology Project (GPCP) has produced a combined satellite and in situ global precipitation estimate, beginning 1979. The annual average GPCP estimates are here analyzed over 1979–2004 to evaluate the large-scale variability over the period. Data inhomogeneities are evaluated and found to not be responsible for the major variations, including systematic changes over the period. Most variations are associated with El Niño/Southern Oscillation (ENSO) episodes. There are also tropical trend-like changes over the period, correlated with interdecadal warming of the tropical SSTs and uncorrelated with ENSO. Trends have spatial variations with both positive and negative values, with a global-average near zero.”

The conclusion of their paper (subscription required) includes the text,

“The merged satellite and in situ GPCP global precipitation annual averages were examined for 1979–2004. Most variations are associated with ENSO and have no trend. A separate mode of variation shows a trend over the period. Testing indicates that this trend is significant and is not caused by data inhomogeneities. The trend mode is associated with simultaneous tropical SST variations over the period, with increased tropical precipitation over the Pacific and Indian Oceans associated with local warming of the SSTs. Increased precipitation in some regions is balanced by decreased precipitation in other regions, and the global average change is near zero. Although the trend mode is strong for this period, the record length is barely long enough to begin evaluation of interdecadal variations.”

This paper is interesting as one of the students in my class in the Spring of 2005 , Sheri Conner Gausepohl, explored this important issue. Her results are consistent with the conclusions in the new Smith et al paper. In our class, we concluded that the more uniform spatial coverage permitted by satellites provides a more accurate diagnosis of trends in a global context, in this case for precipitation, than do studies that focus primarily on in-situ surface data.

In addition, the Smith et al GRL study further documents the loss of information when there is a focus on a global average metric (in this case, they found that “the global average change is near zero”). The long term trends and interannual variability that matter to society are on the regional scale (e.g. droughts and floods) in which the Smith et al study did find important changes (i.e. “Increased precipitation in some regions is balanced by decreased precipitation in other regions…”).

The study also raises questions on the extent to which water vapor content has increased globally, if the global averaged precipitation has not changed significantly. A positive atmospheric water vapor feedback, in response to an increase in the radiative warming of the well-mixed greenhouse gases, is at the foundation of the IPCC perspective on global warming.

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