Monthly Archives: October 2006

The Interlinked Role Of Changes In Radiative Forcings And Hydrology In The Climate System by Dev Niyogi

In 2004 we published a study entitled Direct observations of the Effects of Aerosol loading on Net Ecosystem CO2 Exchanges over Different Landscapes, in Geophysical Research Letters, 31, L20506, doi:10.1029/2004GL020915. The study used observations over different landscapes to show that changes in aerosol levels alter the quantity and quality of radiation (i.e., increased diffuse radiation) reaching the surface. This in turn affects the vegetation response for transpiration and photosynthesis, which can affect the water cycle and the terrestrial carbon cycle (since transpiration is an efficient means of water loss from the earth’s surface; and photosynthesis is a major component of the terrestrial carbon cycle).

We recently expanded this analysis to investigate the effect of radiative changes on the land surface response and the feedback on the regional hydrology. The study appears in the journal ‘Natural Hazards’ (Monsoon special issue) and is entitled Potential Impacts of Aerosol-Land-Atmosphere Interaction on the Indian Monsoonal Rainfall Characteristics. The abstract reads

“Aerosols can affect the cloud – radiation feedback and the precipitation over the Indian monsoon region. In this paper, we propose that another pathway by which aerosols can modulate the multi-scale aspect of Indian monsoons is by altering the land-atmosphere interactions. The nonlinear feedbacks due to aerosol/diffuse radiation on coupled interactions over the Indian monsoon region are studied by: (i) reviewing recent field measurements and modeling studies, (ii) analyzing the MODIS and AERONET aerosol optical depth datasets, and (iii) diagnosing the results from sensitivity experiments using a mesoscale modeling system. Study results suggest that, the large magnitude of aerosol loading and its impact on land-atmosphere interactions can significantly influence the mesoscale monsoonal characteristics in the Indo-Ganges Basin.â€?

There is growing appreciation that aerosols and associated changes in radiative forcing can alter the climate system by interacting with the land surface. The recent issue of the journal Geophysical Research Letters, has two important papers related to this broader perspective.

The first paper is by Alan Robock and Haibin Li titled ‘Solar dimming and CO2 effects on soil moisture trends’. The abstract reads

“Summer soil moisture increased significantly from 1958 to the mid 1990s in Ukraine and Russia. This trend cannot be explained by changes in precipitation and temperature alone. To investigate the possible contribution from solar dimming and upward CO2 trends, we conducted experiments with a sophisticated land surface model. We demonstrate, by imposing a downward trend in incoming shortwave radiation forcing to mimic the observed dimming, that the observed soil moisture pattern can be well reproduced. On the other hand, the effects of upward CO2 trends were relatively small for the study period. Our results suggest tropospheric air pollution plays an important role in land water storage at the regional scale, and needs to be addressed accurately to study the effects of global warming on water resources.â€?

The second paper is by Wilfried Brutsaert titled ‘Indications of increasing land surface evaporation during the second half of the 20th century’. The abstract reads

“It is generally agreed that the evaporation from pans has been decreasing for the past half century over many regions of the Earth. However, the significance of this negative trend, as regards terrestrial evaporation, is still somewhat controversial, and its implications for the global hydrologic cycle remain unclear. The controversy stems from the alternative views that these evaporative changes resulted, either from global radiative dimming, or from the complementary relationship between pan and terrestrial evaporation. Actually, these factors are not mutually exclusive but act concurrently. It is shown quantitatively that, if the presently available data records are taken at face value, despite global dimming, the observed decreases in pan evaporation are generally evidence of increased terrestrial evaporation in those regions. This is consistent with independent hydrologic budget calculations for several large river basins in the USA, and likely further evidence of an accelerating hydrologic cycle in many areas.â€?

These recent studies provide additional evidence regarding the significance of land surface response in the climate system.

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Climate Science Summary

Climate Science has been posting weblogs since July 2005. This weblog highlights the topic areas and the link to each category.

Climate Change Forcings and Feedbacks

Climate Change Metrics

Climate Models

Climate Science Meetings

Climate Science Misconceptions

Climate Science Op-Eds

Climate Science Reporting

Definition of Climate

Guest Weblogs

Q&A on Climate Science

Vulnerability Paradigm

The Major Conclusions of Climate Science

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The Consequences of Nonlinearities in the Earth’s Climate System

In 2003 an article was published in Global Change Newsletter by R.A. Pielke Sr., H.J. Schellnhuber, and D. Sahagian entitled “Non-linearities in the Earth system”.

Excerpts from this article read,

“Research to-date has revealed the need to establish the limits to predictability within the Earth System. It has been shown that climate prediction needs to be treated as an initial value problem with chaotic behaviour. This perspective acknowledges that beyond some time period, our ability to provide reliable quantitative and detailed projections of climate must deteriorate to a level that no longer provides useful information to policymakers.”

The significance of this message has still has not been recognized, as exemplified by the NCAR press release of October 19, 2006 of entitled “Expect a Warmer, Wetter World this Century” which is based exclusively on multi-decadal global climate predictions (the issue of publishing climate predictions as forecasts in peer reviewed journals is a topic we will pursue in depth in a later weblog; we certainly do not publish weather forecasts before the event occurs in a peer reviewed journal!).

In our Global Change newsletter article we advocate for a different approach to environmental risk assessment that does not rely on prediction as the main tool. We write

“Even in the absence of the ability to provide skilful forecasts, there is, however, a critical societal need to identify parts of the Earth System that are particularly vulnerable to environmental variability. As such, the assessment of certain critical components – in the context of the overall non-linear system, may be useful. For example, one critical issue is water resource development, because it is influenced by environmental variability and change, and because it alters the climate system through irrigation, impoundment, draining of wetlands, and deforestation. Such “hot spotsâ€? of Earth System vulnerability need to be identified and monitored so that their non-linear interactions with the rest of the Earth System can be understood in support of policy, strategic land use practices, and general water resource planning.”

Comprehensive vulnerability assessments need to be applied to reduce the societal risk to future environmental conditions, regardless of their cause. This approach should be adopted as the scientific framework to communicate to policymakers, rather than continuing to focus on multi-decadal global climate predictions, which as our article states,

” beyond some time period…. [the] ability to provide reliable quantitative and detailed projections of climate must deteriorate to a level that no longer provides useful information to policymakers.”

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2006 SORCE Science Meeting Overview

An overview of the Solar Radiation and Climate Experiment (SORCE) meeting that has been discussed on the Climate Science Weblog (e.g. see) is given in the September/October SORCE newsletter. A link to the Solar Spectral Irradiance & Climate Modeling Workshop that immediately preceded the SORCE meeting is also provided as part of this overview (see).

The agenda and powerpoint slides (for those that are available) for that excellent workshop are given below:

Presentation 1 Workshop Goals
Peter Pilewskie, LASP

Presentation 2 Welcome / SORCE Update
Tom Woods, LASP

Presentation 3 Overview of measured SORCE SSI and its variability
Jerry Harder, LASP

Presentation 4 Empirical Modes of UV Variations
Alexander Ruzmaikin, JPL, California Institute of Technology

Presentation 5 Overview of models of SSI variability (time scales, users, applications)
Judith Lean, NRL, Washington DC

Presentation 6 Paleoclimate applications
Caspar Ammann, NCAR

Discussion (led by Peter Pilewskie)

Presentation 7 Coupled climate/atmospheric models
WACCM – Dan Marsh, NCAR
HAMMONIA – Hauke Schmidt, Max Planck Institute, Germany & Guy Brasseur, NCAR
GISS – Judith Lean, NRL

Presentation 8 Earth atmospheric radiative transfer applications
Jennifer Delamere, Atmospheric and Environmental Research (AER), Inc.

Presentation 9 Spectral Solar Irradiance Sources
Juan Fontenla, LASP

Presentation 10 LASP Interactive Solar Irradiance Datacenter (LISIRD) Update
Marty Snow, LASP

Discussion (led by Peter Pilewskie)

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A New Dataset for Improved Land-Surface Representation for Climate Studies by Dev Niyogi

A number of articles on this blog highlight the critical role land-surface representation plays in the simulation of climate scenarios. Better land-surface representation is often achieved by enhanced representation of the land-surface processes and land-atmosphere interactions, but despite that, one significant limitation that often remains is the lack of global soil moisture datasets that can be used for developing the model scenarios. Soil moisture representation is considered important since errors in the initializing soil moisture can persist for a significant period (i.e., soil moisture errors have high memory), and lead to higher uncertainty in the model results. Soil moisture availability often controls surface albedo, and the surface Bowen ratio and thus affects the surface temperature but can also contribute to the regional moisture recycling (evaporation and precipitation). Recently new results from the Global Soil Wetness Project (GSWP) were reported by Dirmeyer et al. (GSWP-2: Multimodel Analysis and Implications for Our Perception of the Land Surface, Paul A. Dirmeyer, Xiang Gao, Mei Zhao, Zhichang Guo, Taikan Oki, and Naota Hanasaki, Bulletin of American Meteorological Society, vol. 87 (10), p. 1381-1397 (Available from www.ametsoc.org). The GSWP-2 analysis combines “the simulations of more than a dozen different global land surface models, an unprecedented analysis of terrestrial water and energy budgets is realized.” The GSWP-2 data are available from http://www.iges.org/gswp/ as well as http://haneda.tkl.iis.u-tokyo.ac.jp/gswp2/ and http://www.monsoondata.org:9090/dods/gswp2mma/

Excerpts from the paper read “…the first global multimodel analysis of land surface state variables and fluxes for potential use by meteorologists, hydrologists, engineers, biogeochemists, agronomists, botanists, ecologists, geographers, climatologists, and educators. This is, in many respects, a land surface analog to the atmospheric reanalyses ….but one that encompasses an ensemble of different LSSs (i.e. land surface schemes). Using the results of multiple LSSs provides a result that is not dependent on a single model, is generally superior to the results of any individual model, and is typically as good as or better than the best model at each point and time.” It will be of interest to see how the climate community utilizes this new product and the potential changes that would result in the model scenarios.

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Observational Estimates of Radiative Forcing due to Land Use Change in Southwest Australia

We have a new paper accepted for publication in the Journal of Geophysical Research. It is by U.S.Nair, U.S., D.K. Ray, J. Wang, S.A Christopher, T. Lyons, R.M. Welch, and R.A. Pielke Sr. and is entitled “Observational estimates of radiative forcing due to land use change in southwest Australia”.

In addition to the very important regional scale issues of climate change that this paper illustrates, the study also shows the need to investigate regional scale albedo changes due to land cover change, as necessary to scale up to the global scale in order to more accurately estimate the global average radiative forcing of this particular climate forcing.

The abstract of the paper reads,

“Radiative forcing associated with land use change is largely derived from Global Circulation Models (GCM), and the accuracy of these estimates depends on the robustness of the vegetation characterization used in the GCMs. In this study, we use observations from the Clouds and Earth’s Radiant Energy System (CERES) instrument onboard the Terra satellite to report top-of –the-atmosphere radiative forcing values associated with clearing of native vegetation for agricultural purposes in southwest Australia. Over agricultural areas, observations show consistently higher shortwave fluxes at the top-of-the atmosphere (TOA) compared to native vegetation, especially during the time period between harvest and planting. Estimates using CERES observations show that, over a specific area originally covered by native vegetation, replacement of half the area by croplands results in a diurnally averaged shortwave radiative forcing of approximately -7 Wm-2. GCM-derived estimates for areas with 30% or more croplands range from -1 to -2 Wm-2 compared to observational estimate of -4.2 Wm-2, thus significantly underestimating radiative forcing due to land use change by a factor of 2 or more. Two potential reasons for this underestimation are incorrect specification of the multi-year land use change scenario and the inaccurate prescription of
seasonal cycles of crops in GCM’s.”

One excerpt from the paper reads,

“Betts (2001) and Hansen et al. (1997) both report that the radiative forcing due to land use change is most significant in the mid-latitude agricultural areas that experience snowfall, which accentuates the albedo effect associated conversion of forests to farm lands. The present analysis suggests that shortwave radiative forcing associated with agricultural land use may be significant even in areas without frequent snow cover.”

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A Global Warming Currency

As has been discussed on Climate Science, the focus for policymakers with respect to human-caused climate change has been on predicting the increase in the global average surface temperature in the coming decades (e.g. see). This is presented, for example, as a change in the global average surface temperature due to a doubling of the atmospheric concentrations of CO2.

However, this approach has serious limitations. Not only is the ability to quantitatively measure the global average surface temperature an issue (e.g. see), but there are lags with respect to radiative forcing and the temperature response as well as the “climate sensitivity” [which should be called the "global average surface temperature sensitivity"] to the radiative forcings (e.g. see).

The adoption of heat, however, in units of Joules is a much more robust “currency” to monitor global warming and cooling (see). As has been discussed on Climate Science, most of the heat content changes within the climate system occur in the oceans. The recent Lyman et al paper on the observed ocean cooling between 2003 and 2005 (see) is a very effective example of how the change in ocean heat content can be used to diagnose radiative forcing.

Using Joules as the global warming and cooling unit of “currency”, the scientific community can present projections of its change over the coming decades, as well as the fraction that they attribute to each human- and natural-climate forcing.

For example, Jim Hansen has written (see)

“Contrary to the claim of Pielke and Christy, our simulated ocean heat storage (Hansen et
al., 2005) agrees closely with the observational analysis of Willis et al. (2004)…….
The Willis et al. measured heat storage of 0.62 W/m2 refers to the decadal mean for the
upper 750 m of the ocean. Our simulated 1993-2003 heat storage rate was 0.6 W/m2 in the upper 750 m of the ocean. The decadal mean planetary energy imbalance, 0.75 W/m2, includes heat storage in the deeper ocean and energy used to melt ice and warm the air and land. 0.85 W/m2 is the imbalance at the end of the decade.”

The value presented in this Hansen comment 0f 0.85 W/m2 indicates that for the period 2000-2009, we should see a climate system accumulation of heat of 1.36 X 10**23 Joules.

The observations reported in Lyman et al indicate

“We observe a net loss of 3.2 (± 1.1) X 10**22 J of heat from the upper ocean between 2003 and 2005.” [3.2 X 10**22 Joules = 0.32 X 10 **23 Joules]

Clearly, the radiative imbalance for the remainder of this decade must be greater than presented by Jim Hansen to compensate for this heat loss.

To assess the forecast of climate system heat content change, the frequent update of ocean heat storage change should be a major priority. Rather than focusing on a global average surface temperature to monitor global warming (or cooling), the scientific and policy communities need to move to the assessment of heat itself as the currency.

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