A simple tool for refining GCM water availability projections, applied to Chinese catchments

This is the final version. Available from the European Geosciences Union via the DOI in this record.The discussion paper version of this article was published in Hydrology and Earth System Sciences Discussions and is available in ORE at http://hdl.handle.net/10871/34612There is a growing desire for reliable 21st-century projections of water availability at the regional scale. Global climate models (GCMs) are typically used together with global hydrological models (GHMs) to generate such projections. GCMs alone are unsuitable, especially if they have biased representations of aridity. The Budyko framework represents how water availability varies as a non-linear function of aridity and is used here to constrain projections of runoff from GCMs, without the need for computationally expensive GHMs. Considering a Chinese case study, we first apply the framework to observations to show that the contribution of direct human impacts (water consumption) to the significant decline in Yellow River runoff was greater than the contribution of aridity change by a factor of approximately 2, although we are unable to rule out a significant contribution from the net effect of all other factors. We then show that the Budyko framework can be used to narrow the range of Yellow River runoff projections by 34%, using a multi-model ensemble and the high-end Representative Concentration Pathway (RCP8.5) emissions scenario. This increases confidence that the Yellow River will see an increase in runoff due to aridity change by the end of the 21st century. Yangtze River runoff projections change little, since aridity biases in GCMs are less substantial. Our approach serves as a quick and inexpensive tool to rapidly update and correct projections from GCMs alone. This could serve as a valuable resource when determining the water management policies required to alleviate water stress for future generations.Natural Environment Research CouncilUK–China Research & Innovation Partnership Fun

Are changes in global precipitation constrained by the tropospheric energy budget?

Copyright © 2009 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act September 2010 Page 2 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC §108, as revised by P.L. 94-553) does not require the AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a web site or in a searchable database, or other uses of this material, except as exempted by the above statement, requires written permission or a license from the AMS. Additional details are provided in the AMS Copyright Policy, available on the AMS Web site located at (http://www.ametsoc.org/) or from the AMS at 617-227-2425 or [email protected] tropospheric energy budget argument is used to analyze twentieth-century precipitation changes. It is found that global and ocean-mean general circulation model (GCM) precipitation changes can be understood as being due to the competing direct and surface-temperature-dependent effects of external climate forcings. In agreement with previous work, precipitation is found to respond more strongly to anthropogenic and volcanic sulfate aerosol and solar forcing than to greenhouse gas and black carbon aerosol forcing per unit temperature. This is due to the significant direct effects of greenhouse gas and black carbon forcing. Given that the relative importance of different forcings may change in the twenty-first century, the ratio of global precipitation change to global temperature change may be quite different. Differences in GCM twentieth- and twenty-first-century values are tractable via the energy budget framework in some, but not all, models. Changes in land-mean precipitation, on the other hand, cannot be understood at all with the method used here, even if land–ocean heat transfer is considered. In conclusion, the tropospheric energy budget is a useful concept for understanding the precipitation response to different forcings but it does not fully explain precipitation changes even in the global mean

The missing aerosol response in twentieth-century mid-latitude precipitation observations

Copyright © 2014 Nature Publishing GroupRegional temperature change over the twentieth century has been strongly influenced by aerosol forcing [1, 2]. The aerosol effect is also expected to be pronounced on regional precipitation change [3]. Changes in historical precipitation—for the global mean and land mean of certain regions—should be more sensitive to spatially heterogeneous aerosol forcing than greenhouse gas forcing [4, 5, 6, 7]. Here, we investigate whether regional precipitation and temperature respond predictably to a significant strengthening in mid-twentieth-century Northern Hemisphere mid-latitude (NHML) aerosol forcing. Using the latest climate model experiments, we find that observed regional temperature changes and observed Northern Hemisphere tropical land precipitation changes are consistent with the IPCC Fifth Assessment Report [8] aerosol forcing estimate, but observed NHML land precipitation changes show little evidence of an aerosol response. This may be a result of changes in precipitation measurement practice that increased observed precipitation totals at the same time that aerosol forcing was expected to reduce them [9]. Investigating this inconsistency, we calculate the required increase in early-twentieth-century observed NHML land precipitation to bring this result in line with aerosol forcing. Biases greater than this calculated correction have been identified in countries within the NHML region previously, notably the former Soviet Union [9, 10]. These observations are frequently used as a metric for the quality of model-simulated precipitation. More homogeneity studies would be of huge benefit

Physical Mechanisms of Tropical Climate Feedbacks Investigated Using Temperature and Moisture Trends

ArticleOpen access articleTropical climate feedback mechanisms are assessed using satellite-observed and model-simulated trends in tropical tropospheric temperature from the MSU/AMSU instruments and upper-tropospheric humidity from the HIRS instruments. Despite discrepancies in the rates of tropospheric warming between observations and models, both are consistent with constant relative humidity over the period 1979--2008. Because uncertainties in satellite-observed tropical-mean trends preclude a constraint on tropical-mean trends in models we also explore regional features of the feedbacks. The regional pattern of the lapse rate feedback is primarily determined by the regional pattern of surface temperature changes, as tropical atmospheric warming is relatively horizontally uniform. The regional pattern of the water vapor feedback is influenced by the regional pattern of precipitation changes, with variations of 1--2 W m-2 K-1 across the Tropics (compared to a tropical-mean feedback magnitude of 3.3--4 W m-2 K-1). Thus the geographical patterns of water vapor and lapse rate feedbacks are not correlated, but when the feedbacks are calculated in precipitation percentiles rather than in geographical space they are anti-correlated, with strong positive water vapor feedback associated with strong negative lapse rate feedback. The regional structure of the feedbacks is not related to the strength of the tropical-mean feedback in a subset of the climate models from the CMIP5 archive. Nevertheless the approach constitutes a useful process-based test of climate models and has the potential to be extended to constrain regional climate projections.Natural Environment Research Council (NERC

Network analysis reveals open forums and echo chambers in social media discussions of climate change

Open Access articleAction to tackle the complex and divisive issue of climate change will be strongly influenced by public perception. Online social media and associated social networks are an increasingly important forum for public debate and are known to influence individual attitudes and behaviours yet online discussions and social networks related to climate change are not well understood. Here we construct several forms of social network for users communicating about climate change on the popular microblogging platform Twitter. We classify user attitudes to climate change based on message content and nd that social networks are characterised by strong attitude-based homophily and segregation into polarised "sceptic" and "activist" groups. Most users interact only with like-minded others, in communities dominated by a single view. However, we also nd mixed-attitude communities in which sceptics and activists frequently interact. Messages between like-minded users typically carry positive sentiment, while messages between sceptics and activists carry negative sentiment. We identify a number of general patterns in user behaviours relating to engagement with alternative views. Users who express negative sentiment are themselves the target of negativity. Users in mixed- attitude communities are less likely to hold a strongly polarised view, but more likely to express negative sentiment towards other users with di ering views. Overall, social media discussions of climate change often occur within polarising "echo chambers", but also within "open forums", mixed-attitude communities that reduce polarisation and stimulate debate. Our results have implications for public engagement with this important global challenge.Engineering and Physical Sciences Research Council (EPSRC) - Bridging the Gaps initiativ

Increasing the detectability of external influence on precipitation by correcting feature location in GCMs

Understanding how precipitation varies as the climate changes is essential to determining the true impact of global warming. This is a difficult task not only due to the large internal variability observed in precipitation but also because of a limited historical record and large biases in simulations of precipitation by general circulation models (GCMs). Here we make use of a technique that spatially and seasonally transforms GCM fields to reduce location biases and investigate the potential of this bias correction to study historical changes. We use two versions of this bias correction—one that conserves intensities and another that conserves integrated precipitation over transformed areas. Focussing on multimodel ensemble means, we find that both versions reduce RMS error in the historical trend by approximately 11% relative to the Global Precipitation Climatology Project (GPCP) data set. By regressing GCMs' historical simulations of precipitation onto radiative forcings, we decompose these simulations into anthropogenic and natural time series. We then perform a simple detection and attribution study to investigate the impact of reducing location biases on detectability. A multiple ordinary least squares regression of GPCP onto the anthropogenic and natural time series, with the assumptions made, finds anthropogenic detectability only when spatial corrections are applied. The result is the same regardless of which form of conservation is used and without reducing the dimensionality of the fields beyond taking zonal means. While “detectability” is dependent both on the exact methodology and the confidence required, this nevertheless demonstrates the potential benefits of correcting location biases in GCMs when studying historical precipitation, especially in cases where a signal was previously undetectable

The Relationship between Model Biases in East Asian Summer Monsoon Rainfall and Land Evaporation

This is the final version. Available on open access from Springer via the DOI in this recordThe East Asian Summer Monsoon (EASM) provides the majority of annual rainfall to countries in East Asia. Although state-of-the-art models broadly project increased EASM rainfall, the spread of projections is large and simulations of present-day rainfall show significant climatological biases. Systematic evapotranspiration biases occur locally over East Asia, and globally over land, in simulations both with and without a coupled ocean. This study explores the relationship between evapotranspiration and EASM precipitation biases. First, idealized model simulations are presented in which the parameterization of land evaporation is modified, while sea surface temperature is fixed. The results suggest a feedback whereby excessive evapotranspiration over East Asia results in cooling of land, a weakened monsoon low, and a shift of rainfall from the Philippine Sea to China, further fueling evapotranspiration. Cross-model regressions against evapotranspiration over China indicate a similar pattern of behavior in Atmospheric Model Intercomparison Project (AMIP) simulations. Possible causes of this pattern are investigated. The feedback is not explained by an overly intense global hydrological cycle or by differences in radiative processes. Analysis of land-only simulations indicates that evapotranspiration biases are present even when models are forced with prescribed rainfall. These are strengthened when coupled to the atmosphere, suggesting a role for land-model errors in driving atmospheric biases. Coupled atmosphere-ocean models are shown to have similar evapotranspiration biases to those in AMIP over China, but different precipitation biases, including a northward shift in the ITCZ over the Pacific and Atlantic Oceans.UK-China Research and Innovation Partnership Fun

Dependency of global mean precipitation on surface temperature

Copyright © 2008 American Geophysical UnionWe investigate the causes of temperature dependent changes in global precipitation in contemporary General Circulation Models (GCMs) subjected to a doubling of atmospheric CO2 concentration. By analyzing the energy budget of the troposphere, we find that changes are dominated by processes robustly simulated by GCMs. Importantly, shortwave cloud feedbacks, whose uncertainty is largely responsible for the wide range of GCM temperature climate sensitivities, are shown to have little effect. This is because these mainly arise from the scattering of shortwave radiation that has little impact on the tropospheric heating that controls precipitation. Hence, we expect that the range of simulated precipitation sensitivities to temperature will not change greatly in future GCMs, despite the recent suggestion that satellite observations indicate that GCM precipitation changes are significantly in error

How much will precipitation increase with global warming?

Copyright © 2008 American Geophysical UnionThe advent of meteorological satellites during the 1970s made possible the observation of the seasonally shifting patterns of global precipitation. It was not until recently, however, that the record could be considered long enough to investigate longer-term trends and the relationship between global precipitation and global warming. Using data from the Special Sensor Microwave Imager (SSM/I) instrument, Wentz et al. [2007] reported that global mean precipitation increased at a rate of 7.4±2.6% per °C between 1987 and 2006. Meanwhile, general circulation models (GCMs) used to predict climate change simulate twentieth- and 21st-century mean precipitation increases of about 13% per °C [Held and Soden, 2006]. This difference seems surprising because some GCMs can adequately reproduce the much longer twentieth- century surface-based land-mean precipitation record [Lambert et al., 2005]. Global precipitation changes are tied to the surface energy budget through evaporation and to the tropospheric energy budget through condensation. Thus, if GCMs do underestimate global precipitation changes, the simulation of other climate variables will be affected