The effects of climate change and variation in New Zealand: An assessment using the CLIMPACTS system

Along with a need to better understand the climate and biophysical systems of New Zealand, the need to develop an improved capacity for evaluating possible changes in climate and their effects on the New Zealand environment has been recognised. Since the middle of 1993 the CLIMPACTS programme, has been focused on the development of such a capacity, in the first instance for the agricultural sector. the goals of this present assessment are: 1. To present current knowledge on likely scenarios of climate change and associated uncertainties in New Zealand; 2. To present current knowledge, based on quantitative analyses using a consistent set of scenarios, on the likely effects of climate change on a range of agricultural and horticultural crops of economic importance; 3. To demonstrate, by way of this report and the associated technical report, the capacity that has been developed for ongoing assessments of this kind in New Zealand. This report has been prepared for both the science and policy communities in New Zealand. There are two main components: 1. The detailed findings of the assessment, presented in a series of chapters; 2. An annex, which contains technical details on models used in the assessment

Representing climate and extreme weather events in integrated assessment models: A review of existing methods and options for development

The lack of information about future changes in extreme weather is a major constraint of Integrated Assessment Models (IAMs) of climate change. The generation of descriptions of future climate in current IAMs is assessed.We also review recent work on scenario development methods for weather extremes, focusing on those issues which are most relevant to the needs of IAMs. Finally, some options for implementing scenarios of weather extremes in IAMs are considered

Hydrological droughts in the 21st century, hotspots and uncertainties from a global multimodel ensemble experiment

Increasing concentrations of greenhouse gases in the atmosphere are expected to modify the global water cycle with significant consequences for terrestrial hydrology. We assess the impact of climate change on hydrological droughts in a multimodel experiment including seven global impact models (GIMs) driven by biascorrected climate from five global climate models under four representative concentration pathways (RCPs). Drought severity is defined as the fraction of land under drought conditions. Results show a likely increase in the global severity of hydrological drought at the end of the 21st century, with systematically greater increases for RCPs describing stronger radiative forcings. Under RCP8.5, droughts exceeding 40% of analyzed land area are projected by nearly half of the simulations. This increase in drought severity has a strong signal-to-noise ratio at the global scale, and Southern Europe, the Middle East, the Southeast United States, Chile, and South West Australia are identified as possible hotspots for future water security issues. The uncertainty due to GIMs is greater than that from global climate models, particularly if including a GIM that accounts for the dynamic response of plants to CO2 and climate, as this model simulates little or no increase in drought frequency. Our study demonstrates that different representations of terrestrial water-cycle processes in GIMs are responsible for a much larger uncertainty in the response of hydrological drought to climate change than previously thought. When assessing the impact of climate change on hydrology, it is therefore critical to consider a diverse range of GIMs to better capture the uncertainty

European droughts under climate change: projections and uncertainties

Droughts are one of the most damaging natural hazards, and anthropogenic climate change has and will continue to alter their characteristics. Better understanding of changes in drought characteristics under potential future climates is vital for managing drought risks and impacts, yet projections are very uncertain. This thesis examines the effects of climate change on European drought characteristics through a multi-scenario and multimodel approach. It explores the uncertainty associated with emission scenarios, global and spatial climate projections, and with the identification and characterisation of droughts. Climate projections simulated by the simple climate model MAGICC6.0 and patternscaling climate scenario generator ClimGen are assessed, emulating eighteen CMIP3 general circulation models (GCMs) under ten emission scenarios. Drought severity (magnitude times duration) and spatial extent are analysed for both 3-month and 12-month events. Drought projections vary substantially depending on the GCM, emission scenario, region, season and definition of drought. Overall, climate change enhances drought conditions across the study region, with marked increases simulated for the southern latitudes; reductions are projected for the northern latitudes, especially in winter and spring. Perturbations in the interannual variability of precipitation tend to enhance drought conditions caused by mean precipitation changes, or to moderate or reverse their reductions. Hydrological drought parameters are highly sensitive to potential evapotranspiration (PET), which shows the importance of the PET calculation method. Greater agreement in the direction of change tends to occur in the high- and low-latitudes, and in summer and autumn. Both meteorological and hydrological drought results generally indicate the same direction of change, with the latter having larger magnitudes. Projection ranges tend to increase with time and magnitude of warming; intra-GCM spread dominates other sources of uncertainty. The implications of the large uncertainties include that decision-making should be based on multi-scenario and multi-model results, and with consideration of drought definition

Water Cycle Changes

This chapter assesses multiple lines of evidence to evaluate past, present and future changes in the global water cycle. It complements material in Chapters 2, 3 and 4 on observed and projected changes in the water cycle, and Chapters 10 and 11 on regional climate change and extreme events. The assessment includes the physical basis for water cycle changes, observed changes in the water cycle and attribution of their causes, future projections and related key uncertainties, and the potential for abrupt change. Paleoclimate evidence, observations, reanalyses and global and regional model simulations are considered. The assessment shows widespread, nonuniform human-caused alterations of the water cycle, which have been obscured by a competition between different drivers across the 20th century and that will be increasingly dominated by greenhouse gas forcing at the global scale

Quantifying the extent of change in extreme weather events in response to global warming

Weather extremes have been documented in the context of a warming climate in association with increasing greenhouse gas concentrations. However, there remains much uncertainty as to how these extreme events will respond to future climate warming. In particular, climate modeling studies have predicted changes in the frequency and severity of weather extremes, and the range of changes reported in the literature is very large, and sometimes contradictory, as the nature of many extreme weather phenomena is not fully understood. This uncertainty stems, in part, from the limited ability of coarse resolution climate models to accurately measure and simulate weather events that occur at the microscale level, such as tornadoes and severe thunderstorms. However, some of the range of results reported originates simply from a wide variety of scenarios of future climate change used to drive climate model simulations, which hampers our ability to make generalizations about predicted changes in extreme weather events. The goal of this study is to conduct a meta-analysis of the literature on projected future extreme weather events, so as to identify trends, using global mean temperature change as a common frame of reference. Results indicate that global warming could significantly alter the behavior of multiple extreme weather events, such as mid-latitude drought, severe thunderstorms and tornadoes, as well as the selected important meteorological variables that engender them, into the 21st century

Investigating the link between southern African droughts and global atmospheric teleconnections using regional climate models

Includes bibliographical referencesDrought is one of the natural hazards that threaten the economy of many nations, especially in Southern Africa, where many socio-economic activities depend on rain-fed agriculture. This study evaluates the capability of Regional Climate Models (RCMs) in simulating the Southern African droughts. It uses the Standardized Precipitation-Evapotranspiration Index (SPEI, computed using rainfall and temperature data) to identify 3-month droughts over Southern Africa, and compares the observed and simulated drought patterns. The observation data are from the Climate Research Unit (CRU), while the simulation data are from 10 RCMs (ARPEGE, CCLM, HIRHAM, RACMO, REMO, PRECIS, RegCM3, RCA, WRF, and CRCM) that participated in the Regional Climate Downscaling Experiment (CORDEX) project. The study also categorizes drought patterns over Southern Africa, examines the persistence and transition of these patterns, and investigates the roles of atmospheric teleconnections on the drought patterns. The results show that the drought patterns can occur in any season, but they have preference for seasons. Some droughts patterns may persist up to three seasons, while others are transient. Only about 20% of the droughts patterns are induced solely by El Niño Southern Oscillation (ENSO), other drought patterns are caused by complex interactions among the atmospheric teleconnections. The study also reveals that the Southern Africa drought pattern is generally shifting from a wet condition to a dry condition, and that the shifting can only be captured with a drought monitoring index that accounts for temperature influence on drought. Only few CORDEX RCMs simulate the Southern African droughts as observed. In this regard, the ARPEGE model shows the best simulation. The best performance may be because the stretching capability of ARPEGE helps the model to eliminate boundary condition problems, which are present in other RCMs. In ARPEGE simulations, the stretching capability would allow a better interaction between large and small scale features, and may lead to a better representation of the rain producing systems in Southern Africa. The results of the study may be applied to improve monitoring and prediction of regionally-extensive drought over Southern Africa, and to reduce the socio-economic impacts of drought in the region

The response of southern African vegetation to droughts in past and future climates

Drought and climate change pose a threat to southern African vegetation. This study examines the response of southern African vegetation to drought in both past and future climates. Multiyear and multi-simulation datasets from three dynamic global vegetation models (DGVMs), namely, Community Land Model version 4 (CLM4), Community Land Model version 4 with Variable Infiltration Capacity hydrology (CLM4VIC), and Organising Carbon and Hydrology in Dynamic Ecosystems designed by Laboratoire des Sciences du Climat et de l’Environnement (ORCHIDEE-LSCE). These three DGVMs and the Community Earth System Model (CESM) were analyzed for the study. The DGVM simulations were forced with the reanalysis climate dataset from the National Centers for Environmental Prediction (NCEP) and the Climatic Research Unit - NCEP (CRUNCEP). The simulated climate results were evaluated with observation datasets from the Climatic Research Unit (CRU), while the simulated vegetation index (i.e. Normalized Difference Vegetation Index, NDVI) were evaluated with NDVI data from the Global Inventory Modelling and Mapping Studies (GIMMS). Meteorological droughts were analyzed at different timescales (1- to 18-month timescales), using two drought indexes: the Standardized Precipitation Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI). The responses of vegetation to drought were quantified by means of Pearson Correlation Analysis. The DGVMs were applied to study the sensitivity of vegetation to fire, while the CESM was used to project impact of climate change on the characteristics of southern African vegetation in the future (up to the year 2100) under the 8.5 Representative Concentration Pathway (RCP8.5) scenario, focusing on impacts at 1.5oC and 2.0oC global warming levels (GWLs). Analysis of the observed data shows that the spatial distribution of vegetation across southern Africa is more influenced by the rainfall distribution than by the temperature distribution. The observed correlation between drought index and vegetation index is higher than 0.8 over southeastern part of the region at 3-month drought timescale, and there is no difference between the spatial distribution of the correlation between the SPEI and the vegetation index, and between the SPI and the vegetation index. The three DGVMs failed to capture the response of vegetation to drought; however, the CLM4 shows the best performance while ORCHIDEELSCE fared the worst of the three. The CLM4 simulation show that fire strongly influences growth of vegetation over the summer rainfall region but it has weak influence over vegetation in the western arid zone. The CESM strongly captures the spatial patterns of precipitation and the vegetation index across southern Africa, but it overestimates the magnitudes of the vegetation index across the region, except in Namibia and Angola. The CESM also underestimates the correlation between drought indexes with vegetation, and the timescales at which the vegetation respond to droughts. The CESM projects an increase in the drought intensity as a result of an increased temperature across southern African biomes. However the increase in drought intensity is more pronounced with the SPEI than with the SPI. CESM also projects a future decrease in the vegetation index (i.e. NDVI) in the region except in the dry savanna biome. The impacts of 1.5oC GWLs on the vegetation fluxes vary throughout southern Africa, and the magnitudes of changes in the vegetation fluxes are affected by a further increase in global warming over the region. While there is a good agreement among the CESM simulations on the projected changes in vegetation fluxes across the biomes, the uncertainty in the projections is higher with 1.5oC than with 2.0oC GWL. The results of the study can be applied to mitigate the impacts of climate variability and change on southern African vegetation. Specific mitigation efforts that could be applied to reduce the impacts of droughts and climate change are watershed management, improved vegetation management, impact monitoring, environmental awareness, and remote sensing tools

Extreme hydrological events during the last 3500 years and their changes in the future

The Earth's climate has constantly varied over time due to the influence of internal processes and external factors. Together with the mean climate, spatial and temporal characteristics of extreme hydrological events, such as droughts and extreme precipitation, have also changed over time. For instance, droughts with multi-decadal duration, which have never been detected in the instrumental era, occurred during the Medieval climate anomaly (approximately 950-1200 CE) in North America and northern Europe. Devastating floods associated with heavy rainfall occurred more frequently during the Little Ice Age (about 1250-1850 CE) compared to other periods in the Common Era (the last 2k years) over Europe. These past extreme events significantly impacted the environment and ancient societies, sometimes influencing societal disruptions. Despite their impacts on the environment and society, until now, less attention has been paid to the variations of past extreme hydrological events and their drivers compared to the variations in mean precipitation. Among many factors that hinder the investigations of past extreme hydrological events, the main problem arises from the limited availability of observations and past reconstructions for this kind of sporadic events. Nowadays, complex climate models have become essential tools to examine the underlying dynamics of the Earth's climate. As these models can simulate the response of the climate to internal and external perturbations, they offer a possibility to address responsible drivers of climate variations and also extreme events on the global scale. In addition, simulations with climate models cover long time periods that can go far beyond the modern instrumental era. Hence, information from simulations can complement observations and reconstructions to illustrate better the characteristics of past droughts and extreme precipitation. This thesis uses a state-of-the-art earth system model, the Community Earth System Model (CESM), as the main investigation tool to understand the variability and dynamics of past extreme hydrological events, namely droughts and extreme precipitation, during the past three millennia. It also aims to address the effects of an external factor, i.e., volcanic eruptions, on the climate and the impacts of past change in the climate on ancient European society. The thesis mainly consists of three studies. The first study focuses on the dynamics of persistent Mediterranean droughts during 850-2099 CE. The Mediterranean region is one of the drought hot spots which is projected to experience an intensified drying by the end of the 21st century compared to the historical period. Hence, a more comprehensive understanding of the underlying mechanisms of past droughts over the region is necessary to better assess the changes of drought drivers in the historical and future period. In the study, temporal characteristics of Mediterranean droughts are assessed, and drivers of persistent long droughts are identified. In addition, the sensitivity of Mediterranean droughts to various drought metrics is tested. The second study deals with daily extreme precipitation during the last three millennia previous to the Industrial Era (1501 BCE-1849 CE). The study is based on the newly conducted 3500-year long CESM simulations, which include a new proxy record of reconstructed volcanic eruptions. Internal and externally generated processes that influence the long-term variability of extreme precipitation are identified across the globe using a statistical method based on the extreme value theory. Among the externally generated processes, the impacts of volcanic eruptions on extreme precipitation are analyzed in more detail. The third research topic concentrates on examining the impacts of the 43 BCE Okmok eruption in Alaska on the climate and early Mediterranean civilization. Abrupt large-scale changes of the Mediterranean climate after this large extratropical volcanic eruption are detected in various climate-related records, and the magnitudes of these changes are quantified with CESM. This change in climate is as a possible driver of the societal changes that occurred during the ancient Roman period. Lastly, an outlook and a general conclusion of the thesis are presented, also proposing some potential follow-up investigations