John Caesar

Global observed changes in daily climate extremes of temperature and precipitation

A suite of climate change indices derived from daily temperature and precipitation data, with a primary focus on extreme events, were computed and analyzed. By setting an exact formula for each index and using specially designed software, analyses done in different countries have been combined seamlessly. This has enabled the presentation of the most up-to-date and comprehensive global picture of trends in extreme temperature and precipitation indices using results from a number of workshops held in data-sparse regions and high-quality station data supplied by numerous scientists world wide. Seasonal and annual indices for the period 1951-2003 were gridded. Trends in the gridded fields were computed and tested for statistical significance. Results showed widespread significant changes in temperature extremes associated with warming, especially for those indices derived from daily minimum temperature. Over 70% of the global land area sampled showed a significant decrease in the annual occurrence of cold nights and a significant increase in the annual occurrence of warm nights. Some regions experienced a more than doubling of these indices. This implies a positive shift in the distribution of daily minimum temperature throughout the globe. Daily maximum temperature indices showed similar changes but with smaller magnitudes. Precipitation changes showed a widespread and significant increase, but the changes are much less spatially coherent compared with temperature change. Probability distributions of indices derived from approximately 200 temperature and 600 precipitation stations, with near-complete data for 1901-2003 and covering a very large region of the Northern Hemisphere midlatitudes (and parts of Australia for precipitation) were analyzed for the periods 1901-1950, 1951-1978 and 1979-2003. Results indicate a significant warming throughout the 20th century. Differences in temperature indices distributions are particularly pronounced between the most recent two periods and for those indices related to minimum temperature. An analysis of those indices for which seasonal time series are available shows that these changes occur for all seasons although they are generally least pronounced for September to November. Precipitation indices show a tendency toward wetter conditions throughout the 20th century.

Large-scale changes in observed daily maximum and minimum temperatures: Creation and analysis of a new gridded data set

Received 27 May 2005; revised 16 September 2005; accepted 4 November 2005; published 1 March 2006. [1] A gridded land-only data set representing near-surface observations of daily maximum and minimum temperatures (HadGHCND) has been created to allow analysis of recent changes in climate extremes and for the evaluation of climate model simulations. Using a global data set of quality-controlled station observations compiled by the U.S. National Climatic Data Center (NCDC), daily anomalies were created relative to the 1961–1990 reference period for each contributing station. An angular distance weighting technique was used to interpolate these observed anomalies onto a 2.5� latitude by 3.75� longitude grid over the period from January 1946 to December 2000. We have used the data set to examine regional trends in time-varying percentiles. Data over consecutive 5 year periods were used to calculate percentiles which allow us to see how the distributions of daily maximum and minimum temperature have changed over time. Changes during the winter and spring periods are larger than in the other seasons, particularly with respect to increasing temperatures at the lower end of the maximum and minimum temperature distributions. Regional differences suggest that it is not possible to infer distributional changes from changes in the mean alone.

Global Land-Based Datasets for Monitoring Climatic Extremes

AMERICAN METEOROlOGICAl SOCIETy | July 2013| 997 PB AFFILIATIONS: Donat, alexanDer, anD Yang—Climate Change Research Centre, and ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney, Australia; Durre anD Vose—NOAA’s National Climatic Data Center, Asheville, North Carolina; Caesar—Met Office Hadley Centre, Exeter, United Kingdom CORRESPONDING AUTHOR: Markus Donat, Climate Change Research Centre, University of New South Wales, Sydney, Australia E-mail: m.donat@unsw.edu.au

Human Contribution to the Lengthening of the Growing Season during 1950–99

Abstract Increasing surface temperatures are expected to result in longer growing seasons. An optimal detection analysis is carried out to assess the significance of increases in the growing season length during 1950–99, and to measure the anthropogenic component of the change. The signal is found to be detectable, both on global and continental scales, and human influence needs to be accounted for if it is to be fully explained. The change in the growing season length is found to be asymmetric and largely due to the earlier onset of spring, rather than the later ending of autumn. The growing season length, based on exceedence of local temperature thresholds, has a rate of increase of about 1.5 days decade−1 over the observation area. Local variations also allow for negative trends in parts of North America. The analysis suggests that the signal can be attributed to the anthropogenic forcings that have acted on the climate system and no other forcings are necessary to describe the change. Model projection...

A review of recent developments in climate change science. Part II: The global-scale impacts of climate change:

This article presents a review of recent developments in studies assessing the global-scale impacts of climate change published since the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Literature covering six main impact sectors is reviewed: sea-level rise (SLR) and coastal impacts, ocean acidification, ecosystems and biodiversity, water resources and desertification, agriculture and food security, and human health. The review focuses on studies with a global perspective to climate change impacts assessment, although in the absence of global studies for some sectors or aspects of impacts, national and regional studies are cited. The review highlights three major emerging themes which are of importance for the policy- and decision-making process: (1) a movement towards probabilistic methods of impacts assessment and/or the consideration of climate modelling uncertainty; (2) a move towards assessing potential impacts that could be avoided under different climate change mitigation scenarios relative to a business-as-usual reference scenario; and (3) uncertainties that remain in understanding the relationship between climate and natural or human systems. Whether recent impact assessments show a changed risk of damage to human or natural systems since the AR4 depends upon the impact sector; whether the assessments are robust or not (i.e. will stand the test of time) requires additional expert judgement. However, using this judgement, overall we find an increased risk to natural systems, and in some components of human systems.

Response of the HadGEM2 Earth System Model to Future Greenhouse Gas Emissions Pathways to the Year 2300

AbstractA new ensemble of simulations from the Earth System configuration of the Hadley Centre Global Environmental Model, version 2 (HadGEM2-ES), is used to evaluate the response to historical and projected future greenhouse gas forcings that follow Representative Concentration Pathways (RCPs). In addition to the projected changes during the twenty-first century, extended simulations to the year 2300 allow an investigation into inertia in the climate system post-2100 that may occur even if atmospheric CO2 concentrations have stabilized. Projections of temperature, precipitation, sea level, permafrost, heat waves, and compatible carbon emissions are analyzed. The low emissions scenario RCP2.6 is the only scenario considered here that is approximately consistent with a 2°C global warming limit, though there are regions where local changes in temperature are projected to considerably exceed 2°C, particularly over northern high-latitude areas. An aggressive mitigation approach, represented here by RCP2.6, co...

A review of recent developments in climate change science. Part I: Understanding of future change in the large-scale climate system:

This article reviews some of the major lines of recent scientific progress relevant to the choice of global climate policy targets, focusing on changes in understanding since publication of the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4). Developments are highlighted in the following major climate system components: ice sheets; sea ice; the Atlantic Meridional Overturning Circulation; tropical forests; and accelerated carbon release from permafrost and ocean hydrates. The most significant developments in each component are identified by synthesizing input from multiple experts from each field. Overall, while large uncertainties remain in all fields, some substantial progress in understanding is revealed.

Reassessing changes in diurnal temperature range: Intercomparison and evaluation of existing global data set estimates

Changes in diurnal temperature range (DTR) over global land areas are compared from a broad range of independent data sets. All data sets agree that global-mean DTR has decreased significantly since 1950, with most of that decrease occurring over 1960–1980. The since-1979 trends are not significant, with inter-data set disagreement even over the sign of global changes. Inter-data set spread becomes greater regionally and in particular at the grid box level. Despite this, there is general agreement that DTR decreased in North America, Europe, and Australia since 1951, with this decrease being partially reversed over Australia and Europe since the early 1980s. There is substantive disagreement between data sets prior to the middle of the twentieth century, particularly over Europe, which precludes making any meaningful conclusions about DTR changes prior to 1950, either globally or regionally. Several variants that undertake a broad range of approaches to postprocessing steps of gridding and interpolation were analyzed for two of the data sets. These choices have a substantial influence in data sparse regions or periods. The potential of further insights is therefore inextricably linked with the efficacy of data rescue and digitization for maximum and minimum temperature series prior to 1950 everywhere and in data sparse regions throughout the period of record. Over North America, station selection and homogeneity assessment is the primary determinant. Over Europe, where the basic station data are similar, the postprocessing choices are dominant. We assess that globally averaged DTR has decreased since the middle twentieth century but that this decrease has not been linear.

Changes in climate extremes, fresh water availability and vulnerability to food insecurity projected at 1.5°C and 2°C global warming with a higher-resolution global climate model

We projected changes in weather extremes, hydrological impacts and vulnerability to food insecurity at global warming of 1.5°C and 2°C relative to pre-industrial, using a new global atmospheric general circulation model HadGEM3A-GA3.0 driven by patterns of sea-surface temperatures and sea ice from selected members of the 5th Coupled Model Intercomparison Project (CMIP5) ensemble, forced with the RCP8.5 concentration scenario. To provide more detailed representations of climate processes and impacts, the spatial resolution was N216 (approx. 60 km grid length in mid-latitudes), a higher resolution than the CMIP5 models. We used a set of impacts-relevant indices and a global land surface model to examine the projected changes in weather extremes and their implications for freshwater availability and vulnerability to food insecurity. Uncertainties in regional climate responses are assessed, examining ranges of outcomes in impacts to inform risk assessments. Despite some degree of inconsistency between components of the study due to the need to correct for systematic biases in some aspects, the outcomes from different ensemble members could be compared for several different indicators. The projections for weather extremes indices and biophysical impacts quantities support expectations that the magnitude of change is generally larger for 2°C global warming than 1.5°C. Hot extremes become even hotter, with increases being more intense than seen in CMIP5 projections. Precipitation-related extremes show more geographical variation with some increases and some decreases in both heavy precipitation and drought. There are substantial regional uncertainties in hydrological impacts at local scales due to different climate models producing different outcomes. Nevertheless, hydrological impacts generally point towards wetter conditions on average, with increased mean river flows, longer heavy rainfall events, particularly in South and East Asia with the most extreme projections suggesting more than a doubling of flows in the Ganges at 2°C global warming. Some areas are projected to experience shorter meteorological drought events and less severe low flows, although longer droughts and/or decreases in low flows are projected in many other areas, particularly southern Africa and South America. Flows in the Amazon are projected to decline by up to 25%. Increases in either heavy rainfall or drought events imply increased vulnerability to food insecurity, but if global warming is limited to 1.5°C, this vulnerability is projected to remain smaller than at 2°C global warming in approximately 76% of developing countries. At 2°C, four countries are projected to reach unprecedented levels of vulnerability to food insecurity. This article is part of the theme issue ‘The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels’.

Temperature and precipitation extremes in the second half of the twentieth century from numerical modeling results and observational data

The procedure and results of an objective comparison of climatologies and historical trends of temperature and precipitation extremes are considered using observations and numerical experiments of 20th-century climate simulations. The experimental climatic realizations of five coupled atmosphere-ocean models represented in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change are used. The effect of using ensembles of climatic realizations (including multimodel ensembles) in the problems of simulating climatology and climate changes in annual meteorological extremes is estimated. The positive effect of the ensemble approach is evident when individual ensemble members show significant success.