Drought Severity Index, 12-Month Accumulations - Projections

What does the data show? The Drought Severity Index is not threshold based. Instead, it is calculated with 12-month rainfall deficits provided as a percentage of the mean annual climatological total rainfall (1981–2000) for that location. It measures the severity of a drought, not the frequency. 12-month accumulations have been selected as this is likely to indicate hydrological drought. Hydrological drought occurs due to water scarcity over a much longer duration (longer than 12 months). It heavily depletes water resources on a large scale as opposed to meteorological or agricultural drought, which generally occur on shorter timescales of 3-12 months. However this categorisation is not fixed, because rainfall deficits accumulated over 12-months could lead to different types of drought and drought impacts, depending on the level of vulnerability to reduced rainfall in a region. The DSI 12 month accumulations are calculated for two baseline (historical) periods 1981-2000 (corresponding to 0.51°C warming) and 2001-2020 (corresponding to 0.87°C warming) and for global warming levels of 1.5°C, 2.0°C, 2.5°C, 3.0°C, 4.0°C above the pre-industrial (1850-1900) period. What are the possible societal impacts? The DSI 12-month accumulations measure the drought severity. Higher values indicate more severe drought. The DSI is based on 12-month rainfall deficits. The impacts of the differing length of rainfall deficits vary regionally due to variation in vulnerability. Depending on the level of vulnerability to reduced rainfall, rainfall deficits accumulated over 12 months could lead to meteorological, agricultural and hydrological drought. What is a global warming level? The DSI 12-month accumulations are calculated from the UKCP18 regional climate projections using the high emissions scenario (RCP 8.5) where greenhouse gas emissions continue to grow. Instead of considering future climate change during specific time periods (e.g. decades) for this scenario, the dataset is calculated at various levels of global warming relative to the pre-industrial (1850-1900) period. The world has already warmed by around 1.1°C (between 1850–1900 and 2011–2020), whilst this dataset allows for the exploration of greater levels of warming. The global warming levels available in this dataset are 1.5°C, 2°C, 2.5°C, 3°C and 4°C. The data at each warming level was calculated using a 21 year period. These 21 year periods are calculated by taking 10 years either side of the first year at which the global warming level is reached. This time will be different for different model ensemble members. To calculate the value for the DSI 12-month accumulations, an average is taken across the 21 year period. We cannot provide a precise likelihood for particular emission scenarios being followed in the real world future. However, we do note that RCP8.5 corresponds to emissions considerably above those expected with current international policy agreements. The results are also expressed for several global warming levels because we do not yet know which level will be reached in the real climate as it will depend on future greenhouse emission choices and the sensitivity of the climate system, which is uncertain. Estimates based on the assumption of current international agreements on greenhouse gas emissions suggest a median warming level in the region of 2.4-2.8°C, but it could either be higher or lower than this level. What are the naming conventions and how do I explore the data? This data contains a field for each global warming level and two baselines. They are named ‘DSI12’ (Drought Severity Index for 12 month accumulations), the warming level or baseline, and 'upper' 'median' or 'lower' as per the description below. E.g. 'DSI12 2.5 median' is the median value for the 2.5°C projection. Decimal points are included in field aliases but not field names e.g. 'DSI12 2.5 median' is 'DSI12_25_median'. To understand how to explore the data, see this page: https://climatedataportal.metoffice.gov.uk/pages/user-guide Please note, if viewing in ArcGIS Map Viewer, the map will default to ‘DSI12 2.0°C median’ values. What do the ‘median’, ‘upper’, and ‘lower’ values mean? Climate models are numerical representations of the climate system. To capture uncertainty in projections for the future, an ensemble, or group, of climate models are run. Each ensemble member has slightly different starting conditions or model set-ups. Considering all of the model outcomes gives users a range of plausible conditions which could occur in the future. For this dataset, the model projections consist of 12 separate ensemble members. To select which ensemble members to use, DSI 12 month accumulations were calculated for each ensemble member and they were then ranked in order from lowest to highest for each location. The ‘lower’ fields are the second lowest ranked ensemble member. The ‘upper’ fields are the second highest ranked ensemble member. The ‘median’ field is the central value of the ensemble. This gives a median value, and a spread of the ensemble members indicating the range of possible outcomes in the projections. This spread of outputs can be used to infer the uncertainty in the projections. The larger the difference between the lower and upper fields, the greater the uncertainty. ‘Lower’, ‘median’ and ‘upper’ are also given for the baseline periods as these values also come from the model that was used to produce the projections. This allows a fair comparison between the model projections and recent past. Useful links This dataset was calculated following the methodology in the ‘Future Changes to high impact weather in the UK’ report. Further information on the UK Climate Projections (UKCP). Further information on understanding climate data within the Met Office Climate Data Portal该数据展示了什么？干旱严重程度指数（Drought Severity Index，DSI）并非基于阈值，而是以12个月降雨亏缺占对应站点1981–2000年多年平均年气候总降雨量的百分比进行计算。该指数用于衡量干旱严重程度，而非发生频率。选择12个月累积量是因其可有效表征水文干旱（hydrological drought）。水文干旱因长期（超过12个月）水资源短缺引发，会大规模严重耗竭水资源，这与通常发生在3-12个月较短时间尺度上的气象干旱（meteorological drought）或农业干旱（agricultural drought）截然不同。不过该分类并非固定不变，因为12个月累积的降雨亏缺可能因区域对降雨减少的脆弱性程度不同，引发不同类型的干旱及干旱影响。 DSI的12个月累积量基于两个基准（历史）时期计算：1981-2000年（对应0.51℃升温）和2001-2020年（对应0.87℃升温），同时也针对较工业化前（1850-1900年）升温1.5℃、2.0℃、2.5℃、3.0℃、4.0℃的全球变暖水平（global warming level）进行了计算。 ### 可能产生的社会影响是什么？ DSI的12个月累积量用于衡量干旱严重程度，数值越高代表干旱越严重。DSI基于12个月降雨亏缺，而降雨亏缺时长的影响因区域脆弱性差异而存在地域分异。根据区域对降雨减少的脆弱性程度，12个月累积的降雨亏缺可能引发气象干旱（meteorological drought）、农业干旱（agricultural drought）及水文干旱（hydrological drought）。 ### 什么是全球变暖水平？ DSI的12个月累积量基于英国气候预测18（UKCP18）的区域气候预估，采用高排放情景（Representative Concentration Pathway，RCP 8.5）——即温室气体排放持续增长的情景。与针对特定时间段（如十年）的未来气候变化分析不同，本数据集基于相对工业化前（1850-1900年）的不同全球变暖水平进行计算。目前全球已较1850–1900年至2011–2020年期间升温约1.1℃，而本数据集可用于探索更高水平的升温情景。本数据集可用的全球变暖水平为1.5℃、2℃、2.5℃、3℃和4℃。 每个变暖水平对应的数据集均采用21年周期计算：以该变暖水平首次达到的第一年为中心，前后各取10年形成21年时段，不同模式集合成员对应的时段存在差异。最终的DSI 12个月累积量为该21年周期的平均值。 我们无法准确预测现实未来中特定排放情景的实现概率，但需指出RCP8.5对应的排放水平远高于当前国际气候政策协定下的预期排放水平。本数据集同时提供多个全球变暖水平下的结果，因为我们尚无法确定现实气候最终将达到何种变暖水平——这取决于未来的温室气体排放选择以及气候系统的敏感性，二者均存在不确定性。基于当前国际温室气体排放协定的预估显示，全球升温中值约为2.4-2.8℃，但实际升温可能高于或低于该区间。 ### 命名规则与数据探索方式 本数据集包含对应每个全球变暖水平及两个基准期的字段，命名格式为「DSI12（12个月累积干旱严重程度指数）_变暖水平/基准期_统计量」，其中统计量分为「upper（上限）」「median（中位数）」与「lower（下限）」。例如「DSI12 2.5 中位数」代表2.5℃预估情景下的中位数结果，其字段别名为「DSI12_25_median」（字段名称中不包含小数点，字段别名可保留小数点）。 如需了解数据探索方法，请参阅此页面：https://climatedataportal.metoffice.gov.uk/pages/user-guide 请注意，若在ArcGIS地图查看器（ArcGIS Map Viewer）中打开本数据集，地图默认将显示「DSI12 2.0°C 中位数」的数值。 ### 「lower」「median」与「upper」值的含义是什么？ 气候模式是气候系统的数值表征。为捕捉未来预估中的不确定性，研究人员会运行多组气候模式集合（ensemble），每个集合成员的初始条件或模式设置存在细微差异。整合所有模式结果可得到未来可能出现的一系列合理情景。本数据集的模式预估包含12个独立的集合成员。 为选取合适的集合成员，研究人员首先为每个集合成员计算DSI 12个月累积量，随后按每个站点的数值从低到高进行排序。其中「lower」字段对应排名第二低的集合成员结果，「upper」字段对应排名第二高的集合成员结果，「median」字段对应集合的中位数结果。该设置可提供中位数以及集合成员的离散区间，用以表征预估结果的可能范围，该离散程度可用于推断预估结果的不确定性：lower与upper字段的差值越大，代表不确定性越高。 基准期的结果同样提供了lower、median与upper值，因为这些数值同样来自用于生成预估的同一套模式，这可确保模式预估与近期历史气候数据之间的对比具备合理性。 ### 有用链接 本数据集的计算遵循《英国高影响天气未来变化》报告中的方法。更多关于英国气候预测（UKCP）的信息，请参阅英国气象局（Met Office）气候数据门户网站内的气候数据相关说明。