Coral Bleaching Events 1998 and 2002, Great Barrier Reef

The extent and intensity of two large-scale (>2,000 km) bleaching events on the Great Barrier Reef in 1998 and 2002 were visually assessed by aerial survey.Out of approximately 2900 reefs in the GBR, the aerial survey covered 641 reefs in 2002 (3-20 March) and 654 reefs in 1998 (9 March to 10 April). Note that the same reefs were not necessarily sampled in both years.There was a difference in the way bleaching category labels were recorded between the two years though the amount of bleaching remained the same.Bleaching categories used in 2002 were:0 (60% bleached).Bleaching categories originally used in 1998 (as referred to in the 2001 paper) were:1 (Extreme >60%), 2 (Very High 30-60%) 3 (High 10-30%), 4 (moderate 1-10%), 5 (Low In the dataset the 1998 values have been modified to match the 2002 values. Therefore data in the e-Atlas are now consistent with one another.The 1998 and 2002 satellite SST data which were used to correlate with bleaching observations were aligned by applying an offset based on weather station data. This alignment was because different AVHRR satellites were used for 1998 and 2002 which had different calibrations.Satellite derived SST variables (maximum temperature, days above threshold, degree-days above threshold, spatial adjustment of temperatures by long-term average) were investigated to see which variable correlated best with bleaching intensity. Max3d (the maximum SST occurring over any 3-day period) was the best predictor of the presence or absence of bleaching. SST data period covers 20 December to 7 March in both 1998 and 2002.Geographical range cover the whole Great Barrier Reef: inshore (fringing reefs around islands and isolated reefs close to the mainland) and offshore (located on the mid- and outer-shelf); and regions (northern, central and southern). To determine scale and intensity of coral bleaching on the Great Barrier Reef and its relationship to SST variables. Modelling using nearest-neighbour-analyses was then used to predict bleaching status for all GBR reefs.In 1998, ground-truth surveys carried out on 23 reefs (11°S-18°S) indicated that aerial survey data are likely to be underestimates.In 2002, temperature data was collected using an in situ data logger (Magnetic Island) and weather station (Davies Reef). Field observations of bleaching were also recorded at both these sites.The category labels from 1998 were changed to correspond to 2002 labels.160 m was the flying height for aerial surveys in both years.Thumbnail images are of coral bleaching from the air and below water at Halfway Island in the Keppel Group.Data have been used for the e-Atlas:http://e-atlas.org.au/content/coral-mass-bleaching-extent-1998http://e-atlas.org.au/content/coral-mass-bleaching-extent-2002

本数据集针对1998年与2002年发生在大堡礁（Great Barrier Reef, GBR）的两起大规模（范围超2000千米）珊瑚白化事件，通过航空目视调查评估了其发生范围与强度。大堡礁现有约2900座珊瑚礁，2002年的航空调查覆盖641座礁体（调查时段为3月3日至20日），1998年则覆盖654座礁体（调查时段为3月9日至4月10日）。需注意，两年的调查采样礁体并非完全重合。

尽管两年的白化等级标签记录方式存在差异，但白化程度的统计口径保持一致。2002年采用的白化等级为：0级（白化占比≥60%）。1998年最初使用的白化等级（见于2001年发表的相关论文）为：1级（极端白化>60%）、2级（极严重白化30%~60%）、3级（严重白化10%~30%）、4级（中度白化1%~10%）、5级（轻度白化）。本数据集已将1998年的等级标签修正为与2002年统一的标准，因此电子地图集（e-Atlas）内的1998年与2002年数据现已保持一致。

为匹配珊瑚白化观测数据，研究人员基于气象站数据设置偏移量，对1998年与2002年的卫星海表温度（Sea Surface Temperature, SST）数据进行了对齐。此次对齐的原因在于，两年使用的先进甚高分辨率辐射计（Advanced Very High Resolution Radiometer, AVHRR）卫星型号不同，校准参数存在差异。

研究人员对多种卫星反演的SST变量（包括最高温度、阈值以上持续天数、阈值以上积温、基于长期平均值的温度空间校正值）开展相关性分析，以筛选与白化强度相关性最优的变量。结果显示，Max3d（任意3天周期内的最高海表温度）是预测白化发生与否的最佳指标。两次调查对应的SST数据时段均为1998年和2002年的12月20日至次年3月7日。

本数据集的地理覆盖范围涵盖整个大堡礁，包括近岸区域（岛屿周边岸礁及邻近大陆的孤立礁体）与远岸区域（位于陆架中、外区的礁体），以及北部、中部和南部三个分区。本研究的核心目标为明确大堡礁珊瑚白化的规模与强度，及其与SST变量的关联。随后，研究人员通过最近邻分析（nearest-neighbour-analyses）建模，对大堡礁所有礁体的白化状态进行了预测。

1998年，研究人员对23座礁体（纬度范围11°S~18°S）开展了地面验证调查，结果表明航空调查数据大概率存在低估情况。2002年，研究人员利用原位数据记录仪（磁岛站，Magnetic Island）与气象站（戴维斯礁站，Davies Reef）采集了温度数据，并在这两个站点记录了珊瑚白化的野外观测结果。

1998年的白化等级标签已完成修正，以匹配2002年的分级标准。两年航空调查的飞行高度均为160米。缩略图展示了凯普尔群岛（Keppel Group）中途岛（Halfway Island）上空及水下拍摄的珊瑚白化景象。

本数据集已用于电子地图集的以下页面：
http://e-atlas.org.au/content/coral-mass-bleaching-extent-1998
http://e-atlas.org.au/content/coral-mass-bleaching-extent-2002