Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends
Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends
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DOI：
10.1002/qj.2297
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发表时间：
2014-07-01
影响因子：
8.9
通讯作者：
Way, Robert G.
中科院分区：
文献类型：
--
作者：
Cowtan, Kevin;Way, Robert G.
如果未采样的区域不均匀地分布在地球表面，不完整的全球覆盖范围是全球温度重建中的一个潜在偏差来源。近几十年来，广泛使用的哈德利中心-气候研究单元版本4(HadCRUT4)数据集平均覆盖了全球约84%的区域，未采样的区域集中在两极和非洲上空。研究了三种现有的近全球覆盖的重建方法，每一种方法都表明HadCRUT4由于其对未观测区域的处理而受到偏差的影响。探索了两种重建全球温度的替代方法，一种基于最优内插算法，另一种是结合来自卫星温度记录的额外信息的混合方法。这些方法是基于它们重建省略观测集的技能而得到验证的。这两种方法都提供了比排除未采样区域更好的结果，其中混合方法在没有可用观测值的区域周围显示了特殊的技巧。自1997年以来，混合全球重建中被广泛引用的趋势比偏向于复盖面的HadCRUT4数据中的相应趋势大2.5倍。报道偏差导致近期气温相对于20世纪90年代末的冷偏差，这种偏差从1998年左右增加到现在。从1997年或1998年开始的趋势与全球趋势相比尤其偏颇。1997-1998年的强烈厄尔尼诺现象加剧了这一问题，该事件也倾向于抑制从那几年开始的趋势。
Incomplete global coverage is a potential source of bias in global temperature reconstructions if the unsampled regions are not uniformly distributed over the planet's surface. The widely used Hadley Centre-Climatic Reseach Unit Version 4 (HadCRUT4) dataset covers on average about 84% of the globe over recent decades, with the unsampled regions being concentrated at the poles and over Africa. Three existing reconstructions with near-global coverage are examined, each suggesting that HadCRUT4 is subject to bias due to its treatment of unobserved regions.Two alternative approaches for reconstructing global temperatures are explored, one based on an optimal interpolation algorithm and the other a hybrid method incorporating additional information from the satellite temperature record. The methods are validated on the basis of their skill at reconstructing omitted sets of observations. Both methods provide results superior to excluding the unsampled regions, with the hybrid method showing particular skill around the regions where no observations are available.Temperature trends are compared for the hybrid global temperature reconstruction and the raw HadCRUT4 data. The widely quoted trend since 1997 in the hybrid global reconstruction is two and a half times greater than the corresponding trend in the coverage-biased HadCRUT4 data. Coverage bias causes a cool bias in recent temperatures relative to the late 1990s, which increases from around 1998 to the present. Trends starting in 1997 or 1998 are particularly biased with respect to the global trend. The issue is exacerbated by the strong El Nino event of 1997-1998, which also tends to suppress trends starting during those years.