Global and regional climate in 2007

Abstract

The global average near-surface temperature in 2007 was 0.40 ± 0.10 degC above the 1961–1990 average (Figure 1(a)) and 2007 was nominally the seventh-warmest year in the 158-year HadCRUT3 record (Brohan et al., 2006). The year was characterized by a record-low Arctic sea-ice extent and by unusually high temperatures over the North Atlantic and much of Eurasia. The HadCRUT3 dataset is calculated from land-surface air temperature measurements made at about 4350 land stations and a database of approximately 190 million sea-surface temperature (SST) observations collected since 1850. However, the global average temperature cannot be calculated exactly because large areas of the Earth’s surface remain unobserved and measurements are prone to errors and biases. The uncertainties are such that none of the nine warmest years are distinguishable in a statistically meaningful way and therefore it is not possible to state with absolute certainty that 2007 was the seventh-warmest year on record. Nonetheless, it is virtually certain that 2007 was one of the ten warmest. 1998 remains the warmest year on record with a global average near-surface temperature anomaly of 0.52 ± 0.10 degC above the 1961–1990 average (Figure 1(a)). The exceptional warmth of 1998 was a result of the strong El Nino that developed towards the end of 1997. A moderate El Nino developed towards the end of 2006 and in January 2007, which was the warmest January in the record, the Met Office Hadley Centre issued a forecast saying that there was a 60% chance that 2007 would be warmer than 1998. But SSTs in the tropical Pacific fell rapidly, dropping below La Nina thresholds by May and, subsequently, global average temperatures fell too. By January 2008, the La Nina had become the strongest since that of 1988/1989 (Figure 2). The annual average Northern Hemisphere (NH) near-surface temperature anomaly in 2007 was 0.63 ± 0.11 degC (Figure 1(b)). It is virtually certain to be one of the eight warmest years on record. The Southern Hemisphere (SH, Figure 1(c)) was somewhat cooler with an annual average anomaly of 0.18 ± 0.13 degC and, given the uncertainties, it may have been the fourth-warmest or not even in the top twenty. The annual average temperature anomaly for the tropics (20 N– 20 S) was 0.30 ± 0.03 degC, somewhere Figure 1. (a)–(c) Annual combined land-surface air and sea-surface temperature anomalies (degC) for the period 1850–2007, relative to the average for 1961–1990. (a) Globe; (b) Northern Hemisphere; (c) Southern Hemisphere. The uncertainty range represents ± 2 standard errors. The smooth curves were created using a 21-point binomial filter to highlight the interdecadal variations. Data are from Brohan et al. (2006). 186