Influence of Choice of Time Period on Global Surface Temperature Trend Estimates

Annual global surface temperature and global land surface temperature trends are calculated for all possible periods of the historical record between 1850 and 2009. Two-dimensional parameter diagrams show the critical influence of the choice of start and end years on the calculated trend and associated temperature changes and suggest time scales required to establish robust trends. The largest trends and associated temperature changes are all positive and have occurred over periods ending in recent years. Substantial positive changes also occurred from the early twentieth century until the mid-1940s. The continents exhibit greater long-term warming than the global average overall, but less warming in the early part of the century (segments ending in the 1940s). The recent period of short-term cooling beginning in the late 1990s is neither statistically significant nor unusual in the context of trend variability in the full historical record. Global-mean and land surface temperature changes for periods ending in recent years and longer than about 90 years are extremely unlikely to have occurred by chance. In contrast, short-term trends over less than a few decades are generally not statistically significant. This implies significant contributions of decadal variability to trends estimated over such short time periods.