Global warming
No TL;DR found
Abstract
The global-mean surface air temperature record has been broken four times during the 1990s (1990, 1995, 1997 and 1998) ensuring that the present decade has been the warmest since 1860, when the instrumental series of global-mean surface air temperature commences. Estimates of Northern Hemispheric temperature over the last 1000 years (Jones et al., 1998; Mann et al., 1998) also suggest that the 1990s have been the warmest decade this millennium for the Northern Hemisphere. Since global temperature in 1999 is unlikely to be as warm as in 1998 (owing to a reversion of the Pacific from an El Niño to a La Niña mode; NOAA, 1999), it would appear very likely that as we end this millennium the world will have just experienced its warmest year (1998) of the last one thousand years. The 1998 global surface air temperature anomaly of +0.58 °C above the 1961–90 average means that, using the new estimate of 1961–90 average planetary surface air temperature of 14.0 °C (Jones et al., 1999), the surface air temperature of planet Earth has not been warmer than the 1998 value of 14.58 °C since at least the tenth century AD, and may be longer. How does this cluster of warm years in the 1990s compare with climate model predictions? Figure 1 plots the global-mean surface air temperature anomalies for the years since 1990 from observations (to 1998) and from a number of recent simple and global climate model (GCM) simulations (to 2020). Although these model simulations have all been performed since 1995, they all have ‘guessed’ the growth in greenhouse gas and sulphur dioxide emissions after 1990. Comparison of these predictions against the observed temperature of the 1990s therefore provides a relevant, independent test of how ‘good’ climate model predictions are. The ranges in the model predictions partly result from differences between these emissions ‘guesses’ and the actual growth in emissions, but more importantly from different model sensitivities to greenhouse gas forcing. The Intergovernmental Panel on Climate Change (IPCC) curves on Figure 1 bound the influence of these two sources of uncertainty using the systematic assumptions contained in Kattenberg et al. (1996). The point of showing this Progress in Physical Geography 23,2 (1999) pp. 283–291