A CLIMATE-CHANGE SIMULATION STARTING FROM 1935

Due to restrictions in the available computing resources and a lack of suitable observational data, transient climate change experiments wit h global coupled ocean-atmosphere models have been started from an ini tial state at equilibrium with the present day forcing. The historical development of greenhouse gas forcing from the onset of industrializa tion until the present has therefore been neglected. Studies with simp lified models have shown that this ''cold start'' error leads to a ser ious underestimation of the anthropogenic global warming. In the prese nt study, a 150-year integration has been carried out with a global co upled ocean-atmosphere model starting from the greenhouse gas concentr ation observed in 1935, i.e., at an early time of industrialization. T he model was forced with observed greenhouse gas concentrations up to 1985, and with the equivalent CO2 concentrations stipulated in Scenari o A (''Business as Usual'') of the Intergovernmental Panel on Climate Change from 1985 to 2085. The early starting data alleviates some of t he cold start problems. The global mean near surface temperature chang e in 2085 is about 0.3 K (ca. 10%) higher in the early industrializati on experiment than in an integration with the same model and identical Scenario A greenhouse gas forcing, but with a start date in 1985. Com parisons between the experiments with early and late start dates show considerable differences in the amplitude of the regional climate chan ge patterns, particularly for sea level. The early industrialization e xperiment can be used to obtain a first estimate of the detection time for a greenhouse-gas-induced near-surface temperature signal. Detecti on time estimates are obtained using globally and zonally averaged dat a from the experiment and a long control run, as well as principal com ponent time series describing the evolution of the dominant signal and noise modes. The latter approach yields the earliest detection time ( in the decade 1990-2000) for the time-evolving near-surface temperatur e signal. For global-mean temperatures or for temperatures averaged be tween 45-degrees-N and 45-degrees-S, the signal detection times are in the decades 2015-2025 and 2005-2015, respectively. The reduction of t he ''cold start'' error in the early industrialization experiment make s it possible to separate the near-surface temperature signal from the noise about one decade earlier than in the experiment starting in 198 5. We stress that these detection times are only valid in the context of the coupled model's internally-generated natural variability, which possibly underestimates low frequency fluctuations and does not incor porate the variance associated with changes in external forcing factor s, such as anthropogenic sulfate aerosols, solar variability or volcan ic dust.