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Natural Climate Variability on Decade-to-Century Time Scales
Rates of global warming during periods of most rapid warming in the geologic past, gleaned from various sources. Filled triangles represent global mean temperatures as inferred from measurement data. Open triangles indicate north temperate zone measurements. Open circles represent modeled or predicted values. Numbers near each point indicate the period of warming (past century) or thousands of years before present (prior to the past century) when the period of warming began. Letters in parentheses indicate source of data: Balling, 1992; Barnola et al., 1987; Boden et al., 1992; Clark, 1982; IPCC, 1990a,b; Shackleton and Opdyke, 1973; Wu et al., 1990. Solid line marks approximate upper limit of the periods of rapid global warming. Broken line indicates approximate upper limit of warming in the north temperate zone.
and the Southern Oscillation Index, is removed, the increase in global marine air temperature attributable to CO2 increase is reduced to 0.24°C.
How does this recent global temperature change compare with changes found in other periods in the geological past during which temperatures have increased rather more than the average? I have searched historical records for maximum rates of warming for various periods of time.3 From these I have selected periods during which there was sustained warming, and have calculated a warming rate for each. Data from these periods of maximum rates of warming are plotted on Figure 1 as filled triangles. They include the sharp 60-year recovery from the Little Ice Age; the 1.5°C increase over the 250 years at the end of the Little Ice Age; and the average for the 17,000 years since the start of warming at the end of the last glaciation. For comparison, I show several periods of rapid warming in the past century or so, including the largest year-to-year increases.
Shackleton and Opdyke (1973; quoted in Balling, 1992) used oxygen-isotope data from deep-sea cores as a reflection of global ice-volume changes. The ice-volume change can be interpreted in terms of global mean temperature changes over the past 850,000 years. Temperature increases for the periods that show the most rapid warming (interpreted from Balling, 1992, Figure 1) have also been plotted on Figure 1. In all of these cases, I have chosen those periods exhibiting the most rapid temperature increases.
I have used secondary sources for most of the data for two reasons: For this first analysis, I preferred to use data that have been accepted by others as representing global mean temperature; and my lack of access to a major scientific library severely limits the availability of primary data sources.