Climate change the paleoclimate record

In June 1990 scientists were brought up sharp by a graph which appeared in the journal Nature (Figure 1.2). It was evidence from ice core samples which showed a remarkably close correlation between temperature and concentrations of CO2 in the atmosphere from 160 000 years ago until 1989. It also revealed that present concentrations of CO2 are higher than at any time over that period. Since then the rate of increase has, at the very least, been maintained.

Ice core samples give information in four ways. First, their melt layers provide an indication of the time span covered by the core. Second, a measurement of the extent to which ice melted and refroze after a given summer gives a picture of the relative warmth of that summer. A third indicator is the heavy oxygen isotope 18O in air trapped in the ice. It is more abundant in warm years. Finally, the air trapped in the snow layers gives a measurement of the CO2 in the atmosphere in a solar radiation: 178 terrawatts a year

I_ 30% reflected back into space, f —. 50% absorbed by the Earth.

infra-red radiation is emitted by the Earth. Some is reflected back by the greenhouse gases and warms the Earth's surface

Atmosphere containing / greenhouse gases /

EARTH

The greenhouse mechanism

EARTH

The greenhouse mechanism

Figure 1.1

The greenhouse 'blanket'

Figure 1.2

Correspondence between historic temperature and carbon dioxide

Figure 1.2

Correspondence between historic temperature and carbon dioxide

given year. Other data from ice cores show that, at the peak of the last ice age 20 000 years ago, sea level was about 150 m lower than today.

Another source of what is called 'proxy' evidence comes from analysing tree rings. This can give a snapshot of climate going back 6000 years. Each tree ring records one year of growth and the size of each ring offers a reliable indication of that year's climate. The thicker the ring, the more favourable the climate to growth. In northern latitudes warmth is the decisive factor. Some of the best data come from within the Arctic Circle where pine logs provide a 6000-year record.

The Climate Research Unit of the University of East Anglia has made a special study of the evidence for climate changes from different sources and has concluded that there is a close affinity between ice core evidence and that obtained from tree rings. Also instrumental records going back to the sixteenth century are consistent with the proxy evidence.

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