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Earth’s natural greenhouse effect

But how do we know Earth’s temperature before 1880?

To understand climate trends from the past, before reliable measurement methods existed, climate experts rely on biological or physical archives – known as “proxies” – that preserve past temperature. Tree rings, coral skeletons, and glacial ice cores (Figure 3) are proxies for annual temperature records, while boreholes (holes drilled deep into Earth’s crust) can show temperature shifts over longer periods of time.

Figure 3: Ice core can give scientists information about the environment at the time the ice was formed. Photo by Erich Osterberg.

The way scientists measure and interpret various proxies depends on the information each proxy provides. For example, to obtain temperature records from tree rings, scientists drill cores into several trees that are growing in a region. They identify site-specific factors that influence tree growth such as temperature, precipitation, altitude, and tree age, and then compare these factors against the width or density of the tree rings over the lifetime of the tree. Once researchers are confident about how local tree growth correlates to air temperature, they then seek out older trees in the region that are preserved (perhaps in a local swamp or lake). By matching rings from trees that partially overlap life spans, scientists can construct a continuous climate record over thousands of years.

Ice cores have bands of light and dark areas with traces of various substances, which can be analyzed as to composition and age, yielding important information about the environmental conditions throughout time. Antarctic ice core records vividly illustrate that atmospheric carbon dioxide (CO2) levels today are higher than levels recorded over the past 800,000 years(Figure 4). 

Atmospheric CO2 levels have risen 30 percent in the last 150 years, with half of that rise occurring only in the last three decades. It is a well-established scientific fact that CO2 (and other gases emitted from industrial and agricultural sources) traps heat in the atmosphere, so it is no surprise that we are now witnessing a dramatic increase in temperature.