Average Global Temperature

I am taking the Science elective at the moment. In it, @Hugh_Ross shows a graph of the global average temperature going back over thousands of years. My understanding is that we have only been monitoring the global average temperature for the past few decades, or since we have had the technology to do so.
Can anybody help me to understand how we can know what the temperature has been so far in the past?

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@brianlalor Good question! Generally, climate proxies are used. A proxy is something that stands in the place of something else. In this case, a climate proxy is a side-effect of climate that can be used to reconstruct climate conditions in the past. I suspect there are debates about how effective different climate proxies are in providing accurate temperature estimates, but could not say what those debates are since I am not familiar with the field.

Since it is not possible to go back in time to see what climates were like, scientists use imprints created during past climate, known as proxies, to interpret paleoclimate. Organisms, such as diatoms, forams, and coral serve as useful climate proxies. Other proxies include ice cores, tree rings, and sediment cores (which include diatoms, foraminifera, microbiota, pollen, and charcoal within the sediment and the sediment itself).

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