There's a wealth of indirect data that corresponds with many of these variables and they show a direct correlation with the data in question. Examples of this

There's a wealth of indirect data that corresponds with many of these variables and they show a direct correlation with the data in question. Examples of this include ice core data, which has been gathered on multiple sites in the world, and the concentration of gases contained within is a really good indicator of how things were in the past. Another thing that can be used is tree ring data and other information from proxies. You can study the fossils of fauna and flora deposited over different locations and periods of time to get a good idea of the climate conditions in each place. An interesting correlated piece of data is examining human remains from people. Humans have been buried with foods at the time of their death, and the bones can help establish cause of death. If you go to Peru you can see that the food and living conditions of peoples during El Niño match the surrounding geological data very well. Glaciers are also interesting indicators of climate phenomena, since they form and melt under specific conditions.