To compare today’s climate with that of the distant past, scientists use proxy data, which provide indirect evidence of how temperature and other climate properties varied before instrumental measurements became available. Proxy climate data are derived from a diverse range of sources including ice cores, tree rings, corals, and historical documents.
A recent assessment of these data concluded that the past few decades were warmer than any other comparable period during at least the last 400 years, and possibly the last 1,000 years or longer.
Scientists use measurements of tree ring samples collected from numerous locations to reconstruct climate condition trends for the last two millennia. Wider rings correlate to increased temperature and precipitation.
Temperature records generated by humans extend back over 250 years in some locations. Observing stations put in place in the mid-nineteenth century provide data to estimate average temperatures.
Scientists analyze hydrogen and oxygen atoms in ancient ice cores to infer past temperatures. Bubbles trapped in the ice, as shown in the model, are sampled to determine greenhouse gas concentrations when the bubbles were formed.
Annual density bands in coral skeletons provide information about temperature and salinity. Each year is composed of a dark and light band which provides a record of growth rates that change in response to environmental and climatic conditions. Shallow-water corals grow in tropical and subtropical waters and complement records derived from land-based trees.