Scientists use "proxies" to estimate average global temperatures in the era before instrumental measurements were made on a global scale. Proxies are aspects of nature that respond to climate, such as the widths of tree rings. Proxy studies are uncertain, and some suggest the earth is warmer now than 1,000 years ago, while others indicate the opposite. Historical records prove that some regions of the earth were warmer within the past 1,000 years than currently. For example, large areas of Iceland were cultivated in the 10th century, and Norse settlers colonized areas of Greenland. However, these regional indicators may not be representative of the entire globe.