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 show that some regions of the earth were warmer then. For example, large areas of Iceland were cultivated in the 10th century, Norse settlers colonized Greenland, and "a general absence of sea ice allowed regular voyages at latitudes far to the north of what was possible in the colder 14th century." However, the Intergovernmental Panel on Climate Change has concluded that "in medieval times, as now, climate was unlikely to have changed in the same direction, or by the same magnitude, everywhere."