Is the average temperature of the earth at ground level currently warmer than it was 1,000 years ago?
Scientists use "proxies" to estimate temperatures before ground-level temperatures were recorded 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 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." Yet, the IPCC has concluded that "in medieval times, as now, climate was unlikely to have changed in the same direction, or by the same magnitude, everywhere."